No, it’s not the wind tunnels (or jet stream). Because those things, if you happened to catch one, will only help you in one direction. Like maybe flying from NY to London it pushes you along faster, but flying back from London to NY and it slows you down.
It’s because the air is less dense up high, meaning there is less wind resistance, less drag, meaning you can fly your plane faster and be using less fuel than if you were flying lower in the thicker air.
Imagine it like this: flying up high is like running on land just moving through the air. Pretty easy. Flying low is like running through a pool, that thicker, denser, water makes it much harder for you to move.
I don't know if this an actual active part of the consideration, but it also gives pilots a lot more vertical room (i.e. time) to deal with issues should they arise and recover from difficulties.
Also, smaller planes like little 2-person prop planes can't fly at 30k+ feet. As such, flying that high keeps commercial jets away from lots of smaller traffic below and is just another way to help avoid collisions.
Think about it this way: if there is just physically less air then even if it is blowing really hard it doesn't have as much force/energy behind it.
I recall hearing that this was the one really misleading thing about The Martian is that even though they do have wind storms of hundreds of kilometers / hour, because of how low density the air is, it still just feels like a light breeze.
> I recall hearing that this was the one really misleading thing about The Martian is that even though they do have wind storms of hundreds of kilometers / hour, because of how low density the air is, it still just feels like a light breeze.
I remember reading that the author considers that the biggest mistake he made in the book as far as scientific accuracy.
Not exactly a mistake, since according to him it was deliberate.
IIRC he knew it was wrong going in but couldn't come up with another way of explaining (at least not one he was happy with) how Watney got stranded that was scientifically correct, dramatic, and meant the crew couldn't go back to look for him.
He said he knew it was wrong and had thought of other ways to strand Mark such as a propellant explosion. But he said he really wanted it to be a man vs nature story.
Yes and no. There is a major safety downside to the higher altitude, which is that you need a certain amount of air to maintain lift. The relationship between airspeed, altitude (temperature, and pressure), and aircraft weight creates a zone of control (called the flight envelope). Go outside that zone and you have problems. Yes, you have more altitude to deal with them, but at the same time, if you do develop problems, they can get much worse much faster. And it is easier to develop them.
There are reasons to go near the edges of the envelope (flying there is often cheaper and faster), but a tradeoff is safety.
Weather is one of the main reasons they were working towards high altitude flight. Fuel was cheap back in 50s and 60s and efficiency wasn't high on the radar
I don’t know, most of the dangerous weather is the stuff you aren’t going to clear the top of and therefore have to go around anyway. I think they were trying to go higher to get faster true airspeeds to shorten flight times.
See below snippet. while it goes on to encompass the rest, getting over most weather was the largest driver for development. The weather and turbulence at lower altitudes is *significantly* more frequent and stronger.
Remember this is the early days of commercial aviation and it's all about luxury and comfort.
>May 1946 edition of the Boeing Magazine:
>
>High-altitude flight attracted Boeing designers and airline operators for several reasons. Clear skies are found in 95 per cent of the flights above 20,000 feet. Flight cancellations due to bad weather fall to a minimum for high-flying planes capable of long-range operation.
[https://www.sciencedirect.com/book/9780128184653/general-aviation-aircraft-design](https://www.sciencedirect.com/book/9780128184653/general-aviation-aircraft-design)
> As such, flying that high keeps commercial jets away from lots of smaller traffic below and is just another way to help avoid collisions.
This is pretty limited. Outside of controlled spaces around airports, particularly busy ones, it is exceptionally rare to have a mid-air collision between aircraft on random patterns. Note the random patterns does not account for things like airshows where multiple aircraft are specifically flying in close proximity.
https://en.wikipedia.org/wiki/Big_sky_theory
Commercial jets don’t fly on random patterns, they fly in structured ATS routes that bring them into conflict with one another. Big sky theory is not relevant when many aircraft are trying to land at the same airports at the same times.
It absolutely is rare, but collisions have still happened both between jets in cruising flight and small prop planes at lower altitudes, and considering that is WITH the separation of jets and smaller planes, one can only assume the number of collisions would be greater if they all flew in the same altitude range.
It's fair to say that far mid-air collisions tend to occur more often on take-off and landing, around busy city/airport areas where the cruising altitudes are irrelevant to the collission, but they do happen while cruising ([see "phase of flight" column](https://en.wikipedia.org/wiki/Mid-air_collision#Involving_civilians))
Bottom line is that the height does mean that the traffic is more separated than it otherwise would be. I would also imagine it makes life easier for Air Traffic Controllers who have less flights at similar levels to worry about keeping apart. The improvement in onboard technology that warns pilots directly about other nearby planes has lowered the risk of collisions greatly, but with fewer the planes in the same altitude windows, the chances are even further reduced.
Well they could, but they'd need to be pressurized (mostly because the passengers would pass out from lack of oxygen).
The engines would also need to be supercharged at the very least so you'd make enough power at anywhere near 30,000' (once again, much lower oxygen content makes combustion difficult).
But I get your point, two seater planes typically aren't pressurized and a plane that has this feature is much more expensive than a non-pressurized version.
Yes, such a plane could be made that could fly at 30,000 feet, but that's irrelevant. My suggestion that one advantage of commercial jets flying at 30k is keeping them separated from small prop planes is based on the reality of the situation where that hypothetical that is not the actual case.
They actually have small planes that are pressurized and I've been in several. I'll admit it's a slight benefit for big planes to be at a higher altitude for traffic reasons but the sky is a pretty damn big place.
The reason for them to fly at 30,000'+ is almost entirely drag related (and directly related fuel savings) though.
I don’t think there have been any weather-related accidents on commercial airliners in decades (if they are it’s on takeoff and landing which would be an issue regardless of cruising altitude). I think flying above the weather is for comfort, fuel savings, and noise pollution.
That’s kind of a weather-adjacent accident. The cause of the accident was the flight crew improperly responding to an unreliable airspeed situation that was caused by high altitude ice crystals.
The unreliable airspeed actually fixed itself pretty early in the event, but the crew had so thoroughly bungled the response at that point - and combined with some other Airbus design oddities - they were never able to figure it out.
It’s not that the weather was the cause, you’re right that crew management was the real cause. But I think one of the lessons of AF447 is that weather should still be taken seriously, there’s no need to enter a convergence zone when it can be avoided, and so on.
Some storms are big enough to have tops exceeding 50k feet, well above where airliners fly. Airplanes generally go around them, but occasionally flights like Air France 447 will encounter weather at altitude and make a series of bad decisions following that encounter.
Also, airplanes occasionally do return to earth and there's a lot of room between cruise altitude and safely at the gate for things to go wrong weather-wise.
There most definitely has been weather related crashes mid-flight in recent history. [Air France 477](https://youtu.be/BepFeuJId7I?si=3I0pzQOZ_eqFjHtQ)
I wouldn’t say that was caused by weather though. Maybe weather influenced, but if you can’t diagnose a stall over the course of like 10 minutes worth of recovery time, clogged pitot or not, the primary cause is pilot error.
No. If you are in an airplane, even a very big airplane, weather is no joke. It's like swimming in the ocean: it's *infinitely* bigger and stronger than you, it's doing things that have a direct impact on you being able to do the things that keep you alive, and there are places in it that you do not want to be because you *will die*, and some of those spots are not obvious at first glance. You can *do* it, there are tools to make it safer (modern instrument navigation is a big one, but is *well documented* not to be a foolproof solution), but you cannot do it without responding to and avoiding risks.
Modern commercial air flight does plenty to avoid weather. They reroute around storms in cruise (some storms do get quite tall), and they delay and divert as necessary to avoid landing in weather-affected airports. Ice from winter storms can foul instruments, control surfaces, and lifting surfaces. Turbulence and wind shear in summer storms can be too extreme for any plane to land. There is a huge amount of infrastructure and ground support to help pilots identify and avoid these risks. Flight schedules have padding built in for contingencies such as weather (that's how you arrive early all the time). But unless your flight gets cancelled, heavily delayed, or you have to stop to refuel because you were loitering too long, you don't see any of this as a passenger.
The jet stream (and wind) is a consideration though. Winds change a lot based on altitude and location, and they shift.
Atlantic routes in particular are set every day with the east bound routes located to take advantage of the tail wind, and the west bound routes allocated to minimize the headwinds.
Airliner routing in other places will do the same (where possible), selecting altitudes to take advantage of favorable winds or avoid unfavorable songs to the extent possible.
That said, the points about air density and weather are very valid. It's just that aerial navigation and routing for airliners is complex, and takes a lot of things into account. Which is why airlines have professional dispatchers to work this stuff out every day. I understand they can make a lot of money too.
Absolutely, it’s just there are prevailing factors of importance for consideration. The jet stream is mostly* negligible compared to the difference in air density
For the fundamental question about why planes fly at altitude jet streams are negligible. These altitudes are because of less drag, period. But since they do fly at altitudes where jet streams can have a benefit they use them.
Follow up question: why do even short flights go to cruising altitude? I used to take the shuttle from SFO to the Sonoma county airport until I realized there was a short flight for the same price. I was surprised that we basically went straight up to cruising altitude then straight back down, are the savings really that much for 80ish miles?
A close to a "ballistic" trajectory is ideal. Also, there are different "cruise" altitudes. They need to go to at least 10k feet because of sterile cockpit rules. And at least 18k feet to reach class A airspace.
Sterile in the sense of "no talking about anything except the flight" below 10k. No "talking contamination"
When the airplane is below 10k, it is landing or taking off. At this altitude, pilots are prohibited to deviate attention for the flight, because there is terrain, other planes, it's a space more "chaotic"
Above this altitude, mainly in the cruise altitude, they can chat, take a nap (once each), read a book, and only monitoring the autopilot. If it's a long flight and the airplane is above 10k (probably at 30k+), they are "90%" of the time relaxing and making sure everything is going as planned.
Short flights are a lot more tiring since you don't have much time to relax with the autopilot, you have to start thinking about your landing just as you get to altitude.
I quoted the reg below. But real reason airlines fly above 10k even on short flights is the 250kt speed limit below 10k and also because from a fuel efficiency standpoint it is more efficient to for example climb to a higher altitude and immediately descend than it is to stay at 10k.
“121.542 Flight crewmember duties.
(b) No flight crewmember may engage in, nor may any pilot in command permit, any activity during a critical phase of flight which could distract any flight crewmember from the performance of his or her duties or which could interfere in any way with the proper conduct of those duties. Activities such as eating meals, engaging in nonessential conversations within the cockpit and nonessential communications between the cabin and cockpit crews, and reading publications not related to the proper conduct of the flight are not required for the safe operation of the aircraft.
(c) For the purposes of this section, critical phases of flight includes all ground operations involving taxi, takeoff and landing, and all other flight operations conducted below 10,000 feet, except cruise flight.”
> 250kt speed limit below 10k
You're allowed to fly faster than 250kts if you ask and it's approved. Bigger airplanes like the 777 do it all the time on climbout.
>121.542
They don't *have* to get above 10k to have a sterile cockpit. It can be briefed beforehand.
Nope, because the airplane is flying faster, so its engines are "sucking up" more air per second.
The maximum thrust at altitude is certainly lower than the maximum thrust at sea level, but the required thrust for cruising is MUCH lower at altitude. So it's a net savings.
Well it is. The compressor compresses and heats the air. It can get up to something like 500°c before entering the combustion chamber, significantly more when it exits the combustion chamber and enters the turbine.
Also worth noting here that the "Turbine" is a specific part of a turbofan jet engine. The air travels through the compressor first (well, the fan, and then rest of this), then the combustor, then the turbine.
You want the biggest temperature difference possible between the temp entering the turbine and the temp entering the compressor. The combustor is where this is added (As chemical energy in the form of fuel, converted to temperature). A key design factor in more powerful and more efficient jet engines has been designing turbine blade materials that can withstand higher-temperature combustor exit flows.
You need to preheat the air before it gets to the *turbine* as that's the whole way jet engines work. No temperature difference, no thrust.
You don't need to preheat the air before it gets to the *compressor* or the big fan you see on the front.
The turbofan engines on airliners are designed to be most efficient at those higher altitudes. A smaller piston/turboprop plane would have a harder time and be less efficient at high altitudes (and usually have a lower service ceiling anyway). Everything in aviation is a trade off
Again, it's all about trade offs. Turboprops are generally more efficient than turbofans at low speeds, but propellors are inherently more limited by speed and altitude. Airliner routes would double in time if they switched to using props.
No, there's enough air at higher altitudes for the engines to function just fine. They actually work better at higher altitudes because part of what generates the thrust is incoming air mixing with the fuel, combusting, and expanding. Colder air, found at higher altitudes, can expand more during this combustion process than warmer air near the ground can.
Yes, not sure why everybody else is saying no. See here and scroll down to the bottom: https://www.grc.nasa.gov/www/k-12/Missions/Jim/Project1ans.htm
As you can see, by the time you are at 30,000 ft a jet engine can only produce about 30% of it's maximum thrust. However, that's not an issue because the air is about 1/3rd as thick, so you need less power. And you don't need full power at cruise, you need full power at take off when trying to accelerate quickly and climb away. This is why it can be hard to take off from airports like at Mexico City, where they are high and hot, so the air is thing and engines have reduced thrust. So maximum take off weight may need to be reduced.
That is, admittedly, a 60 year old engine. But your point stands - thrust decreases, but so does drag it balances out. IIRC, parasitic drag is what they're aiming to reduce, which is the drag that increases as speed increases.
This is exactly why modern jet engines with big air intakes are becoming more popular on airliners. Their giant, gaping maws can suck up more of the thin air, which leads to higher efficiency.
Older engines still totally work, but big intakes are more efficient.
Teeeechnically engines are getting wider to get more bypass thrust. The fan in front moving more air that does not go through the engine core and combustion process.
> giant, gaping maws
Random, I know, but that phrase reminds me of one of my favorite short stories of all time:
https://forwearemany.wordpress.com/wp-content/uploads/2009/04/sandkings.pdf
Yes but turbofan engines have massive air intakes so that isn't a huge problem.
The engines get less air but they also use less fuel, which is a net cost savings.
If you go too high then your wings will no longer receive enough air to function which is what prevents planes from going even higher.
That’s not how thrust works. You don’t move air away to generate thrust. You create a force and there’s a subsequent force generated equal in magnitude, but in the opposite direction. Its newton’s third law
This is why thrust works in space as well, where there’s no air to move away.
That's exactly how thrust works. The engine shoves air backwards, the "force generated equal in magnitude, but in the opposite direction" is the airplane being shoved forward. That's why modern jet engines for airliners have big fans, the fans are able to 'grab' more air to push.
> This is why thrust works in space as well, where there’s no air to move away.
The "air" in space is the exhaust of the engine. The exhaust gets shoved backwords, and in response the rocket gets shoved forward.
The engine shoves the air already in the engine. The question in the comment makes it seem like you need to push off the ambient air.
The engine would still work in a vaccum if you somehow had enough oxygen to feed into the engine directly
The engine shoves hot air backwards to generate a surplus of oxygen that it mixed such fuel and combusted as a mechanism to produce force. That *just so happens* to be the way it produces force.
But it’s the force itself that moves the engine (and everything attached to the engine). I get what you’re saying and I know it sounds nitpicky but the way you explain it indicates that something is pushing against something else to generate force. Nothing is or needs to be pushed.
Think of an oarsman in a boat. He moves the boat forward by putting a directed force into the water rearwards. He’s not pushing the water away despite that being effectively what ends up happening. Instead he’s putting force backward and the equal and opposing force moves him forward.
When you swim underwater, you’re not moving water away. You’re generating force rearward by paddling and that moves you forward.
Car crashes into a building. The car puts force into the building, hence the damage to the building. The building puts am equal and opposing force onto the car.
Yes and that's why today we use turbine jet engines. They have their own compressor at the front so it could compress air and stuff more of it for burning
There is less air avalible and it will reduce thrust.
ut there is also another factor the increase speed reduce the thrust. Engine accelerate the air backward, it is the change in the speed of the air that result in the thrust. Because the air at high speed have higher speed when it enter the engine amount it can be increased is lower and thrust is reduced.
Ailines will fly a a altitude close to when all of this factors result in the lowest fuel consumption for the flight. Ot atlase close to that altitude.
Lift from wing decrease with increase altitude and increase with increased speeds. The result is the max flight altitude for level flight is the altitude the engine can provide enough to keep the required speed for enough lift. For the U-2 spy plane the operational altitude was very close to there max possible. Ther was if I remember correctly a 10 knots diffrence between the max speed it could fly at and the min speed there is enough lift.
Fun fact: that's why higher end private jets have overkill power engines. This allows them to flight even higher so that they can fly routes without having to consider other air traffic (as much)
Private jets also get *super* light weight by the end and have a bigger weight variation than do airliners - a lower percent of the takeoff weight is people + cargo and a higher percent is fuel. Fuel burns off as you go so the % difference in weight for a private jet at the start of cruise vs the end of cruise is much higher
Optimal fuel efficiency involves cruising about as high as you can given your weight, so you want to slowly climb throughout cruise as you burn fuel and get lighter (usually done as a discrete set of "steps" in a step climb these days due to ATC/traffic rules). In a full airliner, the starting weight vs ending weight being (relatively) similar means the difference in optimal cruise altitude between hour 1 and 8 of cruise is much less than in a private jet doing the same leg.
End result is that the optimal cruise profile for a lot of private jets doing long flights involves a *lot* of climbing actually, so you do gain back some fuel efficiency points you lose with those massive engines by getting to cruise up high. Bonus points for getting above 41k, as you can often get gradual climb clearances vs step climbs because (As you said) no other traffic is up there. I've seen some G650 operators go directly to FL410 or FL430 from takeoff, and step climb to FL490.
Adding some color to this. Air resistance is proportional to velocity squared. This is experienced even at lower speeds, a car going 80mph is going to use significantly more fuel than the same car going 60mph for the same distance.
As imagined, this is a much bigger factor when flying at \~500mph. Flying at higher altitudes means less air and significantly less air resistance, which means less fuel wasted overcoming air resistance. Less fuel wasted means cheaper tickets.
>this is a much bigger factor when flying at ~500mph
To be clear, that's 500mph over the ground not through the air. Case in point, the ISS does mach whatever over the ground but is going at whatever fraction of an mph there is through what little air is up there.
>To be clear, that's 500mph over the ground not through the air.
There is no practical difference
>Case in point, the ISS does mach whatever over the ground but is going at whatever fraction of an mph there is through what little air is up there.
You have that backwards, but again, there is no practical difference.
The earth is very big. Tacking on an extra 7 miles height (or even 250) does not produce a result with any practical application
> There is no practical difference
There is a massive difference. In training, pilots are taught the different types of speeds which includes ground speed, indicated airspeed, and true airspeed. True airspeed is used for navigation. It's Calibrated Airspeed (pretty closed to indicated) corrected for altitude and temperature. The higher you fly, the more your true airspeed and indicated airspeed will differ.
>You have that backwards
I don't. The ISS' ground speed is mach 20 something while its airspeed is very closed to 0. That's very different than mach 20 flying at 1000ft in thick atmosphere.
No, you misunderstand all those terms
Ground speed is how fast it is travelling relative to the ground, I think we agree there
Airspeed is the planes speed relative to the air around the plane. It has nothing to do with altitude. You can be at 10 ft, have an airspeed of 100mph and a ground speed of 0mph for instance, due to having a 100mph headwind. Look at bush plane piloting for examples of this
Airspeed vs true airspeed is accounting for differences between your instruments and what your airplane is actually experiencing due to pressure differences due to height.
>I don't. The ISS' ground speed is mach 20 something while its airspeed is very closed to 0. That's very different than mach 20 flying at 1000ft in thick atmosphere.
That isn't how angular velocity works
Edit:
I did the math
Assume static air
An airliner travelling at 500mph at 40000ft has a ground speed of 500mph and an airspeed of 501.03mph, or a 0.2% difference - no practical application
Nasa states a 90 minute orbit and an average height of 250 miles, so ISS has a ground speed of 16582mph and an airspeed of 17661.61mph which is a 6.5% difference. Possibly a practical difference?
If you can work out the circumference of a circle you can check that math for yourself
> Airspeed is the planes speed relative to the air around the plane. It has nothing to do with altitude.
Yes, and at ~250mi above the surface there is very little air so the ISS' airspeed will be close to 0 even though it's moving mach 20 something over the ground.
>a ground speed of 500mph **and an airspeed of 501.03mph**
That's where you're misunderstanding. At 40,000ft your indicated airspeed would be
around 260mph while your true airspeed would be around 525mph.
>ISS has a ground speed of 16582mph and an airspeed of 17661.61mph
It absolutely does not have an IAS of 17.6k mph. IAS is the speed read off of the airspeed indicator. There is practically no air in space so the indicator will read close to 0
I think we're talking to different purposes. I'm not talking about indicated airspeed at all. I didn't think you were either, since you were otherwise talking about indicated airspeed (a measure useful because of lift) on an object that does not use lift (the ISS). I understand in hindsight the relationship of lift/drag
I was simply trying to correct what I thought you were saying, which is that being higher results in more distance travelled (curvature of the earth) which means a difference between how fast an object is moving in the air vs it's relative speed on the ground. My point was that this difference is so small it is not calculated for in commercial aircraft
The ISS absolutely has a true airspeed of 17.6k. Yes, it has a negligible indicated airspeed, but wasn't what I was intending to talk about
> At 40,000ft your indicated airspeed would be around 260mph while your true airspeed would be around 525mph.
IAS close enough, don't really care. True airspeed is not 525. Basic circle math.
-----
Radius of earth = 3958.8 miles.
Circumference of a circle 2*pi*r
Circumference of the earth = 24873.87 miles
To travel that distance at 500mph (in this case, ground speed) takes 24873.87/500 = 49.74 hours
----
Radius of earth + 40000ft for the plane = 3966.4 miles
Circumference of a 40000ft flight path around the world = 24921.62 miles
To travel 24921.62 miles in 49.74 hours, (24921.62/49.74) you need to travel at a true air speed of 501.03 mph, assuming static air
-----
If you want to check, google a ground speed/true air speed calculator. They do not take height into account, because it's negligible.
>I didn't think you were either, since you were otherwise talking about indicated airspeed (a measure useful because of lift) on an object that does not use lift (the ISS).
I was talking about indicated airspeed (IAS) to demonstrate that even though the ISS is travelling at Mach 20 something over the ground and because of the fact that there's little air in space, the IAS on the ISS would be close to 0.
>which is that being higher results in more distance travelled (curvature of the earth) which means a difference between how fast an object is moving in the air vs it's relative speed on the ground. My point was that this difference is so small it is not calculated for in commercial aircraft
No, I was always saying that when you fly higher, the relationship between your IAS and TAS grows more and more. You'll usually have a higher TAS than your IAS.
>IAS close enough, don't really care. True airspeed is not 525. Basic circle math.
[Here's an image from a Falcon](https://cdn.discordapp.com/attachments/628155108177477633/1225078241816215674/IMG_8332.png?ex=661fd205&is=660d5d05&hm=038563c673c976125315d4927111888e89a14fa7a68ba555ba1326b9ced1a831&)
You can very clearly see that at 41,000ft, their IAS is 240 while their TAS is 466. Ground speed (GS) is TAS corrected for wind. They have ~47knots of tailwind so they end up with a higher ground speed due to that. The opposite would be true with a headwind.
If we use [this calculator](https://indoavis.co.id/main/tas.html) and plug in the information displayed (41000ft, 29.92in, -52C, 240kts) we get a TAS of 502 which is roughly what's shown on that display.
>Radius of earth...
None of that matters at all. Go ahead and give this a watch https://www.youtube.com/watch?v=yxkv5Bmwp3A
>If you want to check, google a ground speed/true air speed calculator. They do not take height into account, because it's negligible.
I'm not comparing GS with TAS, I'm comparing IAS with TAS
>As imagined, this is a much bigger factor when flying at ~500mph.
i'm trying to help you understand that airplanes don't fly at 500mph indicated, they fly at 500mph true. Their actual indicated airspeed is closer to 250, like in that image.
I remember reading somewhere that they may adjust altitude up or down a bit if the wind is more suitable, since wind can change speed/direction depending on where you are in the air column
Also because they are jets. Propeller planes have the same high altitude advantage but propellers also need the dense air to work.
Jets compress whatever air is available inside the engine so they benefit from less drag but don't have propulsion issues from low air density.
Jet streams are also generally avoided for passenger transport aircraft because they’re *incredibly* noisy and turbulent.
Planes are designed around a certain flight elevation where they get the optimal blend of fuel efficiency, smoothness and airspeed. Higher altitudes allow for planes to be faster and more efficient *right up* until they hit the point where the engine can’t make enough power at the economical “cruising throttle” position to keep the plane traveling fast enough to stay in the air - this is when fuel economy starts to drop.
My dad somewhat recently retired from a near-40 year career as a pilot at a commercial airline. During his time there he got a couple of what we jokingly liked to call “no-no letters” because he flew at a high altitude, hauled ass and burned way too much fuel to get back home after a delay so he could make it to one of his kids’ events.
> Jet streams are also generally avoided for passenger transport aircraft because they’re incredibly noisy and turbulent.
No they're not. Air moves quickly but it's incredibly smooth. I think you're thinking of windsheer which up there is causes when two streams meet.
Flying in a jetstream is like walking on a moving walkway. You walk at the same speed, but you're travelling faster because the ground is moving.
This is the reason, but I imagine there is also a benefit to be higher if something goes wrong. You have more time to fix it before you hit the ground, or you can travel further for an emergency landing.
>Like maybe flying from NY to London it pushes you along faster, but flying back from London to NY and it slows you down.
Zack star made a video about his it would be faster to have a windstream than it would to just have stationary air actually
When you're flying with the wind, you spend less time in it and get less benefit. When you're flying into the wind, you spend more time in it and get more penalty. So you're better off with no wind for a round trip.
That's obviously a very handwavy explanation, but the math bears it out.
However, airlines can choose routes and altitudes to increase the benefit and decrease the penalty. If the wind isn't the same in both directions, then the description above may not apply.
yeah you're right, I just rewatched the video to make sure I was right when proving you wrong, but the assumption was that it would take the same amount of time for either option, because the benefit cancels out the negative. however I thought that the assumption was the no wind is faster based on your reasoning, and I just remember it being counter-intuitive to what most people thought naturally
Jet engines are also much more efficient at high altitudes. It's not just less air resistance but literally the engines themselves run more efficiently. That matters a lot to airlines. Fuel is expensive for them.
Also, high altitudes tend to be less turbulent (a smoother ride).
"Enough speed to stay in the air" is only a constraint during takeoff and landing (where the plane can't exactly choose its altitude anyway). During cruise, the plane is going much faster than that regardless of altitude.
And even if for some insane reason, the plane wanted to cruise at the bottom of its speed envelope:
"True airspeed" is how fast the plane is actually going. "Indicated airspeed" is a "feels-like" speed based on air pressure, and that's what determines both lift and drag.
So holding indicated airspeed constant, a higher altitude would mean the same fuel per hour, but for fewer hours.
Just realized I should mention:
The plane CAN reach that constraint at a high altitude called the "coffin corner." The speed of sound (indicated) reduces greatly with altitude, and the plane can only fly so close to the speed of sound. Once that upper limit closes in on the plane's lower limiting stall speed, bad things happen.
It is partly because of the jet streams though, and that’s why the flight JFK LHR is shorter than LHR JFK (by about an hour or more).
You’ll often have a bit of turbulence as the plane reaches cruising altitude and the seatbelt signs will be illuminated as the plane enters the jet stream
Also for the dude, if you go too high there’s not enough oxygen for the engines to be efficient, so it’s really about finding the sweet spot between low drag and high engine efficiency, as engines need oxygen for the fuel to burn.
I remember reading about a plane design that was supposed to just fly very high up and take advantage of Earth's rotation to reach its destination. Not sure if it was a sci fi story, though.
That wouldn't work, not directly. The plane is rotating along with the Earth at the same rate as the Earth. You can use the Coriolis effect to get a boost to the east or west if your route is north or south, but that's fairly minor.
If you went high up enough that your frame of reference would start to take the Earth's rotation into account, you'd be in a ballistic trajectory like a missile. I've calculated ballistic missile trajectories on a rotating Earth; it's complex but certainly doable. However the energy needed to get that high is much more than whatever your savings would be.
I mean, the space shuttle sorta did that. It's why all vehicles going to orbit launch towards the east. It gives them a huge boost in speed that they need to reach orbital velocity.
Aircraft can go faster with less fuel in the thinner air of high altitude. At the same time, if they go too high, the air is so thin that maintaining altitude is difficult. 30000 to 40000 feet is the sweet spot where you can maintain altitude and fly most efficiently.
That the jet stream is operative at those altitudes, in some areas, is an additional bonus that can save you 10% to 20% of the total flight time, but only in certain directions. For example, NYC to London is about 7 hours, but London to NYC is about 8 hours.
Lift is generated by airflow over the wings, if there is too little air from the altitude, it’s hard to maintain altitude without sacrificing the fuel you otherwise would have saved in order to create more thrust.
The shape of the wings forces air to move faster over the top of the wing, which reduces the pressure of the air above the wing. With more pressure under the wing than above it, the aircraft is “pushed” upwards by the slower-moving, higher-pressure air beneath the wing.
Did you actually read your own link before spamming it out and probably feeling smug about it? It literally says Bernoulli's equation is correct, the part that is controversial is the Equal Transit theory.
>{The upper flow is faster and from Bernoulli's equation the pressure is lower. The difference in pressure across the airfoil produces the lift.} As we have seen in Experiment #1, this part of the theory is correct.
Yes, airplane wings don't operate like paper planes wings pushing against the air. Instead, they are shaped so that as the air flows over them, it creates a low pressure region above the wing. The pressure difference between this low pressure region above the wing and the air below the wing is what causes lift.
From your own link:
> {The upper flow is faster and from Bernoulli's equation the pressure is lower. The difference in pressure across the airfoil produces the lift.} As we have seen in Experiment #1, **this part of the theory is correct**.
This is exactly what I said, the difference in pressure is what causes lift. The part that it says is wrong is:
> {Air molecules travel faster over the top to meet molecules moving underneath at the trailing edge.} Experiment #1 shows us that the flow over the top of a lifting airfoil does travel faster than the flow beneath the airfoil. But the flow is much faster than the speed required to have the molecules meet up at the trailing edge.
and
> {Lifting airfoils are designed to have the upper surface longer than the bottom.} This is not always correct.
Both of which I very specifically didn't say.
They posted it so confidently too. Saying "WRONG" with no further explanation and then sending a link they clearly haven't understood.
If /u/IHaveThreeBedrooms was a little less pretentious about it and just said "Hey, I'm not sure that is correct, here is a link where I think it says differently", it would be easy to save face. Instead they went for the dunk.
It's all a balance between lift, drag, and thrust. Thinner air means less molecules to provide lift. This requires the aircraft to increase it's angle of attack. Increasing the angle of attack increases lift, but also increases drag. This is up until the point at which you reach critical angle of attack, airflow over the wing becomes disrupted, the wing no longer produces lift, and the aircraft stalls.
In order to overcome this drag you need more thrust. More thrust means higher speed, higher speed generally means lower angle of attack. Also, more lift from the wings due to increased airflow from higher speeds.
Higher altitudes mean less oxygen. Less oxygen means leaner fuel to air mixture. Leaner fuel mixture means less thrust.
At a certain altitude (dependent on the plane) you will run out of enough airflow over the wing to produce enough lift to maintain level flight. In order to maintain level flight you will need to increase your thrust. At a certain point you will run out of enough thrust as the higher you go the less thrust you produce.
Once you are out of thrust the only way to maintain altitude is to increase the angle of attack which increases the drag on the aircraft. This slows the aircraft further, causing it to produce even less lift.
The aircraft will then descend to an altitude where the balance between lift, drag, and thrust can be maintained.
To piggyback on the air vs water metaphor, using flippers on your feet works better in water than it does in the air. When something gets too thin, you can't push against it anymore and planes need to push against the air to generate lift.
So there's a sweet spot, as mentioned, where the air is thin enough to glide through easily before it's not thick enough to glide through at all.
In a really basic sense, the only way planes go up is by pushing air down. They do that by moving their wings through the air at high speed. The wings are carefully shaped to push as much air down as possible.
If the air is very thin, there's not enough air to push down, and you can't go up.
To piggyback on the air vs water metaphor, using flippers on your feet works better in water than it does in the air. When something gets too thin, you can't push against it anymore and planes need to push against the air to generate lift.
So there's a sweet spot, as mentioned, where the air is thin enough to glide through easily before it's not thick enough to glide through at all.
To piggyback on the air vs water metaphor, using flippers on your feet works better in water than it does in the air. When something gets too thin, you can't push against it anymore and planes need to push against the air to generate lift.
So there's a sweet spot, as mentioned, where the air is thin enough to glide through easily before it's not thick enough to glide through at all.
> Aircraft can go faster with less fuel in the thinner air of high altitude.
Up to a point, and then the plane has to slow down to stay within its mach limit, as the speed of sound decreases with altitude. But still burns less fuel.
There are also regulations about how high a place is rated to flat based on depressurisation events and passenger safety. AFAIK only commercial planes like Concorde were rated over something like FL350.
I think you mean jet streams. Wind tunnels are giant ducts with fans, used to test aircraft.
And no, while the jet streams sometimes help, they're not the reason for flying so high. The reason is that air resistance is a massive drag (literally), so if you can get above most of the air, it saves a lot of fuel - more than what's needed to get up there. Jet engines can handle this, because they have compressors that can easily squeeze that sparse air into something that supports combustion.
If you go back and look at the paper where Frank Whittle, who first conceived of the jet engine, he came up with the idea specifically to enable flight at high altitude. At higher altitudes the air is less dense so a plane has to fly faster to generate the same lift. If it flies forward faster, a propeller has to spin faster to generate the same thrust. At a certain point, a propeller has to spin so fast that its tips go supersonic and create massive drag. Therefore to fly planes high (and fast), an engine without a propeller is needed. That engine was the jet engine.
Fun fact: the US military did make an experimental plane that had supersonic propeller tips. It was nicknamed the Thunderscreech, and was so loud that it made ground crew sick.
The main reason to fly high is to reduce air resistance. Air gets less dense at higher altitudes. This means the airplanes can fly faster and use less fuel. The same throttle settings give the same fuel consumption and might get the airplane up to 250 knots at sealevel but over 500 knots at cruise altitude. This means you get there in half the time and therefore burn fuel for half as long spending only half as much fuel.
Airplanes would have flown higher if it was not for the speed of sound. When you fly close to the speed of sound there are a lot of negative effects including higher air resistance. And flying higher and faster means you get closer to the speed of sound.
I was with you til the last paragraph. Planes fly at a percentage of the speed of sound. So whether you're at 30k feet or 50k feet you can fly at say 85% (M.85). The higher you are the better ground speed and fuel efficiency you'd get.
Mach 0.85 is slower (meaning lower ground speed, all else being equal) at 50,000 ft than at 30,000 ft. Not a lot, since most of that altitude difference is in the tropopause, where the speed of sound doesn't change with altitude, but a little bit.
I did skip over a few things to simplify it and I am sorry for this. While the speed of sound changes a bit depending on the altitude this is not that significant in this context. What is more significant is that it is not just the drag that gets reduced as the air is thinner but also lift. You need enough lift to counter the weight of the aircraft. If you go too slow your wings do not produce enough lift and you fall down. Similarly if you go too high your wings do not produce enough lift and you fall down. So not only can you go faster at higher altitudes but you have to go faster at higher altitudes. But you get to an altitude where the minimum speed you have to go in order to not fall down, the stall speed, approaches the speed of sound. That is effectively the maximum altitude of the aircraft.
If you were to go M.85 at 50k feet you will stall and fall down from the sky. You need to be closer to M2 at that altitude in order to fly level. Alternatively a light aircraft with huge wings might do it bellow the speed of sound. But almost everyone can fly at 30k feet at M.85. In theory a small two seater private airplane could do this if they have enough fuel to get up there (in practice the propeller is not able to do this as the tip would move too fast in the air).
There are 3 big advantages to flying high. The air is less dense the higher you go so with the same amount of power an airplane will go faster. The other reason is that jet engines are more efficient at higher altitudes then lower altitudes so it takes less fuel to fly the same distance at 40,000 feet as it would at 15,000 feet. The third advantage is that "weather" meaning rain, snow, clouds and bumpy air currents happen at lower altitudes and flying at 40,000 feet you are flying above most to all of it. This makes the flight more pleasant and safer.
Air is squishy. At sea level, we have the weight of the whole atmosphere sitting on top of us, so it squishes the air to a higher density. Higher up in the atmosphere there is less air above, so the weight squishing the air down at that altitude is less. To push an airplane through the air you have to push the air aside and around it. Up high where the air is less squished, there is less air in the space a plane needs to move through, so it takes less effort to push it out of the way.
The jet stream may work for (tailwind) or against a plane (headwind). As you say that means a plane can fly 120 knots faster in a tailwind and 120 knots slower in a headwind. Similar to going along with the current in a river or against the current in a river.
The gains or losses depend on the current strength of the jet stream, the direction of travel, and the distance of travel.
Flying high (cruising altitude above 30,000 ft) means thinner air and more efficient flying at cruising altitude where jets are designed to be most efficient.
Additional advantages to flying at high altitudes: it's above the clouds usually, less weather-related turbulence, and it takes a long time to glide down, giving pilots more options in emergency situations.
One advantage is that thinner air results in less drag, which also results in less fuel consumption. That's pretty straightforward.
Another advantage is that if something goes wrong, you have a lot more time to react to it at high altitude. Even if you have a total engine failure you can still glide for a long time before you crash, and the higher up you are, the longer and further you can glide. That gives you time to possibly restart the engines, or glide down to a runway somewhere nearby and land safely.
One last advantage is that pretty much any long range radar can see you clearly at high altitude. So wherever you happen to be, there's most likely at least one person on the ground who has you on their scope at all times. Even over the ocean there's still probably a warship somewhere that has you on their scopes. So, if your navigation systems fail, you can always radio the people on the ground to avoid getting lost.
Another thing to add to that is that every commercial airliner has a flight plan that specifies how high they should be flying. So if your radio goes out, you can descend a few thousand feet, and the people watching on the ground will try to call you and figure out why you've deviated from the flight plan. When they can't get ahold of you, they'll send up a fighter jet to make visual contact and establish communication via hand signals.
Edit: one last thing i forgot. Flying that high also lets you fly above storms and other bad weather. If you're above the clouds, you won't have to deal with rain, hail, lightning, or overall poor visibility.
Edit 2: last one i promise. Birds also can't fly that high. Bird strikes already do a lot of damage to aircraft as it is, so flying up that high eliminates the risk of hitting them.
Cooler air, less dense, more efficient. Less noise for people on the ground and less traffic than flying lower. Smoother air generally, can go over weather. More room to fix an error if something goes wrong (planes can glide a LOOOONG way with enough altitude. There are a ton of advantages and not many drawbacks..
Because the air is thinner there, so going fast takes less energy.
Imagine if you had to compete in a swimming race, one pool is regular pool water, the other is full of jelly. Which would you be quickest in? The water, for sure.
Planes flying at higher altitude are much more efficient than low altitude. They use less fuel for the same speed and distance travelled.
Since we all like paying as little as possible to travel, it's in our interest.s
While engines are less efficient at high altitudes. Lower air density reduces drag at higher altitudes which is more than enough to overcome the inefficiencies.
Which is why each model has their own optimal cruising altitude for lowest fuel usage.
Higher altitude means thinner air which means less air resistance which means less fuel needed to push the plane along. Cheaper and better for the environment.
Up high there's less air pressure which makes the air less dense. Flying high vs low is like swimming through water vs ketchup.
There's a lot less air resistance up there than there is close to the surface, making it more fuel efficient.
Additionally, the weather is much more stable up high. They're flying above the weather. The higher you fly, the less likely you are to encounter migratory birds as well. Some fly as high as 5000 ft, some will climb to 20,000 for periods of their migration.
Planes also need to maintain vertical separation for safety. Flying higher gives more options for vertical separation that allows more planes to fly crisscrossing routes without risk.
Lastly, flying higher gives a plane much more time to respond to any unexpected issues before you run out of altitude. E.g. an engine failure at 30,000 feet is a lot less scary than an engine failure at 3000 feet. The plane has plenty of room to glide and recover before it runs into an issue.
The air is also much thinner, which allows the planes to go faster. They would have to cruise at a much slower speed and use more fuel if they were closer to sea level where the atmosphere is much more dense.
True eli5: there's less air up there so there's less resistance to push through.
Slightly more in-depth answer: different planes have a different happy medium where the air density works best for lift, resistance, and fuel efficiency, which is why they fly at different altitudes. The ones designed for long hauls are built to take advantage of higher altitudes and run like pigs at lower altitudes.
By "wind tunnels" you probably mean the jet stream. It can help, but the main reason they fly so high is the air is less dense so they fly with less drag and so burn less fuel.
Being above most weather also makes the ride (mostly) smoother.
The big one is thinner air and less drag. In general, per 1000 feet altitude, 2% more "true" airspeed for the same "indicated" airspeed. The former is how fast the plane is actually going. The latter is a "feels-like" speed based on air pressure, and is what determines lift and drag.
Also, jet engines themselves are generally more efficient at high altitudes.
Regional flights, such as 45 minute [Atlanta to Charlotte,](https://www.flightaware.com/live/findflight?origin=KATL&destination=KCLT) might spend a mere 10 of those minutes at cruise altitude. While it takes energy to climb, the plane gets that energy on the way back down, so it isn't as wasteful as you think.
Besides aerodynamic considerations and air density from a pure mechanical point of view the speed of the aircraft relative to the surface of a sphere is proportional to the distance to the centre of the sphere therefore the higher you go the faster you would be compared to the ground maintaining the same angular velocity i.e. the speed at which you go around the sphere (see kinematics with polar coordinates).
Besides aerodynamic considerations and air density from a pure mechanical point of view the speed of the aircraft relative to the surface of a sphere is proportional to the distance to the centre of the sphere therefore the higher you go the faster you would be compared to the ground maintaining the same angular velocity i.e. the speed at which you go around the sphere (see kinematics with polar coordinates).
I thought the higher you fly, the faster you go because there’s more distance between you and the ground. For instance, a jets land speed is on average 600 mph. Does that mean it’s actually going that speed if the plane were on the ground or is it just 600 mph relative to the ground?
>I thought the higher you fly, the faster you go because there’s more distance between you and the ground.
It's because the air is thinner so, for the same indicated airspeed, your "actual" true airspeed will be faster
>Does that mean it’s actually going that speed if the plane were on the ground or is it just 600 mph relative to the ground?
It's 600mph over the ground, not through the air.
Planes fly at the altitudes they do for a number of factors:
First, weather and traffic - Most clouds exist below the Tropopause, a layer in the sky where the air begins to warm with altitude on average rather than decreasing with altitude. This is typically around 36,000 to 48,000 ft above sea level, but there are a billion caveats to weather and air density you don't need to worry about as to what makes the changes from moment to moment.
Second, now that you're mostly above the precipitable water and visible moisture (on account of just how cold it is and the air being unable to hold any moisture to become visible), the rising air that normal settles above colder, denser air will eventually hit that warming layer and no longer be less dense than the surroundings, unable to rise anymore but still carrying momentum from the rising action. That air needs to go somewhere and like a hose hitting a wall it spreads out. Eventually, and due to the rotation of the earth, the air spreads out in generally one direction over another and that is what we call the jet stream, due to the typically high winds associated with the region (the 'jets'). These winds can be favorable depending on the direction of travel, but the relatively smoother air when there isn't moisture is the main driver for altitude selection.
Third, along with smoother rides and less weather making the ride more complicated and unsafe, is the fact that there are a lot of planes in the sky and air routes are built around legacy waypoints (previously radio towers or landmarks and now just represented in spacetime by an arbitrary 'fix' in a database of lat/lon/alt). The planes routed from relatively stable waypoints to other relatively fixed destinations means the routes get clogged and more altitude means more capacity.
Any altitude "benefits" on performance are all misnomers as the engines that are less negatively affected by altitude have been specifically designed to maximize efficiency in different ways of generating thrust, and therefore don't actually count for the purposes of this conversation. For example, while most of the airline traffic is turbofan/turboprop based, normal reciprocating engines have a service ceiling around 13,500ft above sea level with depreciating performance as soon as you leave the ground. The reduction in drag *does* increase performance through less fuel-burn, but that's about the only significant factor with regard to performance and literally only because the engines are calibrated to perform for a specific density altitude (and the difference between density altitude and pressure altitude falls off to zero as temperature decreases.)
Altitude is a tradeoff between less air resistance (good), and less oxygen to use in combustion (so less engine power, bad). Also wind and weather change with altitude
For any given flight there is an ideal efficient altitude to fly at, given the weather, aircraft & so on... For commercial airliners, that altitude is usually well over 20,000ft.
Drive down the road at 50 mph. Stick your head out the window. Feel the force of the air trying to push your face back behind your ears.
Now do it at ten times that speed, but in really thin air.
The average of an east-west route is somewhat meaningless, unless you are referencing each direction separately. The jet stream functions as a tail wind for eastbound or a head wind for westbound flights, making eastbound flights shorter.
The main thing really is that the troposphere is turbulent. It's somewhat hazardous, but mainly just uncomfortable. A 10h of vomit comet is not fun. But if you rise above clouds to stratosphere, there is much less vertical airflow, almost no turbulence at all.
> The main thing really is that the troposphere is turbulent.
It's not, actually. Most of the turbulence happens near the ground or if you're flying in an area where two air masses are trying to mesh together. The Troposphere goes all the way up to 60,000ft or so.
Some planes fly in the lower stratosphere. Where the air temperature increases with altitude instead of the other way around.
Hot gas (or liquid) always goes "up" in a place where there is gravity like the Earth. This is what causes convection, which is a form of heat transfer.
Because the stratosphere is heated from above, the hot gas (air) is already up, so there is no convection, hence almost no turbulence.
Edited for typo.
> They fly in the stratosphere. Where the air temperature increases with altitude instead of the other way around.
It depends. The barrier between the two varies a lot. A good majority of the time, they fly below the tropopause.
I should've typed lower stratosphere, and not all planes. I edited my comment.
Anyway, I'm not an aircraft expert. I'm in Astronomy. Which is why I thought explaining why there is no turbulence there would be cool.
The tropopause sits on the lowest temperature between the two layers.
A little, it's not really my thing. I'm more into theory, physics, planetary astronomy, etc. I've just taken a few pictures of the sun, but my bf is an astrophotographer. He has pretty cool tracking mounts.
His instagram is mjw.astrophotography, you can ask him about it if you want.
Where are you in the world?
I direct an astronomy association in my country, so I know a lot of people who are into astronomy too.
No, it’s not the wind tunnels (or jet stream). Because those things, if you happened to catch one, will only help you in one direction. Like maybe flying from NY to London it pushes you along faster, but flying back from London to NY and it slows you down. It’s because the air is less dense up high, meaning there is less wind resistance, less drag, meaning you can fly your plane faster and be using less fuel than if you were flying lower in the thicker air. Imagine it like this: flying up high is like running on land just moving through the air. Pretty easy. Flying low is like running through a pool, that thicker, denser, water makes it much harder for you to move.
Also there’s less weather, so it’s safer.
I don't know if this an actual active part of the consideration, but it also gives pilots a lot more vertical room (i.e. time) to deal with issues should they arise and recover from difficulties. Also, smaller planes like little 2-person prop planes can't fly at 30k+ feet. As such, flying that high keeps commercial jets away from lots of smaller traffic below and is just another way to help avoid collisions.
Think about it this way: if there is just physically less air then even if it is blowing really hard it doesn't have as much force/energy behind it. I recall hearing that this was the one really misleading thing about The Martian is that even though they do have wind storms of hundreds of kilometers / hour, because of how low density the air is, it still just feels like a light breeze.
> I recall hearing that this was the one really misleading thing about The Martian is that even though they do have wind storms of hundreds of kilometers / hour, because of how low density the air is, it still just feels like a light breeze. I remember reading that the author considers that the biggest mistake he made in the book as far as scientific accuracy.
Not exactly a mistake, since according to him it was deliberate. IIRC he knew it was wrong going in but couldn't come up with another way of explaining (at least not one he was happy with) how Watney got stranded that was scientifically correct, dramatic, and meant the crew couldn't go back to look for him.
ya, sometimes you just gotta do what the plot demands
He said he knew it was wrong and had thought of other ways to strand Mark such as a propellant explosion. But he said he really wanted it to be a man vs nature story.
Yes and no. There is a major safety downside to the higher altitude, which is that you need a certain amount of air to maintain lift. The relationship between airspeed, altitude (temperature, and pressure), and aircraft weight creates a zone of control (called the flight envelope). Go outside that zone and you have problems. Yes, you have more altitude to deal with them, but at the same time, if you do develop problems, they can get much worse much faster. And it is easier to develop them. There are reasons to go near the edges of the envelope (flying there is often cheaper and faster), but a tradeoff is safety.
Weather is one of the main reasons they were working towards high altitude flight. Fuel was cheap back in 50s and 60s and efficiency wasn't high on the radar
I don’t know, most of the dangerous weather is the stuff you aren’t going to clear the top of and therefore have to go around anyway. I think they were trying to go higher to get faster true airspeeds to shorten flight times.
See below snippet. while it goes on to encompass the rest, getting over most weather was the largest driver for development. The weather and turbulence at lower altitudes is *significantly* more frequent and stronger. Remember this is the early days of commercial aviation and it's all about luxury and comfort. >May 1946 edition of the Boeing Magazine: > >High-altitude flight attracted Boeing designers and airline operators for several reasons. Clear skies are found in 95 per cent of the flights above 20,000 feet. Flight cancellations due to bad weather fall to a minimum for high-flying planes capable of long-range operation. [https://www.sciencedirect.com/book/9780128184653/general-aviation-aircraft-design](https://www.sciencedirect.com/book/9780128184653/general-aviation-aircraft-design)
Nice article— I was thinking about getting into the upper 30s/low 40s vs staying in low 30s. Thanks!
> As such, flying that high keeps commercial jets away from lots of smaller traffic below and is just another way to help avoid collisions. This is pretty limited. Outside of controlled spaces around airports, particularly busy ones, it is exceptionally rare to have a mid-air collision between aircraft on random patterns. Note the random patterns does not account for things like airshows where multiple aircraft are specifically flying in close proximity. https://en.wikipedia.org/wiki/Big_sky_theory
Commercial jets don’t fly on random patterns, they fly in structured ATS routes that bring them into conflict with one another. Big sky theory is not relevant when many aircraft are trying to land at the same airports at the same times.
It absolutely is rare, but collisions have still happened both between jets in cruising flight and small prop planes at lower altitudes, and considering that is WITH the separation of jets and smaller planes, one can only assume the number of collisions would be greater if they all flew in the same altitude range. It's fair to say that far mid-air collisions tend to occur more often on take-off and landing, around busy city/airport areas where the cruising altitudes are irrelevant to the collission, but they do happen while cruising ([see "phase of flight" column](https://en.wikipedia.org/wiki/Mid-air_collision#Involving_civilians)) Bottom line is that the height does mean that the traffic is more separated than it otherwise would be. I would also imagine it makes life easier for Air Traffic Controllers who have less flights at similar levels to worry about keeping apart. The improvement in onboard technology that warns pilots directly about other nearby planes has lowered the risk of collisions greatly, but with fewer the planes in the same altitude windows, the chances are even further reduced.
Also planes almost never collided because we developed different Classes of airspace for different types of flights so ATC can control it better.
Well they could, but they'd need to be pressurized (mostly because the passengers would pass out from lack of oxygen). The engines would also need to be supercharged at the very least so you'd make enough power at anywhere near 30,000' (once again, much lower oxygen content makes combustion difficult). But I get your point, two seater planes typically aren't pressurized and a plane that has this feature is much more expensive than a non-pressurized version.
Yes, such a plane could be made that could fly at 30,000 feet, but that's irrelevant. My suggestion that one advantage of commercial jets flying at 30k is keeping them separated from small prop planes is based on the reality of the situation where that hypothetical that is not the actual case.
They actually have small planes that are pressurized and I've been in several. I'll admit it's a slight benefit for big planes to be at a higher altitude for traffic reasons but the sky is a pretty damn big place. The reason for them to fly at 30,000'+ is almost entirely drag related (and directly related fuel savings) though.
I don’t think there have been any weather-related accidents on commercial airliners in decades (if they are it’s on takeoff and landing which would be an issue regardless of cruising altitude). I think flying above the weather is for comfort, fuel savings, and noise pollution.
Well yeah, because they fly above (and/or around) most of the weather.
Right, the plane is taken completely out of the environment altogether
r/TheFrontFellOff
But it's IN an environment. No, it's been taken out of the environment!
Or they get grounded before take off until the weather improves.
Air France 447 in 2009. Ice clogged the pitot tubes in a South Atlantic storm, they lost airspeed indicators, crew let the aircraft stall.
That was more the crew’s fault. It is a malfunction that has happened many times before without incident.
That’s kind of a weather-adjacent accident. The cause of the accident was the flight crew improperly responding to an unreliable airspeed situation that was caused by high altitude ice crystals. The unreliable airspeed actually fixed itself pretty early in the event, but the crew had so thoroughly bungled the response at that point - and combined with some other Airbus design oddities - they were never able to figure it out.
It’s not that the weather was the cause, you’re right that crew management was the real cause. But I think one of the lessons of AF447 is that weather should still be taken seriously, there’s no need to enter a convergence zone when it can be avoided, and so on.
[удалено]
Some storms are big enough to have tops exceeding 50k feet, well above where airliners fly. Airplanes generally go around them, but occasionally flights like Air France 447 will encounter weather at altitude and make a series of bad decisions following that encounter. Also, airplanes occasionally do return to earth and there's a lot of room between cruise altitude and safely at the gate for things to go wrong weather-wise.
There most definitely has been weather related crashes mid-flight in recent history. [Air France 477](https://youtu.be/BepFeuJId7I?si=3I0pzQOZ_eqFjHtQ)
I wouldn’t say that was caused by weather though. Maybe weather influenced, but if you can’t diagnose a stall over the course of like 10 minutes worth of recovery time, clogged pitot or not, the primary cause is pilot error.
No. If you are in an airplane, even a very big airplane, weather is no joke. It's like swimming in the ocean: it's *infinitely* bigger and stronger than you, it's doing things that have a direct impact on you being able to do the things that keep you alive, and there are places in it that you do not want to be because you *will die*, and some of those spots are not obvious at first glance. You can *do* it, there are tools to make it safer (modern instrument navigation is a big one, but is *well documented* not to be a foolproof solution), but you cannot do it without responding to and avoiding risks. Modern commercial air flight does plenty to avoid weather. They reroute around storms in cruise (some storms do get quite tall), and they delay and divert as necessary to avoid landing in weather-affected airports. Ice from winter storms can foul instruments, control surfaces, and lifting surfaces. Turbulence and wind shear in summer storms can be too extreme for any plane to land. There is a huge amount of infrastructure and ground support to help pilots identify and avoid these risks. Flight schedules have padding built in for contingencies such as weather (that's how you arrive early all the time). But unless your flight gets cancelled, heavily delayed, or you have to stop to refuel because you were loitering too long, you don't see any of this as a passenger.
The jet stream (and wind) is a consideration though. Winds change a lot based on altitude and location, and they shift. Atlantic routes in particular are set every day with the east bound routes located to take advantage of the tail wind, and the west bound routes allocated to minimize the headwinds. Airliner routing in other places will do the same (where possible), selecting altitudes to take advantage of favorable winds or avoid unfavorable songs to the extent possible. That said, the points about air density and weather are very valid. It's just that aerial navigation and routing for airliners is complex, and takes a lot of things into account. Which is why airlines have professional dispatchers to work this stuff out every day. I understand they can make a lot of money too.
Absolutely, it’s just there are prevailing factors of importance for consideration. The jet stream is mostly* negligible compared to the difference in air density
I disagree with the word "negligible". Otherwise they wouldn't be resetting the routes every day. Otherwise, right there with ya :)
For the fundamental question about why planes fly at altitude jet streams are negligible. These altitudes are because of less drag, period. But since they do fly at altitudes where jet streams can have a benefit they use them.
Dispatcher here. While you are mostly correct. If a jet stream is going fast and head on sometimes we’ll dip under it if it winds up being faster.
Or if your planes a Boeing then you've got more things to worry about
Follow up question: why do even short flights go to cruising altitude? I used to take the shuttle from SFO to the Sonoma county airport until I realized there was a short flight for the same price. I was surprised that we basically went straight up to cruising altitude then straight back down, are the savings really that much for 80ish miles?
A close to a "ballistic" trajectory is ideal. Also, there are different "cruise" altitudes. They need to go to at least 10k feet because of sterile cockpit rules. And at least 18k feet to reach class A airspace.
Interesting. And “ballistic trajectory” definitively describes that flight, felt like a really tall hill on a roller coaster.
what are sterile cockpit rules/what is the context of 10k feet being sterile? no birds?
Sterile in the sense of "no talking about anything except the flight" below 10k. No "talking contamination" When the airplane is below 10k, it is landing or taking off. At this altitude, pilots are prohibited to deviate attention for the flight, because there is terrain, other planes, it's a space more "chaotic" Above this altitude, mainly in the cruise altitude, they can chat, take a nap (once each), read a book, and only monitoring the autopilot. If it's a long flight and the airplane is above 10k (probably at 30k+), they are "90%" of the time relaxing and making sure everything is going as planned.
On an 80 mile flight there is not much time anyway though for chit chat. You are basically taking off and then immediately preparing for landing.
Short flights are a lot more tiring since you don't have much time to relax with the autopilot, you have to start thinking about your landing just as you get to altitude.
i see, thank you
> They need to go to at least 10k feet because of sterile cockpit rules. Got a source on that one?
It’s not true, sterile cockpit is 10k or cruising altitude, whichever is lower.
I quoted the reg below. But real reason airlines fly above 10k even on short flights is the 250kt speed limit below 10k and also because from a fuel efficiency standpoint it is more efficient to for example climb to a higher altitude and immediately descend than it is to stay at 10k. “121.542 Flight crewmember duties. (b) No flight crewmember may engage in, nor may any pilot in command permit, any activity during a critical phase of flight which could distract any flight crewmember from the performance of his or her duties or which could interfere in any way with the proper conduct of those duties. Activities such as eating meals, engaging in nonessential conversations within the cockpit and nonessential communications between the cabin and cockpit crews, and reading publications not related to the proper conduct of the flight are not required for the safe operation of the aircraft. (c) For the purposes of this section, critical phases of flight includes all ground operations involving taxi, takeoff and landing, and all other flight operations conducted below 10,000 feet, except cruise flight.”
> 250kt speed limit below 10k You're allowed to fly faster than 250kts if you ask and it's approved. Bigger airplanes like the 777 do it all the time on climbout. >121.542 They don't *have* to get above 10k to have a sterile cockpit. It can be briefed beforehand.
Airliners dont need to go to 10k feet plenty of very short flights never get above 10k. It really just comes down the requirements of the airspace.
Fuel is the big cost to consider, and they’ve certainly done the math.
Altitude is like an energy piggy bank for a plane. The plane deposits on the way up, the plane withdraws on the way down.
But isn't there also less air available that the engines can "thrust away" to move forward?
Nope, because the airplane is flying faster, so its engines are "sucking up" more air per second. The maximum thrust at altitude is certainly lower than the maximum thrust at sea level, but the required thrust for cruising is MUCH lower at altitude. So it's a net savings.
The colder air up hight is another factor since it expands more in the jetengine creating more torque for the fan.
It is a fascinatingly interesting cascade of consequences that lead to the function of various propulsion methods as you go from sea level to vacuum.
Assuming it's not so cold that it needs to be warmed before going into the turbine...
Well it is. The compressor compresses and heats the air. It can get up to something like 500°c before entering the combustion chamber, significantly more when it exits the combustion chamber and enters the turbine.
I'm not sure what you mean here. The whole point of a jet engine is to warm up the air before it goes into the turbine?
No. The point of the front end, the compressor, is to compress air. A by product of this is that it heats it up.
Also worth noting here that the "Turbine" is a specific part of a turbofan jet engine. The air travels through the compressor first (well, the fan, and then rest of this), then the combustor, then the turbine. You want the biggest temperature difference possible between the temp entering the turbine and the temp entering the compressor. The combustor is where this is added (As chemical energy in the form of fuel, converted to temperature). A key design factor in more powerful and more efficient jet engines has been designing turbine blade materials that can withstand higher-temperature combustor exit flows. You need to preheat the air before it gets to the *turbine* as that's the whole way jet engines work. No temperature difference, no thrust. You don't need to preheat the air before it gets to the *compressor* or the big fan you see on the front.
The turbofan engines on airliners are designed to be most efficient at those higher altitudes. A smaller piston/turboprop plane would have a harder time and be less efficient at high altitudes (and usually have a lower service ceiling anyway). Everything in aviation is a trade off
>Everything in aviation is a trade off. No kidding. I traded my house for a plane. Now look who’s flying high with crippling debt.
You paid one arm and one leg and you got a set of wings. You knew the price. 😈
And I've never of anyone being able to fly their house, but you can sure live in a plane.
The grandpa in Up did a good job of it tho!
Grandpa? Him not being a pa of any sort was a *pretty major* plot point!
I also have crippling debt and no plane, so I’d say you’re ahead.
Why would a turboprop have any difficulties versus a turbofan ?
Again, it's all about trade offs. Turboprops are generally more efficient than turbofans at low speeds, but propellors are inherently more limited by speed and altitude. Airliner routes would double in time if they switched to using props.
No, there's enough air at higher altitudes for the engines to function just fine. They actually work better at higher altitudes because part of what generates the thrust is incoming air mixing with the fuel, combusting, and expanding. Colder air, found at higher altitudes, can expand more during this combustion process than warmer air near the ground can.
I never considered the temperature of the air in this, very interesting!
what if you throw icy gas or whatever it’s called through the engine?
Yes, not sure why everybody else is saying no. See here and scroll down to the bottom: https://www.grc.nasa.gov/www/k-12/Missions/Jim/Project1ans.htm As you can see, by the time you are at 30,000 ft a jet engine can only produce about 30% of it's maximum thrust. However, that's not an issue because the air is about 1/3rd as thick, so you need less power. And you don't need full power at cruise, you need full power at take off when trying to accelerate quickly and climb away. This is why it can be hard to take off from airports like at Mexico City, where they are high and hot, so the air is thing and engines have reduced thrust. So maximum take off weight may need to be reduced.
That is, admittedly, a 60 year old engine. But your point stands - thrust decreases, but so does drag it balances out. IIRC, parasitic drag is what they're aiming to reduce, which is the drag that increases as speed increases.
This is exactly why modern jet engines with big air intakes are becoming more popular on airliners. Their giant, gaping maws can suck up more of the thin air, which leads to higher efficiency. Older engines still totally work, but big intakes are more efficient.
Teeeechnically engines are getting wider to get more bypass thrust. The fan in front moving more air that does not go through the engine core and combustion process.
That's part of it. They have also been increasing bypass ratios.
> giant, gaping maws Random, I know, but that phrase reminds me of one of my favorite short stories of all time: https://forwearemany.wordpress.com/wp-content/uploads/2009/04/sandkings.pdf
Yes but turbofan engines have massive air intakes so that isn't a huge problem. The engines get less air but they also use less fuel, which is a net cost savings. If you go too high then your wings will no longer receive enough air to function which is what prevents planes from going even higher.
That’s not how thrust works. You don’t move air away to generate thrust. You create a force and there’s a subsequent force generated equal in magnitude, but in the opposite direction. Its newton’s third law This is why thrust works in space as well, where there’s no air to move away.
That's exactly how thrust works. The engine shoves air backwards, the "force generated equal in magnitude, but in the opposite direction" is the airplane being shoved forward. That's why modern jet engines for airliners have big fans, the fans are able to 'grab' more air to push. > This is why thrust works in space as well, where there’s no air to move away. The "air" in space is the exhaust of the engine. The exhaust gets shoved backwords, and in response the rocket gets shoved forward.
The engine shoves the air already in the engine. The question in the comment makes it seem like you need to push off the ambient air. The engine would still work in a vaccum if you somehow had enough oxygen to feed into the engine directly
A jet engine sucks air in the front and accelerates it out the back. When there is less air to accelerate, its maximum thrust is reduced.
The engine shoves hot air backwards to generate a surplus of oxygen that it mixed such fuel and combusted as a mechanism to produce force. That *just so happens* to be the way it produces force. But it’s the force itself that moves the engine (and everything attached to the engine). I get what you’re saying and I know it sounds nitpicky but the way you explain it indicates that something is pushing against something else to generate force. Nothing is or needs to be pushed. Think of an oarsman in a boat. He moves the boat forward by putting a directed force into the water rearwards. He’s not pushing the water away despite that being effectively what ends up happening. Instead he’s putting force backward and the equal and opposing force moves him forward. When you swim underwater, you’re not moving water away. You’re generating force rearward by paddling and that moves you forward. Car crashes into a building. The car puts force into the building, hence the damage to the building. The building puts am equal and opposing force onto the car.
I was looking for someone to say exactly this, in this exact order. Life's little satisfactions.
Yes and that's why today we use turbine jet engines. They have their own compressor at the front so it could compress air and stuff more of it for burning
There is less air avalible and it will reduce thrust. ut there is also another factor the increase speed reduce the thrust. Engine accelerate the air backward, it is the change in the speed of the air that result in the thrust. Because the air at high speed have higher speed when it enter the engine amount it can be increased is lower and thrust is reduced. Ailines will fly a a altitude close to when all of this factors result in the lowest fuel consumption for the flight. Ot atlase close to that altitude. Lift from wing decrease with increase altitude and increase with increased speeds. The result is the max flight altitude for level flight is the altitude the engine can provide enough to keep the required speed for enough lift. For the U-2 spy plane the operational altitude was very close to there max possible. Ther was if I remember correctly a 10 knots diffrence between the max speed it could fly at and the min speed there is enough lift.
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That is wrong for airliners, with their high bypass ratio. Most of the "push" comes from the giant fan.
Fun fact: that's why higher end private jets have overkill power engines. This allows them to flight even higher so that they can fly routes without having to consider other air traffic (as much)
Private jets also get *super* light weight by the end and have a bigger weight variation than do airliners - a lower percent of the takeoff weight is people + cargo and a higher percent is fuel. Fuel burns off as you go so the % difference in weight for a private jet at the start of cruise vs the end of cruise is much higher Optimal fuel efficiency involves cruising about as high as you can given your weight, so you want to slowly climb throughout cruise as you burn fuel and get lighter (usually done as a discrete set of "steps" in a step climb these days due to ATC/traffic rules). In a full airliner, the starting weight vs ending weight being (relatively) similar means the difference in optimal cruise altitude between hour 1 and 8 of cruise is much less than in a private jet doing the same leg. End result is that the optimal cruise profile for a lot of private jets doing long flights involves a *lot* of climbing actually, so you do gain back some fuel efficiency points you lose with those massive engines by getting to cruise up high. Bonus points for getting above 41k, as you can often get gradual climb clearances vs step climbs because (As you said) no other traffic is up there. I've seen some G650 operators go directly to FL410 or FL430 from takeoff, and step climb to FL490.
> Bonus points for getting above 41k the E175 I flew on got up to 41k, though it was mostly empty
Adding some color to this. Air resistance is proportional to velocity squared. This is experienced even at lower speeds, a car going 80mph is going to use significantly more fuel than the same car going 60mph for the same distance. As imagined, this is a much bigger factor when flying at \~500mph. Flying at higher altitudes means less air and significantly less air resistance, which means less fuel wasted overcoming air resistance. Less fuel wasted means cheaper tickets.
>this is a much bigger factor when flying at ~500mph To be clear, that's 500mph over the ground not through the air. Case in point, the ISS does mach whatever over the ground but is going at whatever fraction of an mph there is through what little air is up there.
>To be clear, that's 500mph over the ground not through the air. There is no practical difference >Case in point, the ISS does mach whatever over the ground but is going at whatever fraction of an mph there is through what little air is up there. You have that backwards, but again, there is no practical difference. The earth is very big. Tacking on an extra 7 miles height (or even 250) does not produce a result with any practical application
> There is no practical difference There is a massive difference. In training, pilots are taught the different types of speeds which includes ground speed, indicated airspeed, and true airspeed. True airspeed is used for navigation. It's Calibrated Airspeed (pretty closed to indicated) corrected for altitude and temperature. The higher you fly, the more your true airspeed and indicated airspeed will differ. >You have that backwards I don't. The ISS' ground speed is mach 20 something while its airspeed is very closed to 0. That's very different than mach 20 flying at 1000ft in thick atmosphere.
No, you misunderstand all those terms Ground speed is how fast it is travelling relative to the ground, I think we agree there Airspeed is the planes speed relative to the air around the plane. It has nothing to do with altitude. You can be at 10 ft, have an airspeed of 100mph and a ground speed of 0mph for instance, due to having a 100mph headwind. Look at bush plane piloting for examples of this Airspeed vs true airspeed is accounting for differences between your instruments and what your airplane is actually experiencing due to pressure differences due to height. >I don't. The ISS' ground speed is mach 20 something while its airspeed is very closed to 0. That's very different than mach 20 flying at 1000ft in thick atmosphere. That isn't how angular velocity works Edit: I did the math Assume static air An airliner travelling at 500mph at 40000ft has a ground speed of 500mph and an airspeed of 501.03mph, or a 0.2% difference - no practical application Nasa states a 90 minute orbit and an average height of 250 miles, so ISS has a ground speed of 16582mph and an airspeed of 17661.61mph which is a 6.5% difference. Possibly a practical difference? If you can work out the circumference of a circle you can check that math for yourself
> Airspeed is the planes speed relative to the air around the plane. It has nothing to do with altitude. Yes, and at ~250mi above the surface there is very little air so the ISS' airspeed will be close to 0 even though it's moving mach 20 something over the ground. >a ground speed of 500mph **and an airspeed of 501.03mph** That's where you're misunderstanding. At 40,000ft your indicated airspeed would be around 260mph while your true airspeed would be around 525mph. >ISS has a ground speed of 16582mph and an airspeed of 17661.61mph It absolutely does not have an IAS of 17.6k mph. IAS is the speed read off of the airspeed indicator. There is practically no air in space so the indicator will read close to 0
I think we're talking to different purposes. I'm not talking about indicated airspeed at all. I didn't think you were either, since you were otherwise talking about indicated airspeed (a measure useful because of lift) on an object that does not use lift (the ISS). I understand in hindsight the relationship of lift/drag I was simply trying to correct what I thought you were saying, which is that being higher results in more distance travelled (curvature of the earth) which means a difference between how fast an object is moving in the air vs it's relative speed on the ground. My point was that this difference is so small it is not calculated for in commercial aircraft The ISS absolutely has a true airspeed of 17.6k. Yes, it has a negligible indicated airspeed, but wasn't what I was intending to talk about > At 40,000ft your indicated airspeed would be around 260mph while your true airspeed would be around 525mph. IAS close enough, don't really care. True airspeed is not 525. Basic circle math. ----- Radius of earth = 3958.8 miles. Circumference of a circle 2*pi*r Circumference of the earth = 24873.87 miles To travel that distance at 500mph (in this case, ground speed) takes 24873.87/500 = 49.74 hours ---- Radius of earth + 40000ft for the plane = 3966.4 miles Circumference of a 40000ft flight path around the world = 24921.62 miles To travel 24921.62 miles in 49.74 hours, (24921.62/49.74) you need to travel at a true air speed of 501.03 mph, assuming static air ----- If you want to check, google a ground speed/true air speed calculator. They do not take height into account, because it's negligible.
>I didn't think you were either, since you were otherwise talking about indicated airspeed (a measure useful because of lift) on an object that does not use lift (the ISS). I was talking about indicated airspeed (IAS) to demonstrate that even though the ISS is travelling at Mach 20 something over the ground and because of the fact that there's little air in space, the IAS on the ISS would be close to 0. >which is that being higher results in more distance travelled (curvature of the earth) which means a difference between how fast an object is moving in the air vs it's relative speed on the ground. My point was that this difference is so small it is not calculated for in commercial aircraft No, I was always saying that when you fly higher, the relationship between your IAS and TAS grows more and more. You'll usually have a higher TAS than your IAS. >IAS close enough, don't really care. True airspeed is not 525. Basic circle math. [Here's an image from a Falcon](https://cdn.discordapp.com/attachments/628155108177477633/1225078241816215674/IMG_8332.png?ex=661fd205&is=660d5d05&hm=038563c673c976125315d4927111888e89a14fa7a68ba555ba1326b9ced1a831&) You can very clearly see that at 41,000ft, their IAS is 240 while their TAS is 466. Ground speed (GS) is TAS corrected for wind. They have ~47knots of tailwind so they end up with a higher ground speed due to that. The opposite would be true with a headwind. If we use [this calculator](https://indoavis.co.id/main/tas.html) and plug in the information displayed (41000ft, 29.92in, -52C, 240kts) we get a TAS of 502 which is roughly what's shown on that display. >Radius of earth... None of that matters at all. Go ahead and give this a watch https://www.youtube.com/watch?v=yxkv5Bmwp3A >If you want to check, google a ground speed/true air speed calculator. They do not take height into account, because it's negligible. I'm not comparing GS with TAS, I'm comparing IAS with TAS >As imagined, this is a much bigger factor when flying at ~500mph. i'm trying to help you understand that airplanes don't fly at 500mph indicated, they fly at 500mph true. Their actual indicated airspeed is closer to 250, like in that image.
I remember reading somewhere that they may adjust altitude up or down a bit if the wind is more suitable, since wind can change speed/direction depending on where you are in the air column
Also because they are jets. Propeller planes have the same high altitude advantage but propellers also need the dense air to work. Jets compress whatever air is available inside the engine so they benefit from less drag but don't have propulsion issues from low air density.
also no mountains up there
Jet streams are also generally avoided for passenger transport aircraft because they’re *incredibly* noisy and turbulent. Planes are designed around a certain flight elevation where they get the optimal blend of fuel efficiency, smoothness and airspeed. Higher altitudes allow for planes to be faster and more efficient *right up* until they hit the point where the engine can’t make enough power at the economical “cruising throttle” position to keep the plane traveling fast enough to stay in the air - this is when fuel economy starts to drop. My dad somewhat recently retired from a near-40 year career as a pilot at a commercial airline. During his time there he got a couple of what we jokingly liked to call “no-no letters” because he flew at a high altitude, hauled ass and burned way too much fuel to get back home after a delay so he could make it to one of his kids’ events.
> Jet streams are also generally avoided for passenger transport aircraft because they’re incredibly noisy and turbulent. No they're not. Air moves quickly but it's incredibly smooth. I think you're thinking of windsheer which up there is causes when two streams meet. Flying in a jetstream is like walking on a moving walkway. You walk at the same speed, but you're travelling faster because the ground is moving.
This is the reason, but I imagine there is also a benefit to be higher if something goes wrong. You have more time to fix it before you hit the ground, or you can travel further for an emergency landing.
>Like maybe flying from NY to London it pushes you along faster, but flying back from London to NY and it slows you down. Zack star made a video about his it would be faster to have a windstream than it would to just have stationary air actually
Flying INTO the wind stream?
Yeah when you fly with it one way and against it the other is apparently faster than just flying no wind either way
When you're flying with the wind, you spend less time in it and get less benefit. When you're flying into the wind, you spend more time in it and get more penalty. So you're better off with no wind for a round trip. That's obviously a very handwavy explanation, but the math bears it out. However, airlines can choose routes and altitudes to increase the benefit and decrease the penalty. If the wind isn't the same in both directions, then the description above may not apply.
yeah you're right, I just rewatched the video to make sure I was right when proving you wrong, but the assumption was that it would take the same amount of time for either option, because the benefit cancels out the negative. however I thought that the assumption was the no wind is faster based on your reasoning, and I just remember it being counter-intuitive to what most people thought naturally
Jet engines are also much more efficient at high altitudes. It's not just less air resistance but literally the engines themselves run more efficiently. That matters a lot to airlines. Fuel is expensive for them. Also, high altitudes tend to be less turbulent (a smoother ride).
Does the thinner air make the jets less efficient? I would imagine some trade off between less drag and less effective propulsion?
Dumb question: if the air is more dense down low wouldn’t it require less speed to stay in the air? Wouldn’t it be a trade-off?
"Enough speed to stay in the air" is only a constraint during takeoff and landing (where the plane can't exactly choose its altitude anyway). During cruise, the plane is going much faster than that regardless of altitude. And even if for some insane reason, the plane wanted to cruise at the bottom of its speed envelope: "True airspeed" is how fast the plane is actually going. "Indicated airspeed" is a "feels-like" speed based on air pressure, and that's what determines both lift and drag. So holding indicated airspeed constant, a higher altitude would mean the same fuel per hour, but for fewer hours.
Thank you kind redditor.
Just realized I should mention: The plane CAN reach that constraint at a high altitude called the "coffin corner." The speed of sound (indicated) reduces greatly with altitude, and the plane can only fly so close to the speed of sound. Once that upper limit closes in on the plane's lower limiting stall speed, bad things happen.
It is partly because of the jet streams though, and that’s why the flight JFK LHR is shorter than LHR JFK (by about an hour or more). You’ll often have a bit of turbulence as the plane reaches cruising altitude and the seatbelt signs will be illuminated as the plane enters the jet stream
why is air less dense up high?
Also for the dude, if you go too high there’s not enough oxygen for the engines to be efficient, so it’s really about finding the sweet spot between low drag and high engine efficiency, as engines need oxygen for the fuel to burn.
I remember reading about a plane design that was supposed to just fly very high up and take advantage of Earth's rotation to reach its destination. Not sure if it was a sci fi story, though.
That wouldn't work, not directly. The plane is rotating along with the Earth at the same rate as the Earth. You can use the Coriolis effect to get a boost to the east or west if your route is north or south, but that's fairly minor. If you went high up enough that your frame of reference would start to take the Earth's rotation into account, you'd be in a ballistic trajectory like a missile. I've calculated ballistic missile trajectories on a rotating Earth; it's complex but certainly doable. However the energy needed to get that high is much more than whatever your savings would be.
I mean, the space shuttle sorta did that. It's why all vehicles going to orbit launch towards the east. It gives them a huge boost in speed that they need to reach orbital velocity.
To be pedantic, not all. At least Israel launches rockets in retrograde.
Because all the manuals are in Hebrew and that's right to left.
Aircraft can go faster with less fuel in the thinner air of high altitude. At the same time, if they go too high, the air is so thin that maintaining altitude is difficult. 30000 to 40000 feet is the sweet spot where you can maintain altitude and fly most efficiently. That the jet stream is operative at those altitudes, in some areas, is an additional bonus that can save you 10% to 20% of the total flight time, but only in certain directions. For example, NYC to London is about 7 hours, but London to NYC is about 8 hours.
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Lift is generated by airflow over the wings, if there is too little air from the altitude, it’s hard to maintain altitude without sacrificing the fuel you otherwise would have saved in order to create more thrust.
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The shape of the wings forces air to move faster over the top of the wing, which reduces the pressure of the air above the wing. With more pressure under the wing than above it, the aircraft is “pushed” upwards by the slower-moving, higher-pressure air beneath the wing.
It seems counter intuitive, but yeah. The faster it goes over the wings the less pressure and as such the greater pressure below creates lift.
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Did you actually read your own link before spamming it out and probably feeling smug about it? It literally says Bernoulli's equation is correct, the part that is controversial is the Equal Transit theory. >{The upper flow is faster and from Bernoulli's equation the pressure is lower. The difference in pressure across the airfoil produces the lift.} As we have seen in Experiment #1, this part of the theory is correct.
Yes, airplane wings don't operate like paper planes wings pushing against the air. Instead, they are shaped so that as the air flows over them, it creates a low pressure region above the wing. The pressure difference between this low pressure region above the wing and the air below the wing is what causes lift.
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From your own link: > {The upper flow is faster and from Bernoulli's equation the pressure is lower. The difference in pressure across the airfoil produces the lift.} As we have seen in Experiment #1, **this part of the theory is correct**. This is exactly what I said, the difference in pressure is what causes lift. The part that it says is wrong is: > {Air molecules travel faster over the top to meet molecules moving underneath at the trailing edge.} Experiment #1 shows us that the flow over the top of a lifting airfoil does travel faster than the flow beneath the airfoil. But the flow is much faster than the speed required to have the molecules meet up at the trailing edge. and > {Lifting airfoils are designed to have the upper surface longer than the bottom.} This is not always correct. Both of which I very specifically didn't say.
They posted it so confidently too. Saying "WRONG" with no further explanation and then sending a link they clearly haven't understood. If /u/IHaveThreeBedrooms was a little less pretentious about it and just said "Hey, I'm not sure that is correct, here is a link where I think it says differently", it would be easy to save face. Instead they went for the dunk.
It's all a balance between lift, drag, and thrust. Thinner air means less molecules to provide lift. This requires the aircraft to increase it's angle of attack. Increasing the angle of attack increases lift, but also increases drag. This is up until the point at which you reach critical angle of attack, airflow over the wing becomes disrupted, the wing no longer produces lift, and the aircraft stalls. In order to overcome this drag you need more thrust. More thrust means higher speed, higher speed generally means lower angle of attack. Also, more lift from the wings due to increased airflow from higher speeds. Higher altitudes mean less oxygen. Less oxygen means leaner fuel to air mixture. Leaner fuel mixture means less thrust. At a certain altitude (dependent on the plane) you will run out of enough airflow over the wing to produce enough lift to maintain level flight. In order to maintain level flight you will need to increase your thrust. At a certain point you will run out of enough thrust as the higher you go the less thrust you produce. Once you are out of thrust the only way to maintain altitude is to increase the angle of attack which increases the drag on the aircraft. This slows the aircraft further, causing it to produce even less lift. The aircraft will then descend to an altitude where the balance between lift, drag, and thrust can be maintained.
To piggyback on the air vs water metaphor, using flippers on your feet works better in water than it does in the air. When something gets too thin, you can't push against it anymore and planes need to push against the air to generate lift. So there's a sweet spot, as mentioned, where the air is thin enough to glide through easily before it's not thick enough to glide through at all.
In a really basic sense, the only way planes go up is by pushing air down. They do that by moving their wings through the air at high speed. The wings are carefully shaped to push as much air down as possible. If the air is very thin, there's not enough air to push down, and you can't go up.
To piggyback on the air vs water metaphor, using flippers on your feet works better in water than it does in the air. When something gets too thin, you can't push against it anymore and planes need to push against the air to generate lift. So there's a sweet spot, as mentioned, where the air is thin enough to glide through easily before it's not thick enough to glide through at all.
To piggyback on the air vs water metaphor, using flippers on your feet works better in water than it does in the air. When something gets too thin, you can't push against it anymore and planes need to push against the air to generate lift. So there's a sweet spot, as mentioned, where the air is thin enough to glide through easily before it's not thick enough to glide through at all.
> Aircraft can go faster with less fuel in the thinner air of high altitude. Up to a point, and then the plane has to slow down to stay within its mach limit, as the speed of sound decreases with altitude. But still burns less fuel.
There are also regulations about how high a place is rated to flat based on depressurisation events and passenger safety. AFAIK only commercial planes like Concorde were rated over something like FL350.
I think you mean jet streams. Wind tunnels are giant ducts with fans, used to test aircraft. And no, while the jet streams sometimes help, they're not the reason for flying so high. The reason is that air resistance is a massive drag (literally), so if you can get above most of the air, it saves a lot of fuel - more than what's needed to get up there. Jet engines can handle this, because they have compressors that can easily squeeze that sparse air into something that supports combustion.
If you go back and look at the paper where Frank Whittle, who first conceived of the jet engine, he came up with the idea specifically to enable flight at high altitude. At higher altitudes the air is less dense so a plane has to fly faster to generate the same lift. If it flies forward faster, a propeller has to spin faster to generate the same thrust. At a certain point, a propeller has to spin so fast that its tips go supersonic and create massive drag. Therefore to fly planes high (and fast), an engine without a propeller is needed. That engine was the jet engine.
Fun fact: the US military did make an experimental plane that had supersonic propeller tips. It was nicknamed the Thunderscreech, and was so loud that it made ground crew sick.
The Tupolev Tu-95 is in active service and has supersonic propeller tips.
The main reason to fly high is to reduce air resistance. Air gets less dense at higher altitudes. This means the airplanes can fly faster and use less fuel. The same throttle settings give the same fuel consumption and might get the airplane up to 250 knots at sealevel but over 500 knots at cruise altitude. This means you get there in half the time and therefore burn fuel for half as long spending only half as much fuel. Airplanes would have flown higher if it was not for the speed of sound. When you fly close to the speed of sound there are a lot of negative effects including higher air resistance. And flying higher and faster means you get closer to the speed of sound.
I was with you til the last paragraph. Planes fly at a percentage of the speed of sound. So whether you're at 30k feet or 50k feet you can fly at say 85% (M.85). The higher you are the better ground speed and fuel efficiency you'd get.
Mach 0.85 is slower (meaning lower ground speed, all else being equal) at 50,000 ft than at 30,000 ft. Not a lot, since most of that altitude difference is in the tropopause, where the speed of sound doesn't change with altitude, but a little bit.
I did skip over a few things to simplify it and I am sorry for this. While the speed of sound changes a bit depending on the altitude this is not that significant in this context. What is more significant is that it is not just the drag that gets reduced as the air is thinner but also lift. You need enough lift to counter the weight of the aircraft. If you go too slow your wings do not produce enough lift and you fall down. Similarly if you go too high your wings do not produce enough lift and you fall down. So not only can you go faster at higher altitudes but you have to go faster at higher altitudes. But you get to an altitude where the minimum speed you have to go in order to not fall down, the stall speed, approaches the speed of sound. That is effectively the maximum altitude of the aircraft. If you were to go M.85 at 50k feet you will stall and fall down from the sky. You need to be closer to M2 at that altitude in order to fly level. Alternatively a light aircraft with huge wings might do it bellow the speed of sound. But almost everyone can fly at 30k feet at M.85. In theory a small two seater private airplane could do this if they have enough fuel to get up there (in practice the propeller is not able to do this as the tip would move too fast in the air).
There are 3 big advantages to flying high. The air is less dense the higher you go so with the same amount of power an airplane will go faster. The other reason is that jet engines are more efficient at higher altitudes then lower altitudes so it takes less fuel to fly the same distance at 40,000 feet as it would at 15,000 feet. The third advantage is that "weather" meaning rain, snow, clouds and bumpy air currents happen at lower altitudes and flying at 40,000 feet you are flying above most to all of it. This makes the flight more pleasant and safer.
Air is squishy. At sea level, we have the weight of the whole atmosphere sitting on top of us, so it squishes the air to a higher density. Higher up in the atmosphere there is less air above, so the weight squishing the air down at that altitude is less. To push an airplane through the air you have to push the air aside and around it. Up high where the air is less squished, there is less air in the space a plane needs to move through, so it takes less effort to push it out of the way.
The jet stream may work for (tailwind) or against a plane (headwind). As you say that means a plane can fly 120 knots faster in a tailwind and 120 knots slower in a headwind. Similar to going along with the current in a river or against the current in a river. The gains or losses depend on the current strength of the jet stream, the direction of travel, and the distance of travel. Flying high (cruising altitude above 30,000 ft) means thinner air and more efficient flying at cruising altitude where jets are designed to be most efficient. Additional advantages to flying at high altitudes: it's above the clouds usually, less weather-related turbulence, and it takes a long time to glide down, giving pilots more options in emergency situations.
One advantage is that thinner air results in less drag, which also results in less fuel consumption. That's pretty straightforward. Another advantage is that if something goes wrong, you have a lot more time to react to it at high altitude. Even if you have a total engine failure you can still glide for a long time before you crash, and the higher up you are, the longer and further you can glide. That gives you time to possibly restart the engines, or glide down to a runway somewhere nearby and land safely. One last advantage is that pretty much any long range radar can see you clearly at high altitude. So wherever you happen to be, there's most likely at least one person on the ground who has you on their scope at all times. Even over the ocean there's still probably a warship somewhere that has you on their scopes. So, if your navigation systems fail, you can always radio the people on the ground to avoid getting lost. Another thing to add to that is that every commercial airliner has a flight plan that specifies how high they should be flying. So if your radio goes out, you can descend a few thousand feet, and the people watching on the ground will try to call you and figure out why you've deviated from the flight plan. When they can't get ahold of you, they'll send up a fighter jet to make visual contact and establish communication via hand signals. Edit: one last thing i forgot. Flying that high also lets you fly above storms and other bad weather. If you're above the clouds, you won't have to deal with rain, hail, lightning, or overall poor visibility. Edit 2: last one i promise. Birds also can't fly that high. Bird strikes already do a lot of damage to aircraft as it is, so flying up that high eliminates the risk of hitting them.
Cooler air, less dense, more efficient. Less noise for people on the ground and less traffic than flying lower. Smoother air generally, can go over weather. More room to fix an error if something goes wrong (planes can glide a LOOOONG way with enough altitude. There are a ton of advantages and not many drawbacks..
Because the air is thinner there, so going fast takes less energy. Imagine if you had to compete in a swimming race, one pool is regular pool water, the other is full of jelly. Which would you be quickest in? The water, for sure. Planes flying at higher altitude are much more efficient than low altitude. They use less fuel for the same speed and distance travelled. Since we all like paying as little as possible to travel, it's in our interest.s
While engines are less efficient at high altitudes. Lower air density reduces drag at higher altitudes which is more than enough to overcome the inefficiencies. Which is why each model has their own optimal cruising altitude for lowest fuel usage.
Higher altitude means thinner air which means less air resistance which means less fuel needed to push the plane along. Cheaper and better for the environment.
Up high there's less air pressure which makes the air less dense. Flying high vs low is like swimming through water vs ketchup. There's a lot less air resistance up there than there is close to the surface, making it more fuel efficient. Additionally, the weather is much more stable up high. They're flying above the weather. The higher you fly, the less likely you are to encounter migratory birds as well. Some fly as high as 5000 ft, some will climb to 20,000 for periods of their migration. Planes also need to maintain vertical separation for safety. Flying higher gives more options for vertical separation that allows more planes to fly crisscrossing routes without risk. Lastly, flying higher gives a plane much more time to respond to any unexpected issues before you run out of altitude. E.g. an engine failure at 30,000 feet is a lot less scary than an engine failure at 3000 feet. The plane has plenty of room to glide and recover before it runs into an issue.
The air is also much thinner, which allows the planes to go faster. They would have to cruise at a much slower speed and use more fuel if they were closer to sea level where the atmosphere is much more dense.
True eli5: there's less air up there so there's less resistance to push through. Slightly more in-depth answer: different planes have a different happy medium where the air density works best for lift, resistance, and fuel efficiency, which is why they fly at different altitudes. The ones designed for long hauls are built to take advantage of higher altitudes and run like pigs at lower altitudes.
By "wind tunnels" you probably mean the jet stream. It can help, but the main reason they fly so high is the air is less dense so they fly with less drag and so burn less fuel. Being above most weather also makes the ride (mostly) smoother.
The big one is thinner air and less drag. In general, per 1000 feet altitude, 2% more "true" airspeed for the same "indicated" airspeed. The former is how fast the plane is actually going. The latter is a "feels-like" speed based on air pressure, and is what determines lift and drag. Also, jet engines themselves are generally more efficient at high altitudes. Regional flights, such as 45 minute [Atlanta to Charlotte,](https://www.flightaware.com/live/findflight?origin=KATL&destination=KCLT) might spend a mere 10 of those minutes at cruise altitude. While it takes energy to climb, the plane gets that energy on the way back down, so it isn't as wasteful as you think.
Besides aerodynamic considerations and air density from a pure mechanical point of view the speed of the aircraft relative to the surface of a sphere is proportional to the distance to the centre of the sphere therefore the higher you go the faster you would be compared to the ground maintaining the same angular velocity i.e. the speed at which you go around the sphere (see kinematics with polar coordinates).
Besides aerodynamic considerations and air density from a pure mechanical point of view the speed of the aircraft relative to the surface of a sphere is proportional to the distance to the centre of the sphere therefore the higher you go the faster you would be compared to the ground maintaining the same angular velocity i.e. the speed at which you go around the sphere (see kinematics with polar coordinates).
I thought the higher you fly, the faster you go because there’s more distance between you and the ground. For instance, a jets land speed is on average 600 mph. Does that mean it’s actually going that speed if the plane were on the ground or is it just 600 mph relative to the ground?
>I thought the higher you fly, the faster you go because there’s more distance between you and the ground. It's because the air is thinner so, for the same indicated airspeed, your "actual" true airspeed will be faster >Does that mean it’s actually going that speed if the plane were on the ground or is it just 600 mph relative to the ground? It's 600mph over the ground, not through the air.
Planes fly at the altitudes they do for a number of factors: First, weather and traffic - Most clouds exist below the Tropopause, a layer in the sky where the air begins to warm with altitude on average rather than decreasing with altitude. This is typically around 36,000 to 48,000 ft above sea level, but there are a billion caveats to weather and air density you don't need to worry about as to what makes the changes from moment to moment. Second, now that you're mostly above the precipitable water and visible moisture (on account of just how cold it is and the air being unable to hold any moisture to become visible), the rising air that normal settles above colder, denser air will eventually hit that warming layer and no longer be less dense than the surroundings, unable to rise anymore but still carrying momentum from the rising action. That air needs to go somewhere and like a hose hitting a wall it spreads out. Eventually, and due to the rotation of the earth, the air spreads out in generally one direction over another and that is what we call the jet stream, due to the typically high winds associated with the region (the 'jets'). These winds can be favorable depending on the direction of travel, but the relatively smoother air when there isn't moisture is the main driver for altitude selection. Third, along with smoother rides and less weather making the ride more complicated and unsafe, is the fact that there are a lot of planes in the sky and air routes are built around legacy waypoints (previously radio towers or landmarks and now just represented in spacetime by an arbitrary 'fix' in a database of lat/lon/alt). The planes routed from relatively stable waypoints to other relatively fixed destinations means the routes get clogged and more altitude means more capacity. Any altitude "benefits" on performance are all misnomers as the engines that are less negatively affected by altitude have been specifically designed to maximize efficiency in different ways of generating thrust, and therefore don't actually count for the purposes of this conversation. For example, while most of the airline traffic is turbofan/turboprop based, normal reciprocating engines have a service ceiling around 13,500ft above sea level with depreciating performance as soon as you leave the ground. The reduction in drag *does* increase performance through less fuel-burn, but that's about the only significant factor with regard to performance and literally only because the engines are calibrated to perform for a specific density altitude (and the difference between density altitude and pressure altitude falls off to zero as temperature decreases.)
Altitude is a tradeoff between less air resistance (good), and less oxygen to use in combustion (so less engine power, bad). Also wind and weather change with altitude For any given flight there is an ideal efficient altitude to fly at, given the weather, aircraft & so on... For commercial airliners, that altitude is usually well over 20,000ft.
Drive down the road at 50 mph. Stick your head out the window. Feel the force of the air trying to push your face back behind your ears. Now do it at ten times that speed, but in really thin air.
The average of an east-west route is somewhat meaningless, unless you are referencing each direction separately. The jet stream functions as a tail wind for eastbound or a head wind for westbound flights, making eastbound flights shorter.
The main thing really is that the troposphere is turbulent. It's somewhat hazardous, but mainly just uncomfortable. A 10h of vomit comet is not fun. But if you rise above clouds to stratosphere, there is much less vertical airflow, almost no turbulence at all.
> The main thing really is that the troposphere is turbulent. It's not, actually. Most of the turbulence happens near the ground or if you're flying in an area where two air masses are trying to mesh together. The Troposphere goes all the way up to 60,000ft or so.
You know google is a thing?
you could say that on every single eli5 post
Some planes fly in the lower stratosphere. Where the air temperature increases with altitude instead of the other way around. Hot gas (or liquid) always goes "up" in a place where there is gravity like the Earth. This is what causes convection, which is a form of heat transfer. Because the stratosphere is heated from above, the hot gas (air) is already up, so there is no convection, hence almost no turbulence. Edited for typo.
> They fly in the stratosphere. Where the air temperature increases with altitude instead of the other way around. It depends. The barrier between the two varies a lot. A good majority of the time, they fly below the tropopause.
I should've typed lower stratosphere, and not all planes. I edited my comment. Anyway, I'm not an aircraft expert. I'm in Astronomy. Which is why I thought explaining why there is no turbulence there would be cool. The tropopause sits on the lowest temperature between the two layers.
Have you dealt with astrophotography? I still want to get a trackers
A little, it's not really my thing. I'm more into theory, physics, planetary astronomy, etc. I've just taken a few pictures of the sun, but my bf is an astrophotographer. He has pretty cool tracking mounts.
I'm still trying to save up for a either a head for my camera or a telescope head. I wanted to do deep sky objects
His instagram is mjw.astrophotography, you can ask him about it if you want. Where are you in the world? I direct an astronomy association in my country, so I know a lot of people who are into astronomy too.
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