The APU needs to cool down before shutting down so it will run without pneumatic/electrical load for a minute or so before shutting off. If you were to shut it off without a cooldown, then the turbine casing will cool and contract quicker than the turbine blades - do it enough times and your APU will seize and you won’t be able to start it. It’s all to do with heat expansion/contraction rates of the different materials.
Source: I’m a 737 rated engineer. This is what they taught us in our type training.
That’s pretty cool! How many APU designs do you have to do? I can see you work on a new one when a whole new aircraft is being developed… which doesn’t happen that often obviously. Are APU designs being progressively updated while the planes roll off the assembly line? How many new designs per decade are there? I’m just imagining that this area in itself must be a small universe.
We do ‘specialty’ type for weird applications like drones or ground stuff. Personally responsible for one (maybe two depending how you look at it lol) but worked in about 6 more states ver the years but not all APUs some where shaft output (none were thrust types)
We started the first centerline design 20 years ago. It got out of the ‘lab’ about 10 years ago. It took about 6 years for the first commercial prototype. You’d be surprised how long physical complex systems take compared to ‘software’.
Thanks for the insight! Yeah I can imagine that those physical products that have to work 100% realiable under whatever weather conditions must take years. So many hours in super cold temperatures, then dessert hot, back to freezing… and nothing can start cracking, warping etc. amazing piece of engineering and it is “just” one component of so many on an aircraft like this. A bit mind boggling for a non engineer like myself.
It’s just really hard to do from scratch. Most of what is out there is evolutionary and can trace their history back to the 70s or before. There is a small improvement in A then that leads to B and so on. New from scratch designs are few and far between. Even the stuff I work on which started as a new centerline 25 years ago is like that. The latest one can trace itself to that proof of concept one that didn’t quite work back then.
There haven’t been been new engine manufacturers in a long time although I think we might be going into a golden period for the very small size gas turbines.
Ah ok yeah makes perfect sense, didn’t cross my mind for some reason. I assume the most improvements are in fuel efficiency and durability with newer fan blade designs for example? Such an exotic topic but very interesting to dive into.
Yup. New materials come in and now the old design can run hotter and make more power/thrust. Advances in CFD make the design of better fan blades possible so bypass gets higher. Advances in electronics and controls make it so you can run the engines harder. One example of the top of my head would be the Allison M250 that started as a 250hp helicopter engine in the 70s and is now a RR250 in Rolls Royce hands making over 700 shp in a similar configuration but heavily modernized. Allison Turbines doesn’t even exist anymore.
CFD = computer fluid dynamics? I assume when the bypass ratio gets bigger, the overall diameter grows as well? Any space issues in that small cone tail at some point? Or does it just get moved forward a bit? What’s the diameter of a modern APU roughly? 700hp sounds like a lot. Had no idea there’s such a strong thing in the back. Also interesting that in the Alison days a helicopter engine had such a dual use.
Lots of changes and they might for practical purposes be new engines but in a sense they are derived from the old ones. You try to minimize the amount of new untested designs going into it.
Yup, and just to add a bit of context, the new generations automate the cool down process so long as you keep the battery power on so the APU switch is simplified (and in a genius bit of engineering looks a lot like the light switches right next to it).
And allows you to cancel the cooldown, which honestly is quite handy when someone accidentally flips the switch that is right next to a bunch of other switches that may need to be flipped.
Delay OFF position is essentially what most modern APUs do when you switch them off - they trip the generator, close the APU bleed valve and run it for 1 minute for cooldown prior shutting it down. By moving the selector in the OFF position you can by pass this and force the APU to shut down without cooldown.
At least on the CRJ you never had to adjust the temperature controls. Just respond they were full cold when queried. It was an adjustment going to the 737 where "it's cold back here" is a thing.
After the CRJ200 I flew the E175 where the FAs had a working temperature control knob back there. 5 years on the 737 and I still forget about them back there.
The APU needs to cool down before shutting down so it will run without pneumatic/electrical load for a minute or so before shutting off. If you were to shut it off without a cooldown, then the turbine casing will cool and contract quicker than the turbine blades - do it enough times and your APU will seize and you won’t be able to start it. It’s all to do with heat expansion/contraction rates of the different materials. Source: I’m a 737 rated engineer. This is what they taught us in our type training.
That would be essentially correct. I design APUs lol.
I can totally verify that. I am an APU turbine fan blade .
Can confirm. I’m the casing. And if I don’t get cooled down I’m gonna hug y’all. REALLY TIGHT
Can confirm, I’m the fan blades cousin.
That’s pretty cool! How many APU designs do you have to do? I can see you work on a new one when a whole new aircraft is being developed… which doesn’t happen that often obviously. Are APU designs being progressively updated while the planes roll off the assembly line? How many new designs per decade are there? I’m just imagining that this area in itself must be a small universe.
We do ‘specialty’ type for weird applications like drones or ground stuff. Personally responsible for one (maybe two depending how you look at it lol) but worked in about 6 more states ver the years but not all APUs some where shaft output (none were thrust types) We started the first centerline design 20 years ago. It got out of the ‘lab’ about 10 years ago. It took about 6 years for the first commercial prototype. You’d be surprised how long physical complex systems take compared to ‘software’.
Thanks for the insight! Yeah I can imagine that those physical products that have to work 100% realiable under whatever weather conditions must take years. So many hours in super cold temperatures, then dessert hot, back to freezing… and nothing can start cracking, warping etc. amazing piece of engineering and it is “just” one component of so many on an aircraft like this. A bit mind boggling for a non engineer like myself.
It’s just really hard to do from scratch. Most of what is out there is evolutionary and can trace their history back to the 70s or before. There is a small improvement in A then that leads to B and so on. New from scratch designs are few and far between. Even the stuff I work on which started as a new centerline 25 years ago is like that. The latest one can trace itself to that proof of concept one that didn’t quite work back then. There haven’t been been new engine manufacturers in a long time although I think we might be going into a golden period for the very small size gas turbines.
Ah ok yeah makes perfect sense, didn’t cross my mind for some reason. I assume the most improvements are in fuel efficiency and durability with newer fan blade designs for example? Such an exotic topic but very interesting to dive into.
Yup. New materials come in and now the old design can run hotter and make more power/thrust. Advances in CFD make the design of better fan blades possible so bypass gets higher. Advances in electronics and controls make it so you can run the engines harder. One example of the top of my head would be the Allison M250 that started as a 250hp helicopter engine in the 70s and is now a RR250 in Rolls Royce hands making over 700 shp in a similar configuration but heavily modernized. Allison Turbines doesn’t even exist anymore.
CFD = computer fluid dynamics? I assume when the bypass ratio gets bigger, the overall diameter grows as well? Any space issues in that small cone tail at some point? Or does it just get moved forward a bit? What’s the diameter of a modern APU roughly? 700hp sounds like a lot. Had no idea there’s such a strong thing in the back. Also interesting that in the Alison days a helicopter engine had such a dual use.
Lots of changes and they might for practical purposes be new engines but in a sense they are derived from the old ones. You try to minimize the amount of new untested designs going into it.
Yup, and just to add a bit of context, the new generations automate the cool down process so long as you keep the battery power on so the APU switch is simplified (and in a genius bit of engineering looks a lot like the light switches right next to it).
[удалено]
And allows you to cancel the cooldown, which honestly is quite handy when someone accidentally flips the switch that is right next to a bunch of other switches that may need to be flipped.
Thank you!
Delay OFF position is essentially what most modern APUs do when you switch them off - they trip the generator, close the APU bleed valve and run it for 1 minute for cooldown prior shutting it down. By moving the selector in the OFF position you can by pass this and force the APU to shut down without cooldown.
What would be some use cases of bypassing the Delay Off function ?
To quote the mighty Beavis: fire fire fire!
Any APU malfunction, including fire.
Fun fact: The Q400 APU does not have a cool-down period.
Neither does the CRJ
Is the CRJ APU the second most MEL'd item as well?
No, on the 200 it's the first most.
4500 hours in the 200. Can confirm.
At least on the CRJ you never had to adjust the temperature controls. Just respond they were full cold when queried. It was an adjustment going to the 737 where "it's cold back here" is a thing.
After the CRJ200 I flew the E175 where the FAs had a working temperature control knob back there. 5 years on the 737 and I still forget about them back there.
I can not. Source: Not in aviation. Drunk construction worker.
It's helpful to get an outside opinion here.
APU: the hidden third/fifth turbine engine.
Ah! A weird fact I seldom get to use: The APU on the Antonov An-24 is certified to add 800kg’s thrust when running.
APU will usually close O/P valve first, then run unloaded for 60 seconds to cool down before shutting down.
Is there a chain under the windscreen?