Hey IFC! I was wondering if in the real world if they turn off the APU after just starting up one engine and if they turn it on after clearing the runway after landing? Thanks!
There isn’t a one size fits all for every plane
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To be more specific I’m talking about commercial airliner jets.
If I’m correct, it depends on the operation. While I believe most use APU for both engine, you can definitely use one engines bleed air for the other engines start up. They may do this during one engine taxis?
In terms of after landing, the APU is turned on when clear of the runway.
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Ok sounds good, thank you!
some also turn their apu’s off after takeoff (i think)
I think they do that to have more power in the engines during takeoff but I’m not completely sure
yes you can the engine should give enough power for the whole aircraft
oops when you get close to the gate turn it on
That’s more in the business jet area. Most commercial airlines rely solely on their engines for power/pressurization
To comment on this: I know that on the A320 family pilots sometimes opt to keep the APU on for bleed air (to power the PACKS) during TO/GA thrust takeoffs instead of handing it over to the engines. This is to prevent unnecessary wear and tear on the engines during high power settings. Shortly after takeoff, the APU will be switched off. This procedure rarely applies to FLEX takeoffs but may be used if deemed necessary.
It is also possible to turn off the PACKS completely and have the APU turned off for TO/GA takeoffs.
I imagine other aircraft sometimes do this as well.
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I know from my Operating Manual for the 757/767 you leave the APU on until around flap retraction after takeoff. This way, if there is a catastrophic engine failure on takeoff roll or initial climb out, the aircraft controls and communication systems are still live for pilots to manage the emergency.
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In contrast to the statement that the Challenger 350 has its APU on during takeoff, an article on APU’s in general in Flying Magazine states:
“In most cases, the APU is shut down before takeoff and reignited when the aircraft clears the runway after landing.”
How an Auxiliary Power Unit Works - FLYING Magazine
Elsewhere I read:
- APU’s have been around since WWII aircraft
- The 707 and Concorde had no APU
- The Space Shuttle had 3 redundant APU’s
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