Engine/Apu bleed

APU bleed is the alternate source for pressurization and cooling..

You would use it say if you were on the ground to cool the airplane.

Or, if you had a pac failure while flying, you could use APU bleed to provide air conditioning for the airplane.

APU (auxilliry Power Unit) is a mini jet engine in most cases and is for ground use only. You will see exhaust exiting the real of most Commercial aircraft as they leave the APU running to provide power on the ground for things like Air conditioning/lighting and starting the engines (depending on what type engine starters) without a ground power hookup. You are usually prohibited from using a APU in flight. rob

Yes, if I am not mistaken there are a few specific uses for the APU in mid-flight and they all have to do with emergencies.

Yes, if I am not mistaken there are a few specific uses for the APU in mid-flight and they all have to do with emergencies.

After talking to a close friend of mine who is an Avionics Engineer at Air Canada it seems that on large jets they can and do use the APU in flight in the event of more than one of the PACS failing or even if, say a generator went off line, the APUs on these airplanes are cabable of backing up pretty well everything..
On the other hand, the smaller multi engine airplanes like the Dash Eight do not have the capacity to run the APU in flight. They won't even fire up if there is no weight on the wheels..