Hi All,I know the APU is used for safety on takeoff's and landings, but how is it used during cruise flight? Is it left running? Or, is it shut down until the descent phase of the flight? Any ideas about this would be appreciated. Best. :DAl Whitney - Bacal Aircraft Lab

"I know the APU is used for safety on takeoff's and landings"You sure? I dont think it would make any difference to saftey as all the APU does is supply the Bleed air and Electricity when the engines arent running. When the engines are running,they can supply this bleed air and power comes from the Engine Generators. I know that above 10,000ft-the APU must be turned off. So that eliminates any use during cruise.As far as I am aware,the APU is usually turned off during the taxi phase on departure...Then again-I could be wrong ;)RegardsJohnhttp://www.bavirtual.co.uk 650Hours ;)P 2.53 GHZ512 RAMWINXPGFORCE 4 128MB Ti4600http://vatsim.pilotmedia.fi/statusindicato...tor=OD1&a=a.jpg

From what my experience shows, the APU is turned off just after engine start and it is turned on again after you have vacated the runway after landing. I also think some airlines turn the APU on again on approach.Someone correct me if I'm wrong.John/AFG

Hi all. Well i can tell you that most apu designs will not allow apu starting above 25000ft. That is the case with the dc-10. We normally shut down the apu during the before take-off check and restart it during the after landing check. Up at altitude the apu is useless even at 25000ft. It doesnt supply enough volume for pressurization or anti-ice. We use the apu below 10000ft for additional back up power in case of generator or engine failure. As for take-off and landing i know it is possible but there is a penalty that has to be applied for take-off data for apu running. There are some cases where a maximum thrust take-off is required. That would be a airconditioning packs off take-off. Some airliners would leave the apu on to supply airconditioning bleed air while the engine pneumatics are turned off. In the dc-10 if this was done the caution system would remind you to switch to engine pneumatics during the climb. Though manuals say its useful for pneumatics inflight i wouldnt try it....Better to descend below 10000ft and save the apu for electrical if needed. Ive heard of times when the apu was started on short final but we dont anymore.

Thank You Everyone for your help understanding when the APU should be used. Its usage seems to vary greatly across different aircraft. If there were total engine failure below 10,000 ft, then the APU can be used for hydralics and electrics so the plane can be glided down, at least, as in the 767, "Gimli glider". But that plane had no fuel to even run the APU. Anyone remember this? :-hmmmBest to all,Al Whitney - Bacal Aircraft Lab :-)

Greetings,I believe you may be confusing the RAT (Ram Air Turbine) wich can provide limited air and electricity with the APU (Auxilary Power Unit). The APU is basically an engine, located in the tailcone of the aircraft, and as you point out requires fuel to run.Not all aircraft have RATs. The 747, for example, doesn't. However, with 4 engines and 4 sets of bleed air, a situation where you lost one engine and it's air/electricity generator isn't quite as big a problem as one failing in a twin engine aircraft.Every AOM or checklist I have/have seen for the 7(3/4/5/6/7)7 airframe has the APU used during engine start to provide air to turn the turbines to the point where they can be made self-sustaining, or in limited situations during takeoff (APU->PACK takeoff in a 747 for example) so that you can get a little extra power from the engines for really heavy weights, or really hot days. And even that's not doable for any aircraft.In fact, the 747 series aircraft have cutouts which will not allow the APU to be started during flight. The 757/767 will let you start it in flight (usually reserved for the 0case of engine failure so that you can replace some of the power and bleed air lost), but as has been previously mentioned, only under a certain altitude.Luck!P

Actually the the APU on the 737NG, 757,767 and 777 have to be certified to start at cruise as part of the ETOPS program. Would be a bad thing if you lost an engine and had to descend to 18000ft to fire up the APU for a backup generator. :-)'RegardsPaul Gollnick:-coolTechnical Operations/Customer Operational SupportPrecision Manuals Development Groupwww.precisionmanuals.com