rami Posted February 24, 2011 Share Posted February 24, 2011 hi all why we can not use bleed air from APU during flight? Quote Link to comment Share on other sites More sharing options...
NATOPS1 Posted February 25, 2011 Share Posted February 25, 2011 Easy answer... Your book says not too, the reason... you have four other sources of bleed air all with higher outputs so there is no reason to use the APU air. If you have the engine air off it would be for a reason... If you do turn on the APU air inflight you WILL exceed the 650 EGT max as the APU output could not meet both of the AC's bleed air demand. (and the FIRE light will illuminate or so I have been told...) Quote Link to comment Share on other sites More sharing options...
n1dp Posted February 25, 2011 Share Posted February 25, 2011 This was demonstrated many moons ago with a CG herk taking off on a hot day. Good intentions by the FE, wanted to keep the pax cool. Short field takeoff, engine bleeds closed, left the APU running and got a fire light on climb out. By design, not enough cooling to provide bleed air in flight. Quote Link to comment Share on other sites More sharing options...
pjvr99 Posted February 25, 2011 Share Posted February 25, 2011 Also APU FCU cannot compensate enough for altitude/temperature change. Even big commercial aircraft are limited to about 15000ft. It may run at higher altitudes but will flame out at the first application of load - even a shift in generator load can be too much Quote Link to comment Share on other sites More sharing options...
Chris Down Under Posted February 26, 2011 Share Posted February 26, 2011 Because of the extra EGT when you use Bleed Air. The Fire wire is very close to the exhaust pipe inside the compartment. when Airborne the air flow going past does not allow exhaust gas to be dispersed as quickly, with the higher EGT of Bleed Air, this extra build up of temp puts the APU fire light on. Many years back when we had to replace all our X wing bleed air pipes due corrosion, one of our FE's also tried to use the APU and just the flt deck pack on. Got airborne and yep the APU light came on with the increse in airspeed. Quote Link to comment Share on other sites More sharing options...
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