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APU


rami
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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...)

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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.

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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.

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