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On a warm day, having the APU1 bleed on provides pressurised air for the air conditioning packs to operate, cooling the cabin. During engine start, this bleed air is redirected to the engine starter valve to initiate the main engines. (Frequent flyers might notice passengers reaching for the nozzles, not realising the air is now used for engine start.) After the engines start, they supply bleed air for air conditioning, allowing the APU bleed air to be switched off.

My question concerns taxiing back to the stand after a flight. The APU is typically started during this time preemptively, as it will be needed to supply electrics on stand, bridging the gap until ground power is connected. The APU provides power during this transition, which frequent flyers might notice as a brief flicker of the lights – the switch from engine power to APU power.

On warmer days (but not necessarily markedly "Hot Weather" days2), there's also a need to transition from engines supplying bleed air for air conditioning to the APU doing so.

On the 737, the bleed air duct is shared by engine number 1 and the APU. If too much air pressure from engine 1 hits the line, it might cause "blow back" into the APU if the APU BLEED switch is on.

In my company, we avoid turning the APU bleed air switch ON until both engines are below 20% N2 to prevent this "blow back." However, this procedure is not referenced in our SOPs, and to my knowledge, it isn't mentioned in the 737-800 AFM, FCOM, FCTM, or QRH.

So, is this danger of "blow back" a genuine risk to the APU bleed system operation, or is it a practice that has been informally passed down among pilots without substantial evidence?enter image description here


1 An APU (Auxiliary Power Unit) is a small engine which sits in the rear of the 737-800 which can provide electrical power and compressed air for cabin air conditioning and engine starting. This is typically a Allied Signal 131-9(B) for the Boeing 737-NG

2 The “Hot Weather Operations” (Outside Air Temp above 40oC/104oF) supplementary checklist refers to turning the engine bleed air switches OFF before turning the APU switches ON. (The APU does a better job of providing bleed air in this scenario compared other the engine).

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So, is this danger of "blow back" a genuine risk to the APU bleed system operation, or is it a practice that has been informally passed down among pilots without substantial evidence?


There are two related paragraphs in the Boeing FCOM, and one in the AFM.

Boeing FAA-approved AFM (D631A001 revision 31, 2011)

Except for engine start do not open APU bleed air valve when LH engine bleed air valve is open, or when isolation and RH engine bleed air valves are open.

Boeing FCOM (D6-27370 revision 26, 2010)

AFM operational (see a copy here):

The DUAL BLEED light illuminates whenever the APU bleed air valve is open and the position of the engine bleed air switches and isolation valve would permit possible backpressure of the APU. Therefore, thrust must be limited to idle with the DUAL BLEED light illuminated.

Non–AFM operational information (see a copy here):

APU bleed valve may be open during engine start, but avoid engine power above idle.


Note there is also a possibility of backpressure of the engines 9th stage by the APU at idle thrust, causing the engine valve to close:

With both the APU and engine bleed air valves open, and the engines operating at idle thrust, there is a possibility of APU bleed air backpressuring the 9th stage modulating and shutoff valve. This would cause the 9th stage valve to close.

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