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Guest schuurman01

Low bleed air pressure from APU.

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Guest schuurman01

Hi,I notice before engine start that the bleed air pressure from the APU is about 15 PSI with all PACK's "OFF" and isolation valve open and that I think is not enough to start the engine's, according to a Colleague of my, how is engineer, you need at least 25 PSI to open the starter valve. but the pressure needed to start the engine's is around 30 to 35 PSI according to the engineer. If the APU can only deliver 15 PSI, this APU needs to be replaced "in real life".I don't now if anyone else having this problem?Thanks.Greetings Mark

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Guest wkzzo

In main engine start mode, the APU ECU opens the IGVs to meet theincreased airflow requirements.The IGV's(inlet guide vanes) will open or close dependent on signal logic from the ECU. When the starter is energized the duct pressure will usually go to around 40PSI on start with a healthy APU.

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"If the APU can only deliver 15 PSI, this APU needs to be replaced "in real life"."Ref Boeing Maintenance Manual (D&O) 49-50-00... around page 14... for various APU Bleed modes.BTW, 15 psi is acceptable for "Duct Pressurization Mode" (otherwise every NG operator in the world would have to change all their NG APU's).737 Classics are somewhat different, however.Cheers.Ian (Newbie NG "mechanic"... but, hey, even I knew this :()

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Guest schuurman01

"BTW, 15 psi is acceptable for "Duct Pressurization Mode" (otherwise every NG operator in the world would have to change all their NG APU's)."Hi Ian I was only figure of speaking,But it's our company pollesy to find the problem or even to replace the APU in the worst case scenario, Acccording to the company's DDG the pilots can accept the aircraft for two more flight'sthan the problem needs to resolved. I don't how you company handle's problem's like this. but we have strand maintenance program.GreetingsMark.

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"But it's our company policy to find the problem or even to replace the APU in the worst case scenario, Acccording to the company's DDG the pilots can accept the aircraft for two more flight'sthan the problem needs to resolved. "I'm still not sure you understand what we are talking about, Mark. "Duct Pressurization Mode" is the second lowest APU bleed pressure output mode. That is... APU Bleed selected ON, but packs OFF (The lowest pressure mode is APU running, but APU Bleed selected OFF).The NG APU bleed system is a "demand" system. The more bleed you want, the more it will try to give you. If you don't want any bleed air at all, it will give you only a low air pressure.IF I were only getting 15psi for main engine start, then YES, there would be problem which would need to be resolved. Hope you understand (Please read the manual section I recommended for further details).Cheers.Ian.

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