anybody can explain this?(737ng /apu bleed air goes off )

[eagle737]

i'm a 737ng guy, last sunday-a raining day, we found the apu's bleed air went off while we were doing "before start preparation" ,and apu was still running. we turned the packs and apu bleed air switches off .after the rain stopped,we turned the packs and apu bleed air switches back to "on", and the air pressure came back.
anybody can explain this? is this a part of 737ng design consideration? if so, what is the reason?

[Loony_Pilot]

The APU on the NG is FADEC controlled and the bleed air works on a demand basis.. no demand for bleed air.. no duct pressure.

[Flight Detent]

Hey Looney....

That should read: "small amount of pressure",

It will show approx. 18 psi with no pneumatic demand!

Cheers,

FD;)

[CaptainSandL]

The APU will load-shed its air (before electrics) if the EGT gets too high. This is quite rare and I have only seen it during engine start and when all the galley ovens were in use. Dont know the significance of the rain though:D

[eagle737]

thank u for your replies.
the situation was: there was "no" bleed air pressure.
we suddenly couldn't feel the air conditioning noise, we checked the air supply duct pressure gauge, it showed "0".
i know "the bleed air works on a demand basis..", so why did it go off while we did need it----air conditioning demand.
waiting for any further explaination.

[CV Donator]

Must have been a pack problem. The left one.

[alexban]

did the apu make a laud noise-like a bang- when the air came off?...it sometimes happen with the ng apu...but then it comes back on after few sec.

[eagle737]

reply to alexban:
there was no significant evidence when the apu bleed air went off except unusual silence and duct pressure gauge showed "0".

[Terraplaneblues]

Eagle 737

This problem has been reported to Boeing by other operators, however the article is subject to, BOEING PROPRIETARY, CONFIDENTIAL, AND/OR TRADE SECRET.
All I can recommend is that you report this incident to your MCC or Tech services group and they should easily find the article on MBF.com with recomendations of which actions to take.

737NG-FTD-49-02002 may or may not be what they are looking for ;)