737NG APU warm up - 1 minute vs. 3 minutes

[skywalker001]

Hi everyone,

I'm trying to track down a reference in either a Boeing or Allied Signal manual which states that the 737NG APU warm up time before using the APU bleed air for pneumatics is 3 minutes (recommended), or an absolute minimum of 1 minute.

Chris Brady's b737.org.uk has the answer I'm looking for (as always) but I was wondering which official publication specifies these limitations.

Much appreciated. :ok:

[nick14]

I'm sure it states a 1 minute in our fcoms although I have never heard 3 minutes mentioned. We routinely use 1 minute.

[skywalker001]

As far as I know the 3 minute warm-up is an airmanship recommendation and not a hard and fast limitation. Have a look at The 737 APU (http://www.b737.org.uk/apu.htm#Limitations_&_Operating_Techniques).

Nick, I've found the 1 minute limitation FCOM vol.2 7.30.3 but still no clue where the 3 minutes came from...

Thanks though.

[Capt Chambo]

Never heard of 3 minutes, only ever heard of waiting one minute.

Operate the APU for one full minute before using it as a bleed air source. This one minute stabilization is recommended to extend the service life of the APU.

Ref: FCOM (2) Engines/APU, APU start last paragraph.

[skywalker001]

Thanks for that reference Chambo.

Has anyone else heard of this 3 minute recommended warm up time before?

[mangatete]

Maybe being confused with Engine warm up... Before Take Off Procedure " run engines for at least 3 minutes....
Cheers

[nick14]

And engine cool down 3min, nothing about the apu in my mind. I fear it might be an over cautious pilots response?

[skywalker001]

Perhaps the 3 minutes theory is the product of a severe case of 'broken telephone'. I hear what you guys are saying about the 2 minute warm-up and 3 minute cooldown of the engines (remembering of course that these aren't limitations but recommendations to avoid engine degradation - see FCOM 1 NP21.37 and 56).

But the 3 minutes I'm referring to is what I stumbled across on Chris Brady's B737 . org . uk website. Have a look at this link and go to "APU Limitations and Operating Techniques" - The 737 APU (http://www.b737.org.uk/apu.htm#Limitations_&_Operating_Techniques)

Nick, I think you may be right, the response of an over cautious pilot.
Thanks for the help, chaps.
Fly safe.

[JQKA]

what i know from theory is that what he said Skywalker is truee.
Yje Apu warm up need from 1 to 3 minutes, but obviously every one nowadays as standard they use to wait just 1 minute!;)

[flyby797]

Hi Skywalker,
Actually there is a BOEING FLIGHT OPERATIONS TECHNICAL BULLETIN which deals with APU ops: "Improved APU Operating Practices". There you'll find your 3' warm up. Hopes this helps. The reference of the FOTB is 99-1 dated feb 12, 1999.