B737NG minimum start pressure

[POL.777]

From a B737NG typerating questionaire.

The minimum start pressure at 2000 ft above sea level (prior to starter engagement) is?
a) 39 psi
b) 29 psi
c) no limit

I can't find the answer in the operations manual or the flight crew training manual, but I found something on http://www.b737.org.uk/powerplant.htm

Copy paste "Min duct pressure for start: 30psi at msl, -½psi per 1000ft pressure altitude. Max: 48psi. "
However I am not sure if this is CL or NG.
This question is in a section called NG differences so it might be a trick question...

Any insights are highly appreciated.

Best regards

POL

[purr]

hope this helps
__classic____
CLEARED FOR START
DOORS .... CLOSED
FlLT DECK WINDOWS.... LOCKED
AIR CONDITIONING PACKS ...OFF
START PRESSURE........_________-psi
ANTICOLLISION LIGHT .....ON

__ NG ___
The start pressure line is missing

This is because the bleed pressure from the classic APU is fixed and NG is as per demand so if you looked up at the NG pressure guage prior to start it would be close to zero .Only after the starter is engaged does the pressure build up.
I think there is a seperate compressor in the NG APU that does the trick.
Why don't you check with the engineering they might be more precise. :ok:

[POL.777]

Thank You very much for the replies.

I guess the big question is; What do they ask for in this particular question?
One thing Capt.
I don't have the reference mentioning that I should subtract 0,5 psi for every 1000 ft. AMSL. My manual just says "advance thrust lever until bleed duct pressure indicates 30 psi" in the ENG Crossbleed start. (Manual D6-27370-TBC)

Thank You for both for the interesting replies.

Bst rgds

[Shaka Zulu]

My manual says the same, subtract 0.5psi per 1000'

[Flight Detent]

The 737NG APU pneumatic supply pressure will read 15-18 psi when the bleed valve is opened, and prior to selecting a service, i.e. aircon pack, engine starter.

And yes, the NG APU employs a seperate compressor, thats why Boeing recommends operating both packs on the ground, its more economical with fuel!

This is my understanding of the system.

Cheers :ok:

[POL.777]

Still can't find the reference to "Subtract 0,5 psi" in my manual. Anyone care to supply me with a direct reference to where I can find it? Maybe this Boeing manual needs an update (or maybe I need spectacles)

Best regards

POL

[Alaskan Timber]

Hey Pol,

Capt. Marvellous is right. The answer to your question is: no limit.

Regarding the 30 psi etc, that is when you do a crossbleed start.
You can find this in vol. 1 B737NG page SP 7.3. The 0,5 psi I can't find either in the B737NG. In the 'classic' it is written.

Good luck