737NG APU in Main Engine Start (MES) mode

[allendc]

Hello
I have a quick question about the 737NG APU in Main Engine Start (MES) mode. In main engine start mode, the APU ECU opens the APU Inlet guide vanes to meet the increased airflow requirements.

Anyone have the specification on the increase in APU bleed air flow and pressure during MES mode?

Thanks in advanced.

David
www.737FlightSim.com

[allendc]

Hello
I am still trying to find an answer about the 737NG APU in Main Engine Start (MES) mode. In main engine start mode, the APU ECU opens the APU Inlet guide vanes to meet the increased airflow requirements.

Anyone have the specification on the increase in APU bleed pressure during MES mode?

Thanks in advanced.

David
www.737FlightSim.com

[Escape_Slide]

Ask Maintenance

The B737 maintenance manual should answer this for APU change testing. Check with the mechanics.

[allendc]

Hi Escape_Slide
I do not have access to any 737NG's or maintenance personal, thanks to 911.

David
www.737FlightSim.com

[Escape_Slide]

Oh OK

Can anyone help with the maintenance manual stuff?

[DDG]

AlliedSignal 131-9b APU Operating Bleed Load Limit

AlliedSignal 131-9b APU as fitted to the B737NG has a bleed load operating limit at sea level,60F day of 160pounds per minute at 60psi.The ECU will modify this output pressure and flow during MES mode depending on actual atmospheric conditions to allow for changes in surge margins.
Best info i could find at hand,referance was AMM part 1 Chapter 49-00-00 page 5.
Cheers

[Contract Con]

Gday,

In really simple terms, it goes from about 20PSIG (before you move the start switch), up to around 40PSIG when you crank it.

Cheers,

Con

[allendc]

A big thanks to Contract Con, DDG, & Escape_Slide!

If I may one last bit of info please.

When you crank it, is there a pressure drop as the start valve reaches full open and engine rotation begins.

Or

Can the APU maintain 40 PSI during the total start cycle?

David
www.737FlightSim.com

[Contract Con]

Gday,

I will have a look when I'm in the machine next. That is not for about 5 days though. I'm sure someone else will be in one before me.

However, I'm pretty sure it can hold it, or damn close. I remember the big press. drop in the Classic 73 when you cranked it, but I have a feeling I noticed this new APU can keep the pressure up.

Anyone??

Cheers,

Con

[Flight Detent]

Of cousre the Classic and NG has entirely different air supply systems.

The Classic supplied bleed air from the APU compressor, and the NG APU supplies air from the seperately driven compressor.

Cheers FD

[Contract Con]

gday FD,

Care to ellaborate??

Cheers,

Con

[Flight Detent]

Contract Con,

Sure, but I don't have any technical pubs with me to refer to...

My understanding is that the NG APU drives a seperate common shaft air compressor that supplies all the 'bleed air' for ground aircon and engine starting.
Have you noticed in the NG FCOM that it's recommended to operate both aircon packs on the ground, it's more economical for the APU.

Cheers, FD

[Contract Con]

Gday FD,

Could you point me in the right direction for a ref. re the APU design. I cant seem to find anything in the books nor via the Allied Signal website.

You know how the Boeing books are now, purely a need to know basis, and it seems that they dont think we need to know much anymore

Thanks for your input,

Con

[Contract Con]

Allendc,


Machine I flew recently, the duct press. rose from 20psig unloaded, up to 35psig when cranked and stayed there until "starter cutout".

Hope that helps.

Cheers,

Con

[allendc]

Contract Con
Thanks a million!

David
www.737FlightSim.com