737-800 APU/1 Main Fwd Boost Pump.
 

Hi everyone. What is the advantage of running the #1 Main Fwd Boost Pump instead of the #1 Main Aft Boost Pump when running the APU. I understand the logic of having to run a boost pump from #1 Main, but I don't understand why you'd use the FWD one as it sits higher than the AFT.

Our FCOM changed a while ago to just a main pump. Other than to ensure positive pressure to prolong the APU fuel control unit, there isn't one. Perhaps it always used to , or still does according to the operator, be due to the old boeingFCOM/flying manuals where some sort of restriction existed due to type commonality. Perhaps between the 200s and 3/4/500 efis generation. Or maybe some restriction of operating pumps/apu whilst refuelling.

I have heard that its easier to service and replace than the aft pump - don't know if there is any truth to that though...

Sounds right as access to the FWD boost pump is via deployed LE FLAPS and access to the AFT boost pump is via the wheel well.

So in that case,using the AFT one would make more sense as servicing access would be easier.

Yeah. Thanks for the input. But i'm still wondering why.

How about you select the AFT pump until someone in your Airline comes up with a proper reason to do otherwise?
Seriously,i see no reason to use the FWD pump and there may be none.
Maybe you should ask your maintenance guys as they normally know quite enough :ok:

738 APU FUEL
 

My 738 SDS says #1 Aft Boost Pump.
But in fact you can use any pump as long as there is sufficient fuel in that tank, and the crossfeed valve is open.
Cheers !

Thanks De Facto. Seems i'll have to seek official clarification by my airline.

QF22 - Our aircraft suite of manuals all say pretty much the same thing. Perhaps you should check what pump is used during APU operation on your guys 737's. I think you'd be surprised at your findings.

Irrespective of what pump is used, it doesn't matter much as long as we stick to procedures, which clearly state that any pump can be used (with a very subtle hint to use any one of the #1 Main Tank pumps, preferably #1 AFT, depending on which manual you read and use).

I just need to know... now! :)

The aft pump def sits in lower in the tank compared to the fwd one.But unless Tank fuel is really low,it should not matter.

Could you substantiate that?how much lower?1cm?5?10?

Thanks.

Well did some digging on this trivial matter. Best explanation I got was this - on the CL, the fuel sys would scavenge the remaining useable fuel, if any, from the CWT to the L tank. To do this, #1 FWD boost pump was switched ON (or left on), with CWT boost pumps off. Scavenging will automatically cease after a time period after having switched the cwt pumps off.

The practice of leaving the -800's #1 fwd pump on for APU ops is suspected to have come from, surprise surprise, the CL.

Ultimately in the -800, it doesn't matter which pump is used to assist APU fuel feed, but this offers an explanation why some may revert to using only the #1 fwd.

spitfire..:ok: you're right!

.....and how many engineers would you find bothering with a tank fuel pump?

BOAC - that would be most of those who are diligent enough to follow the recommendations/instructions found in almost all of the planes manuals. I've not met one engineer who doesn't have a pump on when operating the APU during normal operations. It is also common practice with our tech crew from experience/observation, perhaps having a pump running is a part of their APU start/operation procedures/manuals too.

Well, Spitfire, I'm impressed! In my 20 years on 737s I NEVER found a pump on with APU running post engineering inputs, and when asked they would reply - 'cos it don't need it'.

I assume by 'tech crew' you mean pilots, and yes, always one pump there, sometimes crossfed, sometimes front, sometimes rear.