Is it normal that I can turn on the APU without having the left fuel pumps or even all fuel pumps engaged?? This is what happens to me... And I wonder how that can be... Nic

It's called Gravity!

The APU can get its fuel in 3 ways.The first way is a running AC fuel pump on the left side of the airplane. Or any other pump when the Crossfeed valve is open.The second way is the APUs own DC pump. In the left wing a little DC pump is mounted which provides fuel to the APU if the APU is running.The third way is not a common way but the APU may also feeds itself without a pump because the low pressure which the running engine makes in the fuel line causes the fuel to flow to the APU. But probably you can't start your APU that way.However to extend service live of the APU fuel supply unit it is recommended to run at least one AC fuel pump to provide positive fuel pressure.

Well not quite ! The fuel is suction fed from the No 1 tank when no pumps are operating.Boeing does have an option for an APU DC fuel boost pump.Not sure if this is in the NGX.

AFAIK, the NG does also have the DC pump installed......

Thanks to all! Except of the "gravity" point I learned a lot again! :-)Nic

The fuel is suction fed from the No 1 tank when no pumps are operating

Okay my mistake. I thought I remembered seeing something about the engines being able to function without the pumps on because of Gravity, so figured that applied to the APU as well.

The APU can get its fuel in 3 ways.The first way is a running AC fuel pump on the left side of the airplane. Or any other pump when the Crossfeed valve is open.The second way is the APUs own DC pump. In the left wing a little DC pump is mounted which provides fuel to the APU if the APU is running.The third way is not a common way but the APU may also feeds itself without a pump because the low pressure which the running engine makes in the fuel line causes the fuel to flow to the APU. But probably you can't start your APU that way.However to extend service live of the APU fuel supply unit it is recommended to run at least one AC fuel pump to provide positive fuel pressure.

Didn't know

The APU can run perfectly with NO fuel pumps running. As mentinoned before in a previous post, if you start the APU on battery power only, There is no fuel pump's running you need AC electrical power for that. There are 737 NG that have a DC fuel pump installed to keep a positve fuel pressure to the APU. But that is a Airline option, many airlines do not have this DC pump installed. On the APU it self there is a pump that suchs fuel from main tank 1, There is no gravity.

Okay my mistake. I thought I remembered seeing something about the engines being able to function without the pumps on because of Gravity, so figured that applied to the APU as well.

Engines can, but the APU is above the fuel tanks.

but i have a question, can you keep operationg the APU with no fuel pumps on and ofcourse battery on.regards

Yep you can....but to reduce wear on fuel control components it's recommended to have positive fuel pressure delivered to the APU as described above, either via a functioning AC pump or the APU's own DC powered pump.I'm quite surprised a DC APU pump is an airline option on the NG's though....gotta save money everywhere eh?I know on the old Garret APU's if you didn't have positive pressure goin' into the FCU/Gov. you'd have a heck of a time getting above 30%

AFAIK, the NG does also have the DC pump installed......

The DC pump (where installed) operates when LOW pressure condition is present on the left side.As there is no sensor to detect the low pressure in the APU duct, a logic controls if pressure is present or not.The logic controls the pumps low pressure (all) and crossfeed valve position, plus fire and APU switch obviously.Basically, the pump comes on whenever there is a low pressure on the left side fuel ducts.This happen with all pumps off or with RH pump/s on but crossfeed closed.When at least one left pump, or right pump with crossfeed is turned on (with low pressure light extinguished), the DC pump stop operation.When running for long time, it is useful to equalize fuel tanks by turning on the opposite wing pumps plus crossfeed or, if fuel is present in center tank, burn fuel in that tank with the same logic as te wings, left center pump alone, right one with crossfeed.