APU Fuel Feed from CTR Tank

[balaton]

Hi Guys,

In rw, to avoid fuel imbalance on the ground and with fuel in CTR tanks it is a recommended practice to switch on the LH CTR Pump switch and feed the APU from the CTR tank.

With ixeg 737 the APU always uses fuel from the left main tank regardles of switch positions. (Dear mod, if it is Lufthansa specific, please delete my post !)

 

Cheers,

Tamas

[Morten]

As far as I can tell from the schematics, fuel to the APU always draws from tank #1 (left) regardless of pumps, it even suction feeds from there... but I could be wrong offcourse, Jan?

Edited November 4, 2016 by Morten

[mmerelles]

This is my understanding.

 

Fuel to the APU comes always from the left manifold side, which doesn't mean the fuel is coming from left side tank1. It depends on electrical fuel pump position.

When the center electrical pumps are ON, they provide higher pressure flow than wing pumps. This why the engines burns fuel from center tanks first when all 6 pumps are ON and fuel is available on the center tanks. Center pumps win and feed the manifold.

This applies for the APU as well, having center tank pump ON they will feed left side manifold, thus engine and apu will get fuel from center tank not from the wing tank.

 

note: if no electrical fuel pump is ON, then yes, the APU will always suction fuel from tank1 via its APU bypass valve.

 

Summary for the classic (left side):

No pumps turned on at all. Engine and APU will suction fuel from the left tank via their corresponding BYPASS valves

Left pumps on. Engine and APU will receive fuel from left manifold being feed by the electrical pumps. 

Left and Center pumps on. Stronger center pump will feed left manifold. APU & Engine will get fuel from center tank.

 

 

I am also curious to get this clarified

 

 

 

[balaton]

Mmerelles, that's a good and perfect summary of the fuel pumps logic.

Just too small addititions:

If you turn on the Left CTR pump only, it is the only source that pressurizes the left side of the fuel manifold (no need to switch on the LH main pump). Therefore, the APU will draw fuel from the ctr tank.

To be a perfectionist,: if you open the the crossfeed valve and turn on the right CTR pump, the whole manifold will be pressurized and voila, the APU will use ctr tank fuel (follow the scheme drawing by Morten.)

I like your professionalism, cheers,

 

Tamas

P.s sorry for my typos it is late night here in Europe

 

 

[Morten]

BTW Tamas, isn't there a restriction on use of the CTR pumps on the ground?  Has something to do with risk increased of tank explosions due to temp rise is the CTR tank if the tank runs dry. This happened to a Thai 737.  So maybe different policy's also on this around...

[balaton]

Ye, you are right Morten! There is some restriction on the usage of the CTR pump on ground. But this is only a procedural restriction. Technically it possible to feed the APU from the CTR tank.

Here is an extract for you from the Boeing FCOM concerning CTR pump usage:

"If extended APU operation is needed on the ground, position an AC operated fuel pump ON. If fuel is loaded in the center tank, position the left center tank fuel pump switch ON to prevent a fuel imbalance before takeoff.

CAUTION: Center tank fuel pump switches should be positioned ON only if the fuel quantity in the center tank exceeds 453 kgs.

CAUTION: Do not operate the center tank fuel pumps with the flight deck unattended."

Cheers,

 

Tamas

 

[AngelOfAttack]

I noticed in 1.5.1 APU still only draw fuel from No1 rather than CTR tank. My condition is on the ground with engines off and APU on, ELEC established on GPU, only center L PUMP on and all other pumps were off, and center tank have enough fuel, about 1600kg+.

[Litjan]

You are correct, we have not changed that logic yet.

Edited October 14, 2023 by Litjan