757-200 APU fuel feed
 

Can anyone tell me if it is possible to feed to APU from the R/H tank to maintain a balance between mains during prolonged APU operation, preventing a fuel transfer later on.
I believe if you pull 115vac pump cb the dc pump will take over?

Cheers

Not sure about from the R Tank, but if you open the refuel panel, it can be run from the C Tank (the pump inhibit is removed with the refuel panel open).

We do day stops over the winter in Scandinavia and the APU stays on with an engineer onboard to stop the a/c freezing over and this is what he does to prevent imbalance!!

It's has been awhile but as I recall just opening the crossfeed and turning on one boost pump in high tank side would cause the balancing to occur. Single crossfeed configuration on the aircraft in question. Try it and see if it does not work for you. The fueling door also works as mentioned by a previous poster.

due to some of the areas we travel the unavailabilty of suitable servicing stands prevents us from accessing the refuel panel, this being the reason we can't easily do a ground transfer.
Cheers

To get fuel from right tank, turn off left tank boost pumps and open cross feed. Right system will then supply fuel to both motors and APU. The only time the DC pump fires up is when the DC pump presssure switch senses no fuel ( less than 3or4 psi) pressure in the left engine fuel feed manifold. Pulling CB,s on the left boost pumps, all three, will allow the DC pump to operate as long as the cross feed valve remains closed. If my memory serves the APU uses around 150-200 kg per hour.
Hope this helps

This all gets me confused... I tought it is impossible to run the APU without left ac or dc pumps! Now what Lightning 5 tells is definitely not true, the left AC boost pump WILL operate even with the switch in off.

With the airplane on the ground and the APU control selector ON the L FWD boost pump starts. If the boost pump generates sufficient pressure, the DC fuel pump remains off. The L FWD boost pump remains on regardless of its switch position as long as there is 115 vac power, the APU control selector is ON, and the boost pump is not faulty. Pulling the 115 vac circuit breaker for the L FWD boost pump shuts the pump off and starts the DC pump in the left fuel system.

Now, since the APU controller gets a pump pressure signal from the left AC and DC pump, it will never open the APU shutoff valve when both pumps stay off (and you'll get a message DC Pump).

With refuelling door open, you can get the center pumps to run indeed, but the APU will still not run I believe...

This is also what I read in AMM, so can anyone confirm that the AMM could be wrong here?

Piper 19

I understood that the original question related to a flight condition. You are correct in stating that the left forward boost pump will run anytime AC power is on the aircraft, irrespective of the the switch position, but how do you start the APu when no AC power is available. Dc pump kicks in as the left feed manifold is not pressurised until APU is a 95% and on line or ground power is applied.The AC and DC pump messages will be unavailable until AC is on the aircraft and EICAS is powered. In respect to centre overide pumps, you are correct when you say that they will run when the P28 panel is open (assuming that the pumps are switched on) or the engines have reached 50% n3, this signal coming from the speed cards.
Apologies if I have confused you.

no problem:ok:
however, I still don't see a way to feed the APU from right tank only. (on grnd)

Piper 19
Apologies. age is creeping in and brain failure! Should really have read the Q. You are correct, you cannot get the apu to use fuel from the right tank unless you transfer fuel into the left tank. APU gets its fuel from the left tank only. Into the dark room now!:ugh:

What about a CB for the DC pump.does it exist?
regds
MEL.

Can't remember the 757 specifics but the APU uses the dc pump if there is low pressure ie a 'cold start from a dead aeroplane' or, if there is main busses powered it uses the left aft boost pump.
If you select the right aft boost pump ON, open the crossfeed valve and trip the breaker for the left aft pump you will now be using fuel from the right main.:8

due to some of the areas we travel the unavailabilty of suitable servicing stands prevents us from accessing the refuel panel, this being the reason we can't easily do a ground transfer.
:eek:

Who is it I don't want to fly with?

Gas path is correct.

The APU fuel line picks up fuel from the engine fuel feed line, not the left tank. The APU DC pump picks up fuel from the left tank and outputs to the engine fuel feed line. Any pump or tank could be used to feed the APU providing the crossfeed is open. The key point is to pull the circuit breaker for the Left Aft Boost pump especially if the left tank is empty as it can't be controlled from its own switch when the APU is running.

http://i658.photobucket.com/albums/u...FeedMedium.jpg

That's it in a nutshell.
It's possible, but why would you want to do it?! If you fed the APU from the Right main tank, you would only start an imbalance from that tank now, instead of the left!
Running fuel from the CTR tank would be the most logical solution if you were planning a protracted period of APU operation.

Lightning5
Ummm...either I misunderstand your point/question, or I don't think you meant to ask that did you...?! Batteries...perhaps?!!

:ok:

Cheers

The original question was because the operator, who I suspect is the NZ Air Force sits on the tarmac with the APU running for hours on end. My experince is that the 757 APU will burn something slightly less than 200 pph with all the packs and other elecrical draws up and running while waiting for Mr. Big Shot to arrive at the airplane. If no corrective action is taken for several hours a imbalance of five or six hundred pounds will occur and even though within limits, I suspsect the crew will want to balance it out as a matter of good operatng practices.

reply to the posts above, I'm still not convinced. For me it is impossible to use right tank, even with crossfeed open and right pumps running.
Only looking at the feed pipes you can indeed say you can use right tank through crossfeed, but it's the electric control that does not allow to use right tank.
If the left AC pump and DC pump send a signal "I'm not running" , the APU will not start, even with crossfeed open and right pumps on.

1/ left ac pump running = apu gets fuel from left tank
2/ left ac pump off due to no AC power= DC pump takes over = apu gets fuel from left tank
3/ left ac pump and DC pump not running = APU controller doesnt allow fuel to go to the APU, shutoff vlve stays closed. If you would try to start the APU here, the only action you get is an EICAS message.


otherwise, the Boeing schematics we have are wrong. Pity no a/c here to test it.

The APU does not monitor ac or dc pump for starting purposes. The APU SOV is motor operated does not require fuel pressure to open. It opens when the APU start switch is selected to ON.
The APU DC pump only runs if the Fuel System press sw says low press. There is no need to pull the DC pump C/B. (The APU dc pump press sw. only turns on a fault msg).
When the APU is first started, the DC pump will run. When AC power is supplied, the right boost pump will take over. The Fuel System pressure switch will shutoff the DC pump.
If the AC power is already being supplied, and the right boost pump is on and supplying pressure, the DC pump will not run.
I can't be sure of this but if the APU is started and the AC and DC pumps do not operate, the APU fuel pump may be able to suction enough fuel to start.

Who is it I don't want to fly with?


military so no worries there mate

So I take it as long as you pull the L/H CB's and start the centre or R/H pumps when ac power is supplied you will supply fuel to the APU?

Correct!
Dead a/c. Start the apu ...the D.C. pump runs.
A.C. power ON the l/h aft BP runs and the D.C. pump switches off.
To run the APU from the r/h main. Switch ON the r/h aft BP. OPEN the crossfeed valve(s) then trip the l/h aft BP circuit breaker.
Don't forget to log what you've done regarding tripping of CB's! :8
Also correct....up to a point! If you are lucky enough you may just get AC power on in time, otherwise it's look for suitable access to get up there to bleed the FMU :ugh:
That certainly applies to some other Boeings!