Wing tank pump.. ON or OFF??

[Silky]

Here is a one that gets all our engineers going against our pilots.

We shut down the wing tank pumps and they want one on the left wing on to feed the APU. Reason being the apu pump is a DC pump and therefore not as powerful and tends to fail.
Correct me if I'm wrong but that pump only runs when there is insufficent pressure and due to the weight of the fuel in the wing I was always under the impression that it gravity feeds.
:) :)


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Gravity always wins....

[TwoTun]

Silky:
youi said:
<<Correct me if I'm wrong but that pump only runs when there is insufficent pressure and due to the weight of the fuel in the wing I was always under the impression that it gravity feeds.>>

depends on
a) your aircraft type for when that DC pump runs or not, and

b) wether the wing is higher than the APU or vice versa....

regards

[This message has been edited by TwoTun (edited 15 October 2000).]

[Silky]

Should have said...A320 and no the APU is higher than the wing but tell does that prevent gravity feeding.....not

[HugMonster]

If your fuel is lighter than air, having the wing lower than the APU will not prevent gravity feeding.

Otherwise, if you're using Jet A1 like most of the rest of us, you'll find it very difficult for it to run uphill unaided.

[Lucky Strike]

A320;

Has it's own fuel pump and takes fuel from the left inner wing tank. There was an operational engineering bulletin (OEB) a few years back that instucted the use of a wing tank pump to ensure APU supply, but this was deleted/obsolete some time ago. Hangers on to the old way of doing things may persist.

But, maybe the engineers know something from line experience pilots don't

[Brenoch]

Dunno anything about the 320.. On the 75 the L fwd AC pump runs all the time when the APU is running..

[MasterGreen]

Silky my little Muppet. Jet A1 is a bit heavier than air and doesn't run uphill at all well, even in a pipe. So you need a pump of some description. Now the APU has a fuel pump as well and this can "suck" a bit - but not a lot. Certainly not enough to get fuel all the way from the wing tank to the tail (even in a dinky little A320). It needs a positive head pressure to function (cavitation is the word you might be searching for here).

So there is a DC pump for when there is only DC power - battery start for instance. However DC pumps are much more prone to problems (brushes / controllers etc) than an AC pump. Some A/Craft have an automatic selection here, but maybe the A320 doesn't. If the Engineers think it's a "Good Idea" to have the AC pump on - let 'em. They know more about the systems than pilots ever will and besides that - once you have shut it down - it's their aircraft. And further more they have to change the pumps as and when they fail.


MG.

[Boxer]

MG my man...
in defence of good ol Silky for raising the subject, I too have seen great variation in the application of said same proc. As for our "Little 320" well that explains the tone of your response and just to throw a few facts into the mix. Tha DC pump is a suction pump as you rightly stated and is submerged in the fule line, it ONLY operates when insufficent pressure is sensed. It is possible form my days of not paying attention in physics class that if sufficent weight is placed on a fluid in a line/pipe it will travel up hill..... now I am not saying that it is sufficent to supply the APU full time but maybe intermitently (large amounts of fuel in tank). I think that was what good ol Silky was refering to. If I'm wrong I'm sure the likes of you will let me know.
The other reason I have been so well informed by my techy friends is that it is a hell of a job to change this pump.. :) :)

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Dont forget you wont be leaving empty handed!

[WenWe]

A320 DC APU fuel pump will run at any time the APU is switched on & the wing boost pumps (AC) are not supplying pressure.
The APU will run without either, but a low fuel pressure warning will be generated. I don't know if it would be able to supply all elec/pneu loads in this condition & there might be starting probs.

[ManaAdaSystem]

"Whenever the APU is operating and AC electrical power is on the airplane busses, extended service life of the APU fuel control unit can be realized by operating at least one fuel boost pump to supply fuel under pressure to the APU."

Straight from the book guys, 737, but presumably it applies to the Airbus APU as well.

[near enuf is good enuf]

75 dc pump used for starting the apu and once up to speed l ac pump auto switches on and the dc pump switches off.
The dc pump on a 73 is an option believe it or not and the apu does not need a pump in the tank to start.Some 73/300's I work on at present don't even have a dc pump amd quite a few of the 200's didn't either.
APU's will suction feed due to a small pump in the fuel control unit.


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So that you may not be the martyred slaves of Time,
get drunk, get drunk,
and never pause for rest!
With wine, poetry, or vitrtue,
as you choose!"

[ManaAdaSystem]

And the small pump in the fuel control unit is called the .. driven pump?

[near enuf is good enuf]

Actually, it's not called the "anything" driven pump.
It is called a high pressure fuel pump and is located in the FCU which is (75) bolted on the gear box of the APU or (73) bolted on a lube unit which is bolted on the gearbox of the APU and driven by the gear box.
Boeing have obviously decided that there is no need for a dc pump for starting as they have done away with it completly on the 73 NG.
I guess it has taken 30 odd years for them to come full circle and realise that the system that they employed on the 200 works just fine.

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So that you may not be the martyred slaves of Time,
get drunk, get drunk,
and never pause for rest!
With wine, poetry, or vitrtue,
as you choose!"

[ManaAdaSystem]

Near enuf is good enuf,

"Fuel to start and operate the APU comes from the left side of the fuel manifold when the AC fuel pumps are operating. A DC operated APU fuel boost pump is installed to ensure positive fuel pressure to the APU fuel control unit. During APU start and operation, the pump operates automatically when the APU fuel control unit senses low fuel pressure. The pump shuts off automatically when an AC fuel pump pressurizes the fuel manifold. If the AC and DC fuel pumps are not operating, fuel is suction fed from the No.1 tank."

The above is a quote from the 737 NG AOM.
I dont want to sound like a p***k, but there is no mention of another pump anywhere. Of course, this doesnt mean it doesnt excist, the APU section in the AOM covers three(3) whole pages. Its so short, you can actually read it and not fall asleep. :)

[near enuf is good enuf]

ManaAda,
You are obviously a pilot and as a pilot you are not expected to know a/c systems as well as engineers!!!!!

That is why you fly 'em and we fix 'em.

Ops manuals give a very basic overview of the a/c systems and try to keep it simple so you guys can get your heads around it. :) :) :)

Each boeing a/c have subtle differences but in general are the same. Now what you have said is more or less true but I assure you some a/c have dc pumps some don't.
What had not been covered is how the APU "suction feeds".
That is what my last post was trying to convey.Some of the earlier posts seem to believe that "suction feeding" was due to the weight of the fuel in the tanks and heads of pressure etc. Well it is not.
As I've already explained, suction feeding is caused by fuel being sucked from the left hand tank by a high pressure fuel pump, located on the FCU, which is mechanically driven by the APU gearbox.

The ops manual does not mention a lot of things purely because you don't need to know.

If it is important for you to see written confirmation ask one of you engineers to dig out the a/c schematics manual chapter 49. You will find a very pretty picture of the fuel system.





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So that you may not be the martyred slaves of Time,
get drunk, get drunk,
and never pause for rest!
With wine, poetry, or vitrtue,
as you choose!"