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A320 Center Tank Fuel

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A320 Center Tank Fuel

Old 24th Jun 2008, 17:28
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A320 Center Tank Fuel

Hi guys,

I am new on the A320. Am wondering why is it prohibited to use the center tank fuel on take off? (FCOM 3, 3.01.28, page 2, operating limitations)

I know the center tank pumps will automatically be off (if under auto mode) when slats are extended & 2 mins of engines running, but I am trying to understand the logic behind this.

Thank you.
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Old 24th Jun 2008, 17:47
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I understand that in the event of total electrical failure, gravity feed is only provided by the wing tanks. So we don't want to starve the engines on takeoff or go around. But I'm not sure if this is THE REASON why we don't use the center tanks on those critical phases.
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Old 24th Jun 2008, 17:48
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Hi there - I believe the simple answer is that the manufacturer does not want one single source of fuel supplying the two engines at such a critical time of the flight (takeoff). Most manufacturers prefer a dedicated tank to engine arrangement.

OP
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Old 24th Jun 2008, 18:40
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Recirc

And what about the fuel recirculation system for IDG cooling...? If the wing tanks are full, they cannot stow the fuel which comes back from the heat exchanger... Just my thoughts...

Last edited by FLX/MCT; 25th Jun 2008 at 07:07.
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Old 24th Jun 2008, 18:48
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Thumbs up

Gravity feeding from the center tank is not possible (no by-pass valve fitted on the centre tank pumps). So if there is a failure in the climb out on centre feeding there is no alternate route
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Old 25th Jun 2008, 12:36
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matiasbalacco - yes, suction feed is only available from the wing tanks, so in the event of total electric failure, you will not have the wing tank fuel pumps running , ie fuel will only be suction fed from wing tanks. It makes no difference whether the fuel is coming from center tank or wing tank before it fails. So I don't think this is the reason.

Office Pest - It's not single source. The wing tank fuel pumps are running at the same time with the center tank fuel pumps, only at lower pressure. If center tank pumps fail, wing tank pumps automatically take over to supply fuel to the engines.

ASFKAP - Not sure about this. But it doesn't seems to be a valid reason.

FLX/MCT - Fuel recirculation for IDG cooling will be automatically stopped on take off and climb (FCOM1, 1.70.40, page 6 -CFM eng. only). So this is not the reason.

alwaysontime - Not true. Look at the reply to Office Pest.
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Old 25th Jun 2008, 14:58
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I believe it got something to do with the center tank electric fuel pumps as this limitation does not apply to A321 which uses the "newer" jet pumps instead of electric pumps.

Anyone?
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Old 25th Jun 2008, 15:24
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Dang, I thought for sure this would have been tackled well and quickly enough.... but I guess age and experience do have their uses.

CERTIFICATION

You can't get the aircraft type certified using the centertank fuel in that way, that's a generic regulatory NO NO.
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Old 25th Jun 2008, 17:00
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Jaxon - you mean for certification purpose, center tank pump cannot be used?!
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Old 25th Jun 2008, 17:14
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Jaxon

CERTIFICATION

You can't get the aircraft type certified using the centertank fuel in that way, that's a generic regulatory NO NO.
Really?

The B757 uses centre tank fuel on take-off and is certified worldwide.
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Old 26th Jun 2008, 10:41
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Hi there - I believe the simple answer is that the manufacturer does not want one single source of fuel supplying the two engines at such a critical time of the flight (takeoff). Most manufacturers prefer a dedicated tank to engine arrangement.

OP
Don`t know the answer either, but makes sense...


CERTIFICATION

You can't get the aircraft type certified using the centertank fuel in that way, that's a generic regulatory NO NO.
I believe you mean, that for certification you can`t have an engine running on one pump only. That`s what happens on the A320 if using the center tank. But also don`t know if that`s the reason, because if center tank pump fails, the wing tanks would feed the engine, since the 2 wing pumps are always operating. And there would be no interruption to flow, beacause its a question of fuel flow pressure. When center tank pump flow pressure drops bellow the wing tank pressure, you`ll have wing tank jet flowing down the pipe.
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Old 26th Jun 2008, 11:00
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Aikon the point I was (obviously not very well) trying to make is that Airbus thinks it is safer to have a dedicated tank to engine arrangement as that would mean a totally seperate source of fuel for each engine. Obviously the centre tank IS one single source of fuel. I am well aware of the logic regarding the fuel pumps and the sequence valves in the fuel system thanks.

Best Regards
OP
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Old 26th Jun 2008, 12:09
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Its a single tank/source if contamination is involved.... better to reduce the chance of simultaneous failures.
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Old 29th Jun 2008, 13:58
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why not on A321

Hi goeasy, why not use your logic on A321 also ? not so convincing...

lets find a better one
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Old 2nd Jul 2008, 10:04
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Originally Posted by IFLY_INDIGO View Post
lets find a better one
Or perhaps a better differences training.

A321 is not equipped with any CTR tank PUMPs as such, only TRANSFER VALVES. Under ceratin logic they open to enable fuel transfer from the CTR TK to the WING TK. The energy to do so is provided by means of WING TK fuel flow through converging / diverging nozzle (aka jet pump), which by means of suction uplifts CTR TK fuel into stream of and these two combined are then reintroduced into the WING TK.

Picture worth a thousand words is to be found at FCOM 1.28.10 p3.

FD (the un-real)
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Old 2nd Jul 2008, 11:59
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FD - I think what IFLY-Indigo meant was if the reason is to reduce the chances of simultaneous failures in case of fuel contamination, then this logic does not valid in A321 cos the fuel will go into both wing tanks, contaminate the fuel in both wing tanks and then subsequently causing simultaneous engines failure.
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Old 3rd Jul 2008, 04:21
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thats what i meant...

thanks AIKON .. exactly thats what i meant..

by the way flightdetent, I don't know the system differences of A321 at all cause I don't fly it.. I fly A320 only..
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Old 4th Jul 2008, 08:47
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I seem to have come across a little too harsh, apologies IfIg.



The problem being solved by feeding segregation is not the one of fuel contamination.

If you uplift fuel contaminated beyond what is acceptable for the engine to run, you wouldn't start up or alternatively, you have mud in all your tanks and are doomed anyway.

What is avoided is CTR TK contamination in most critical phase of flight. If your CTR TK is carrying problems, (water, other contaminants, low level) you do not want these to be propagated simultaneously into the engines during takeoff. Solved on A320. Yet, sooner or later the CTR TK fuel is introduced into the system anyhow, just not for takeoff.

On the 321 in order to have fuel in CTR, the wing tanks must also full to begin with. Given the pipe connection, the same result is achieved as for A320 without a need to separate the CTR tank. Because only small stream of CTR TK fuel is uplifted at the jet pump into a stream of WING TK fuel, which is then together reintroduced into the approx. 6 tonnes that are now carried at the fuel WING TK and being pumped up to the engines. So the rate of WING TK fuel decay due to CTR TK fuel problem is very small.

True, after some time the mixture in WING TK will reach highes content of CTR TK fuel, approx 50:50 but this will come only after 3 t are consumed by the engine so that 3 t form CTR TK found their way to each onside WING TK. About 2 hours into a flight, I woud suppose, you are running on the richest CTR TK fuel mixture at cruise level.

Compare this to the A320, when after slat retraction (3 minutes, 5000 ft AGL) you are running on 100% CTR TK.

Now, thinking about the take-off part, my guess is that the rate of decay due to CTR TK fuel put up into WING TK is geniously negligable. In fact, is by desing no grater than the fuel flow. Even at TKOF THR FF (which I strongly doubt jet pumps are able to compensate by uplifting CTR TK fuel into WING TK) of let's say 4000 kg/h per engine, in all five minutes only 300 kgs of CTR TK fuel will be transferred in the 6 tonnes originally carried.

FD.
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Old 4th Jul 2008, 13:02
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Thank you Flight Detent for your detail explanation. This seems to make good sense.

However, if we are looking from a Boeing aircraft point of view, then this does not apply. Boeing aircrafts 737/757/767/777 all have center tank with center tank pumps that pump fuel directly to engines, but none is required to switch off the center tank pumps on take off. If contamination problem is so serious, I am sure Boeing would had think of it, isn't it?

Flight Detent, please don't take this the wrong way. I am in no way to dispute your explanation as I myself do not know the answer. What we want is a "healthy" discussion and hopefully can come up with the correct answer.

Anyone has any more input? Thanks.
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Old 4th Jul 2008, 14:57
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Wink re from captain87

Hi all,

CTR tank cannot be used for takeoff simply because fuel coming from WING TKS is partially used to cool IDG's down, then it flows back to the wing tank having the cycle repeated. If we were to use CTR TK during takeoff (inhibited by CONF 1 slats/flaps selection), having both the wing tanks full, the fuel flowing back from IDG's would find no place to be "restored" into the respective wing tank ... So Airbus logic in this phase, automatically inhibits CTR TK and allows WNG TKS to automatically run in order to let place for IDG's fuel.
This automatic process planned by Airbus engineers does not allow us to use CTR TK because it would be automatically excluded again !

That's what I think,
captain87
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