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.

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.

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

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...

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.

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?

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.

Jaxon - you mean for certification purpose, center tank pump cannot be used?!

Jaxon
 

Really?

The B757 uses centre tank fuel on take-off and is certified worldwide.

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


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.

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

Its a single tank/source if contamination is involved.... better to reduce the chance of simultaneous failures.

why not on A321
 

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

lets find a better one:ugh:

Or perhaps a better differences training. :suspect:

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)

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.

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..

I seem to have come across a little too harsh, apologies IfIg.

http://img372.imageshack.us/img372/2...fuelaa0.th.gif

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.

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.

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 !

:ok: That's what I think,
captain87

Yes the 757/767 does takeoff with center fuel pumps on with some exceptions. Naturally the first one is if the fuel in the tanks is less than 1000 lbs you can't run them anyway. The fuel system will scavenge out that fuel. You must have at least 5,000 lbs in the center tank to run them on takeoff. If you have less than 5,000 you'll have less than 1,000 in the tanks before reaching 10,000 which is a distractor in that portion of flight. So what we do is leave them off and once we get to a less busy time we'll turn on the center pumps and burn down to 1,000.

captain87 - That's not the reason. The Fuel recirculation to IDGs is automatically inhibited during take off & climb. FCOM 1, 1.70.40, page 6.

flyr767 - Yes, agreed. The exception is for obvious reason that it does not alert the pilots unnecessary during high workload period and it is not for the reason of contamination.

The op wasn't about B75/76 but one of the reasons you can use centre tank fuel on these aircraft is that all the fuel pumps are operating all the time. The centre tank pumps operate at a higher pressure than the wing tank pumps and thus the aircraft uses centre tank fuel first. If the centre tank pumps fail, the wing tank pumps are already operating and can provide a continuas flow in a very high power and critical situation, without the engine being starved by low pressure. This is not the case in the A320. In this aircraft a low fuel pressure would have to be sensed before the aircraft would switch to wing tank fuel.

Nope, Jonty, that's not the case in A320. A320 center tank fuel operation logic is the same with Boeing aircrafts (737/757/767/777).

"All fuel pumps (wing pumps & center tank pumps) remain on throughtout the flight. They are fitted with pressure relief sequence valves which ensure that, when all pumps are running, the center tank pumps will deliver fuel preferentially"
FCOM 1.28.10, page 2.

Wing Tanks Pump PushButton - On
"Pump is on but fuel feeds only when center tank pumps delivery pressure drops below threshold"
FCOM 1.28.20 page 1.

Learn something new every day, thanks for that.

You are most welcome. It's all about learning and it is what this forum is for.

Very good discussion but just wonder, where's the Airbus Engineer?

My thought on this subject:

Obviously half of the sky (Boeing) does take off on center tank fuel.

To my knowledge this has never caused any accident.

But it is true that in theory there is a risk of contamination going to both engines at a critical time.

Flight Detent has explained very well why this does not apply to the 321.

I think the real reason is Airbus thinks they know better than Boeing, and must make everything better than its competitor.

That's the simple reason why, being their airplanes more sophisticated than Boeing (at least in their mind...), they excluded the remote possibility of this contamination problem with a prohibition of their own.

LEM

Perhaps they require two pumps feeding each engine for takeoff.

Not true!

The QOTS takes off using tank to engine. Once flaps 5 (iirc) achieved pre-departure, the crossfeed valves to the fuel gallery from the centre tank are automatically closed, the providing tank to engine for take off until a suitable period of time afterwards.

Once they are opened again,m the fuel feeds from the centre tank (and the stab tank feeds the ctr tank) until empty, at whihc point the inners feed all engines untl mains 1-4 are equal at which point it is tank to engine again (when at about 52 tonnes remaining).

Che, this point has already been discussed, as all pumps are pumping together, the pressure is not the issue.

Top Bunk, forgive my ignorance, but what is QOTS? 747 maybe ??

LEM

QOTS = Queen of rhe Skies = B747:ok:

Aikon the A320 Centre Tank pumps run for 2 mins after engine start and then, as Capt87 says, shut off till slats are retracted
Ref 1.28.10 CT Pumps Control logic diagram
regards

Aikon
A320 - Clarification - IDG cooling is continuous, the fuel return to tank is inhibited during takeoff and climb, when it does start there has to be room in the outer or inner wing tank. Hence the wing tank fuel is used until the 500 kg. underfull sensor operates, then the CTR pumps run again.
See FCOM 1.28.10 p.6

it says the fuel return valve is controlled by the fadec but doesn't specify that its inhibited during T/O and climb.

where can i find these facts.. :)

Hi guys,

I almost forgot about this until you guys brought it up again.

Anyway, this is the answer I got from Airbus. There's a requirement for certification that the engines are not fed by the same fuel tank. A321 meet this requirement because center tank fuel is transferred to the wing tanks first, then from wing tanks, fuel is supplied to both engines. But for A320, center tank pumps will have to be off until take off is completed to meet the requirement.

I do not know how Boeing managed to "circumvent" this requirement but probably like LEM said, Airbus always think they planes are better than Boeing thus their requirement are more stringent.

If so, all 737's (at least) are illegal.

So it can't be a legal requirement, I think just french perfectionism...


Btw, I'd be happy if they applied the same perfectionism to warm my feet on those lousy (when it comes to warming feet :mad:) cockpits!

LEM

LEM,

Like I said, I do not know how Boeing managed to circumvent the requirement.
It maybe just french "perfectionism", but those are the answers I got back from Airbus.

Yeah that's what they told me too...

Well, to be more precise, what my instructor told me...

A wannabee who thought none could take off on center tank.

When I told him that all 737's do that, he didn't know what to reply :ugh:

CTR tank feeding
 

Hello Mr. Aikon

CTR tank feeding during T/O is inhibited because our centre tank is equipped with two pumps and because of which only one pump supplies the fuel to its respective engine. In case of failure there is no backup for fuel feeding to that engine. And we are very well aware that T/O is a critical phase.