View Full Version : A320 Center Tank Fuel
Aikon
24th Jun 2008, 16:28
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.
LA931
24th Jun 2008, 16:47
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.
Office Pest
24th Jun 2008, 16:48
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
FLX/MCT
24th Jun 2008, 17:40
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...
alwaysontime
24th Jun 2008, 17:48
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:ok:
Aikon
25th Jun 2008, 11:36
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.
Aikon
25th Jun 2008, 13:58
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?
Jaxon
25th Jun 2008, 14:24
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.
Aikon
25th Jun 2008, 16:00
Jaxon - you mean for certification purpose, center tank pump cannot be used?!
Starbear
25th Jun 2008, 16:14
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.
TO MEMO
26th Jun 2008, 09:41
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.
Office Pest
26th Jun 2008, 10:00
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
goeasy
26th Jun 2008, 11:09
Its a single tank/source if contamination is involved.... better to reduce the chance of simultaneous failures.
IFLY_INDIGO
29th Jun 2008, 12:58
Hi goeasy, why not use your logic on A321 also ? not so convincing...
lets find a better one:ugh:
FlightDetent
2nd Jul 2008, 09:04
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.
IFLY_INDIGO
3rd Jul 2008, 03:21
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..
FlightDetent
4th Jul 2008, 07:47
I seem to have come across a little too harsh, apologies IfIg.
http://img372.imageshack.us/img372/2893/a321fuelaa0.th.gif (http://img372.imageshack.us/my.php?image=a321fuelaa0.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.
captain87
4th Jul 2008, 13:57
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
flyr767
4th Jul 2008, 21:10
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
Che Guevara
3rd Feb 2010, 18:35
Perhaps they require two pumps feeding each engine for takeoff.
TopBunk
3rd Feb 2010, 18:39
Obviously half of the sky (Boeing) does take off on center tank fuel.
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 ??
TopBunk
3rd Feb 2010, 19:05
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
TyroPicard
4th Feb 2010, 09:19
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
MD83FO
4th Feb 2010, 14:50
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.
There's a requirement for certification that the engines are not fed by the same fuel tank.
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:
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.
Boyington
8th Apr 2015, 12:31
I think regulations require that for takeoff the engines must be supplied from a tank with two pumps.
Volume
8th Apr 2015, 13:52
...more a structures guy than a fuel system specialist...
But for Airbus center fuel tank means center wing box (inside the fuselage), for Boeing the center fuel tank extends to outboard of the landing gear, hence it is significantly larger and closer to the engines. For the 757 the center fuel tank has 1.5 times the capacity of both outer tanks together, for the A320 it is roughly 1/4 (!)
So the same name does not mean the same thing.
Boyington
9th Apr 2015, 11:58
I think the regulations require that during takeoff the engines must be supplied from tanks with two pumps.
DogFather
12th May 2015, 13:04
I think the answer can be found in this link. FAR 23.955 - Fuel Flow
Scroll down to Section 23.955 (f), Turbine engine fuel systems
http://www.risingup.com/fars/info/part23-955-FAR.shtml
vilas
12th May 2015, 14:58
It is forbidden to take off with centre tank fuel. Because in case of contamination of centre tank fuel being single source both engines may flame out.
AeroTech
21st May 2015, 01:04
Hi,
If the engines are not fed by the same fuel tank is a requirement for certification (according to Airbus reply), it will be nice if someone can post this requirement/regulation (EASA or FAA).
a) If there is such requirement for certification why Boeing is not complying with this regulation?
I think the answer can be found in this link. FAR 23.955 - Fuel Flow
Scroll down to Section 23.955 (f), Turbine engine fuel systems
I don't think this link provide an answer to the question. In addition I think Part 23 does not apply to A320. Part 25 applies to A320.
b) I am wondering if engines are not fed by the same tank during take off in other Airbus aircraft.
Feedback appreciated.
The bird
22nd May 2015, 13:54
I noticed that the new A320's in our company got the same fuel system as the A321 so with the transfer valve for the center tank. What is the whole idea behind this system? I know we dont need the center tank pumps anymore and fuel is being fed to the inner tanks by suction but why? Saves a few bucks and weight but I cant imagine that it is the only reason. It is also not very clear if this fuel can be used i.c.o. gravity feeding. Maybe someone that knows this system can clarify these things. Thanks guys
Goldenrivett
22nd May 2015, 14:59
Hi The bird,
It is also not very clear if this fuel can be used i.c.o. gravity feeding.
Gravity feeding simply needs the fuel to run "downhill" to the engine. The engines hang below the wing, so gravity feeding is possible from the wing tanks.
The fuel level in the centre tank will mostly be below the engines and below the fuel inlet from the wing tank, therefore gravity feeding is impossible.
Have you ever tried to syphon water from an aquarium or fuel from a tank?
HeyRogerWilco
28th Mar 2017, 11:06
It's a 14CFR requirement. Each engine must have a unique fuel source for take off. 14 CFR part 23 (I think).
ToniLu88
15th Jun 2017, 14:12
I thought Fuel Feeding from Wing Tanks during T/O was to empty them to make Fuel returning from the IDG Cooling System not go overboard.
FlightDetent
15th Jun 2017, 20:24
That makes absolute sense, the explanation I was given was that it is a regulatory requirement. What you say is an issue any stage of flight, and the central pump tank logic takes care of that.
Of course, takoff with CTR TK fuel and wings not full would be a non-standard fuel loading arrangement, but still possible. That's why the additional logic of no CTR Pump operation with slats out? Sounds good to me.
stilton
17th Jun 2017, 05:22
Don't know about Airbus but you'd have significant ZFW restrictions with that configuration
on a Boeing.