B738 fuel burn

[B777Heavy]

Hi, noticed this for quite abit now, as you are trodding along with fuel in your center tanks, the actual fuel burn and what you should have burnt is pretty much at par....but as soon as center tank is all gone, the fuel burn equilibrium with the computer flight plan is upset and we end up losing quite abit of fuel in comparison with what we should have...Not sure why this would be......:confused:Perhaps CG related?

[BOAC]

My initial reaction would be to check your c tank gauging - do your uplifts cross-check with tank quantities? GofG would make very little difference. What isquite abit of fuel? Are your scavenge pumps working?

[B777Heavy]

BOAC, not sure how u guys cross-check planned burn to actual burn throughout the flight(I'm sure it's pretty standard). Here is the scenario, on a 5hour flight, we were not really losing any fuel as compared to planned burn off at different waypoints when we had fuel in the center, in fact we had a gain, right up to the point where center fuel was depleted...or as the flight progressed, nothing changed dramatically, winds were pretty spot on, sats were give or take....yep scavenge wouldn't be the question here as the center tank was dry on the B738 at this point.
We climbed when the computer flight plan did...(just to figure out why we would lose fuel on these 5hr+ sectors)
Eventually just before landing we were at a dismal -800kgs when compared to the planned. Can't really explain why it would be so....

Also noticed it doesn't just happen on one airplane happens on pretty much all or B738s

[BOAC]

Whoa! Too many variables in there. What about flight times? On PLOG or greater? Were your 'climbs' to optimum levels or max? CI? Actual speeds? Time spent at non-optimum altitudes/speeds - eg arrival routing/speed control, min drag approaches etc etc.

What about your uplift cross-checks?

[B777Heavy]

Climb to optimum, yep min drag CDA carried out. On CI 30. Approach really wasn't the issue, noticed actual v planned was off enroute. However standard speeds, no speed till 250/FL100.

PLOG??

Uplift, we had 100kgs more gas than requested as per flt plan...on rotation.

[BOAC]

Flight time and uplift cross-check?

[majuro]

B777Heavy

I know exactly what you mean and have been pondering this for a while. During the climb till not long after, fuel used plus fuel remaining is a couple hundred more kg than that departed with. I am guessing that this is the result of a guage overread during the climb. This does resolve itself not long after top of climb with fuel used plus fuel remaining equaling the departure fuel less any small divergence due to APU burn (if it is running) UNTILL that last 1000kg of centre tank fuel is used. Once the centre tank is depleted, the sum of fuel and that remaining is a figure 2-300kg shy of what you departed with. This relationship continues right through to shutdown.

If you land with say 9000kg and haven't touched that last 1000 kg or so of centretank fuel this apparent loss of fuel does not occur.

I have seen some documentation from Boeing advising that the reason is the combination of fuel flow transmitter inaccuracy, FQIS reading high in full tank conditions and low at lower fuel levels and fuel remaining in the centretank when pumps are turned off. Apparantly it is all within limits!!!????

If you load on an extra couple of hundred extra kilos of fuel for flights that arrive with the centre tank depleted you seem to arrive with what you should.

B777, that 800kg could well be a good 300kg from what I describe above plus 5 hours of APU burn (say 250kg) plus a bit of rounding at each end.

cheers

[Immigrant]

In our airline all 700 and 800 - the same issue....500 lb lost as soon as CENRT burned off, we fly 7+ hr sectors

My Instr. told that Boeing acknowledged this problem and working on it

this might help...... :ok:

http://www.pprune.org/tech-log/238854-737ng-ctr-tank-density-error.html

[misd-agin]

It's been my experience in Boeing a/c (14,000 hrs) that the fuel total is slightly low in the climb and shortly after level off. After level off it typically adjusts closer to actual fuel on board.

Eg - close to top of climb today fuel was already -700 lbs. After approx. 20 minutes at cruise we were down to -300 lbs. There's no way we were burning 1200 lbs less per hour which is what that fuel change would require.

No observed, repeatable, issues with fuel totals related to center tank fuel use on 737-800's(700+ hrs).

[BOAC]

Thanks 'immigrant' for digging up that 'old' thread - as I said in #2 here it looks like a tank gauging error and SHOULD be sortable by an uplift check which should be pretty standard?

[captjns]

Flown with carriers without aircraft biasing. Makes a difference on long flights when the carrier takes aircraft specific tails performance into consideration.

[KristianNorway]

Hey guys

We have the exact same thing on our fleet. I've been asking around about this, and it seems to be fairly common, but if you get an answer please let me know.

We operate 738's and employ fuel bias calculations.

Often you loose about 500 kgs when center tank is depleted, but approaching the end of the sector (for us everything from 3 to 6 hrs) it rises again to a difference of appr. 2-300 kgs.

I see from CaptainSandL's webpage that the 738 has a accuracy of about 2% of full capacity without densiometer which puts us right there (13066 x 0.02 = 261 kgs).

I've heard a theory that without the densiometers it measures all the fuel from an average value of capacitance between all the three tanks. When you then deplete the center tank, the average specific gravity is higher than the SG for the cold main tanks, and it underreads.

I don't have access to any maintenance manual, so I have no idea, and no opportunity to check this I'm afraid. There is little to deduct from our companys OM's.

I hope some clever heads here figure it out.


Cheers :ok: