FMC Fuel Burn Calculation

[3-2-1-Now 126]

Hi,

 

Noticed a couple of oddities with the FMC fuel burn calculation.

 

Sat at the gate, shutdown, let's say for a given route, it is estimating 15000 lbs fuel at destination. Just because I started the engines, this immediately drops to 8000 lbs (an example - however this is an actual figure). The only thing I did was start the engines!

 

Another oddity I found is in the VNAV CRZ page. Max fuel at dest is attained through either entering M0.80 cruise, or a CI of zero for ECON speed. Selection of ECON results in a cruise of lets say, M0.844 with a CI of 400, and the same max fuel prediction at destination, as if I had changed the CI to zero. If I manually enter M0.844, the predicted fuel at destination drops significantly, even though it matches the ECON speed!

 

EDIT: as a result of my typing that, it seems that ECON is ignoring the CI for the fuel burn calc, even though it is calculating M0.844 for cruise with a CI of 400. This might explain why selection of ECON results in max fuel at dest despite the high cruise speed.

 

What is going on??? Which value(s) can be trusted for fuel planning? Note all these predictions are zero wind, but that doesn't matter when you haven't departed yet.

 

Best regards,

Robin.

[Flamin_Squirrel 71]

777 FMC fuel logic is worth learning.

 

On the ground engines off, the FMC fuel figure (and thus predicted fuel at destination) should be SENSED (and will say SENSED against fuel on the PERF page). This should match the fuel shown on the EICAS and FUEL displays.

 

When the first engine is started, FMC fuel levels are no longer sensed, they're calculated and the FMC displays CALC against the fuel on the PERF page. It does this by subtracting fuel burnt (measured by fuel flow into the engines) from the fuel sensed at engine start. If you then go to the PROGRESS page 2/4, you can easily compare TOTALIZER fuel (same as sensed, just under another name) and CALCULATED fuel.

 

This is an effective way of determining if you've got a fuel leak.

 

Anywhoo, I've noticed the same problem as you. I got around this by looking at the SENSED fuel (shown on the EICAS display) and overwriting the fuel shown on the PERF page (if you do this it'll accept the number you enter and display MANUAL next to it) but not looked at the underlying issue. It might not be updating the FMC fuel on engine start like it's supposed to. I'll investigate later.

[Flamin_Squirrel 71]

I think you're onto something. I landed with 6.8 tonnes of fuel, got to the gate then uplifted to ~54 tonnes for another flight. The PERF page showed 54 CALC, which is the correct amount, but I think it should state 54 SENSED. When I started engines, the PERF page stated ~34 CALC. I've no idea where it got that number.

 

Would others be able to test this? I think something maybe wrong.

[3-2-1-Now 126]

Interesting... I shall do a flight and see what the values do.

 

Best regards,

Robin.

[http404 22]

I think you're onto something. I landed with 6.8 tonnes of fuel, got to the gate then uplifted to ~54 tonnes for another flight. The PERF page showed 54 CALC, which is the correct amount, but I think it should state 54 SENSED. When I started engines, the PERF page stated ~34 CALC. I've no idea where it got that number.

 

Would others be able to test this? I think something maybe wrong.

 

You might have started your Engines with your fuel pumps off. This will give a large reading discrepancy between sensed and calculated fuel. And the FMC will later offer you an option to chose either Sensed or CALC fuel in case you're sure there's no leak.

[3-2-1-Now 126]

That's not the issue, and I fail to see how it could cause what we are seeing anyway???

 

I did however manage to catch the problem!! As you can see here, #2 is running, #1 is shutdown. When the system considered the engine to be running, at the exact moment it zeroized the FUEL USED for the #2 engine, it SUBTRACTED this used value from the CALCULATED value. The value of the #2 fuel used was 6.1 before it was zeroized.

 

Looking at the PROG page, the fuel at dest also dropped from an estimated 10.0 to 3.9.

 

 

Best regards,

Robin.

[hopskip 253]

Tot Used = 6.1
Are you starting off a saved panel profile that was saved after arriving at an airport where you used 6.1 tonnes of fuel to get there?

I did that once... Went back to the Short Turn state that comes with it.

 

I reccon you have that 6.1 used amount saved in there from a 'previous flight' even if that previous flight was weeks ago but saved as your new startup state. I'd also hazard a guess that you inserted the fuel into the FS Actions > Fuel directly and not through ground ops (and waiting for near on an hour for it to fuel the plane & open/close some doors)?

 

 

Open the Short Turn state, do whatever changes you want, save THAT (without ever turning on an engine) and then you should be free of this issue.

 

The problem seems to only occur for me if I:

 

• Fly around for a bit burning fuel, then Shut down and save a panel state - then use that panel state at a later time.
• Don't use the Ground Ops to fuel the plane in real time.

Both of these conditions met = this error here.

[3-2-1-Now 126]

Tot Used = 6.1

.......yes?? Before I started the engine it was 12.2! (6.1 x 2). It too reduced when the #2 fuel used was reset.

 

 

I reccon you have that 6.1 used amount saved in there from a 'previous flight' even if that previous flight was weeks ago but saved as your new startup state.

Uhh no. I loaded a fresh flight from the FLY NOW screen. I flew from Heathrow to Schipol just to get some fuel used value (it's only a 40 minute flight).

 

Best regards,

Robin.

[hopskip 253]

.......yes?? Before I started the engine it was 12.2! (6.1 x 2). It too reduced when the #2 fuel used was reset.

 

 

Uhh no. I loaded a fresh flight from the FLY NOW screen. I flew from Heathrow to Schipol just to get some fuel used value (it's only a 40 minute flight).

 

Best regards,

Robin.

 

When you load from the FLY NOW screen, are the engines running?

(I personally have used a default save state selection so that it always starts engines shut down, ADIRU off, Chocks on, Lower cargo forward/aft open, passenger doors closed if LR model, Main deck cargo closed if F model, APU running.

[3-2-1-Now 126]

When you load from the FLY NOW screen, are the engines running?

Not AFAIK. My load state is one I created whereby the aircraft is in long turn configuration, on GPU, with doors closed.

 

Best regards,

Robin.

[Flamin_Squirrel 71]

I did however manage to catch the problem!! As you can see here, #2 is running, #1 is shutdown. When the system considered the engine to be running, at the exact moment it zeroized the FUEL USED for the #2 engine, it SUBTRACTED this used value from the CALCULATED value. The value of the #2 fuel used was 6.1 before it was zeroized.

 

Excellent work spotting the problem. I've verified your findings and submitted a ticket.

[Vic1 3]

hi

I also have encountered this fmc landing fuel calculation anomaly once a started engines the calculated amount dropped significantly. I ended up adding more for the flight only to discover the correct fuel was actually on-board as planned when I landed. all winds and legs were correct no discontinuities in the plan the route distance was correct. Something was wrong with the fmc . Flight was loaded from a saved panel state. all was correct until the engine was started but was then in theory too late to add more for the flight. did anyway unnecessarily it turned out.

 

Just to be clear here.

 

Is this a bug? and if so in simple terms is there a way to avoid it in the meantime while the team fix it

 

 

 

 

 

 

 

 

 

 

 

 

 

 

tks

 

 

kav

[Flamin_Squirrel 71]

Is this a bug? and if so in simple terms is there a way to avoid it in the meantime while the team fix it

I believe it's a bug.

 

There is an easy workaround for now: go to the PERF page and enter the correct fuel there. it'll change the fuel figure and state manual next to it. Then you're good to go.

[crosdorff 0]

I can confirm this bug.

I see this bug every time when doing a second flight.

 

For example: EDDM => EHAM => KJFK. After starting the engines at EHAM for the second leg the fuel calculation is wrong. Entering the right amount in the PERF page solves the problem for that flight. (The information in FS ACTIONS => FUEL is correct at all times)

 

Kind Regards,
Coen

[HeronVA 0]

Hi,

 

I need to ask if someone has the same problem as i experience with ultra long haul flights, with the remaining fuel at destination. i know that all the calculation through the pfpx are +/- 5% wright! I check with an external client from FSA and the results were astonishing, with a difference of approximately 234 kgs from PFPX. The thing is that i expect to see for instance 12.4 kgs remaining fuel at destination in stead i am reading 20.6, and that is totally wrong. I tried to add the wings at every waypoint and had the same results.

 

Any ideas on how to fix that?

 

My regards

Giorgos