Oki-

FCOM DSC-22_20-50-10-29 any help?

Thanks a lot Moody , I'll fly the same aircraft tomorrow ill check again.

No Airbus-experience here, but this is how it works on the MD-11 and I'm guessing it should be much the same:

/FF+FQ means that the FMS bases its indication of FOB on two information sources: one is the measured fuel quantity (FQ), the other is the fuel flow (FF - the calculation starts off with the total fuel on board before engine start, and then substracts fuel based on actual, measured fuel flow).

/FQ means that the FOB-indication is based only on measured fuel quantity. This could happen if e.g. a fuel flow sensor is u/s, or during or after a fuel dump situation.

/FF means that the FOB-indication is based only on the fuel flow-calculation. This could happen, you guessed it, if e.g. a fuel quantity sensor is u/s.

The crew can enter /FF or /FQ to force either way of calculating FOB. Clearing the line will return the calculation to /FF+FQ.

If /FF is used, there is NO WAY for the crew to enter the fuel flow! The system uses actual, measured fuel flow. Think of it, it would be nonsensical to manually enter a fuel flow. (Don't confuse this with the fuel flow factor, which can be entered into the FMS elsewhere and which corrects predictions for high/low consumption of the particular aircraft).

So, this FMS ended up in /FF for some reason (and I'm thinking it gets reset to /FF+FQ at the beginning of each flight cycle, so it must have happened during this flight?) but the FMS should still be able to make accurate fuel calculations and predictions!

Crucial to me is: why does the FMS think that the aircraft will land with more fuel than it thinks it has on board now?

I can't think of anything else than a software bug, where the FMS stops updating its fuel predictions for some reason. As suggested by ElitePilot, modifying the route might force the FMS to re-calculate its predictions again.

Ops identified the same problem down here.

Here is the thing: on some FMS, if you edited somehow the route to the alternate, you would have no alternate predictions at all. It was a software bug which was fixed later.

It was called something like "Erroneous Alternate Fuel Predictions Upon Modification of a Company Route"

Whenever we found the same situation, we would erase the alternate, add it again, and did touch the route to it (i.e. adding a star to the route).

Hope that helps!

yes
 

Konde thank you ,
I know the OEB , but we didnt use a company route and besides here, there is a prediction .On the mentioned bug, there is no prediction.
But you never know its an Airbus may be it might have worked:)

By the way , i tried to modify the FOB.And it worked . So i confirm , Its blue and modifiable.
When you change it , it turns to /FF from /FF+FQ.
E.g. : Before 3.1/FF+FQ you enter "7.9 " it turns 7.9/FF
Meaning , the fms starts believing what you tell , and disregards the FQ.

Basically when you put in an FOB , the FMS then calculates the new Dest and Alternate EFOB accroding to it.

Everything make sense and logical at this point.

The problem here is, after the calculation EFOB is greater then the FOB.:ugh:

Brings to mind an old saw that dates to the 1960's.

"To err is human. To really screw up, you need a computer."

:ok:

I admit never used or have in depth knowledge about FMS but could a glitch have given a negative fuel flow number? and subsequent subtraction resulted in an addition?

What does the FIM say..........

There is a supplementary procedure FMS specificities named "ERRONEOUS ESTIMATED FUEL ON BOARD (EFOB) WHEN THE FUEL FLOW ONLY (FF) IS USED"
According to which :
FOB is the computed value of the Fuel On Board. It is based on BLOCK entered by the flight crew on the MCDU and : ‐ Data received from Fuel Flow (FF) and Fuel Quantity (FQ) sensors (from Fuel Quantity Indicator System, FQIS), or ‐ Data received from Fuel Flow only (FF), or ‐ Fuel Quantity (FQ) only With the FMS REV 2+ and previous Thales' standards, an anomaly has been reported on EFOB computation in case of FQIS failure, or when the flight crew selects the FF only. In cruise, when the FF only is used by the FMS and is below 2.7 t/h, the FOB becomes frozen for predictions. Therefore, the EFOB constantly increases at all downpath waypoints and the FOB value at the FROM waypoint displayed on the F-PLN B page is frozen. The FMS uses the FOB to compute the Estimated Fuel On Board (EFOB), at each waypoint and at destination. The EFOB is displayed on the MCDU on the page F-PLN A and FUEL PRED. However, the FOB displayed on FUEL PRED page is correctly updated (based on FF only). A manual re-entry of the FOB through the MCDU enables to successfully update the FOB used for predictions, including the EFOB.

So yes it's a known issue ;)

Okivan;
Actually, I wouldn't be interested in what's "wrong" with the FMS.

The FMS system is for predictive estimates, not for official numbers.

The question is rather, what does your OFP - Official Flight Plan state as the FOB over destination? That's the number that governs the operational viability and even the legality of the flight.

The FMS is a planning tool that may not guarantee 100% accuracy but the OFP is expected to be accurate, especially where it concerns fuel on board over destination .

If the discrepancy is large, by all means have a look to see what the problem is and resolve it to the satisfaction of all on the flight deck. For me, 500kg isn't a large discrepancy to see, even in an A319 FMS especially when we don't know the source of the difference and therefore can't know which one is correct.

PJ2 right on as is pilot737

It's almost certainy because of one of the 2 FMS 'specifities' - the one pilot 737 mentioned or 'ERRONEOUS FUEL PREDICTION IN THE CASE OF DESCENT WITH TWO ALTITUDE CONSTRAINTS'.

These are found in FCOM PRO SUP AP section.