View Full Version : FMC fuel predictions based on Calculated or Totalizer fuel

13th Dec 2016, 05:36
Which do you use/prefer/have more confidence in etc?

13th Dec 2016, 09:47
Errr.... Calculated. Else how can you tell if you are using (loosing) fuel which has not been consumed by the engines (leak)?

13th Dec 2016, 11:27
Mostly a combination of both.

If I want to examine the current situation, itīs usually the totalizer that is first checked.

For predictions and checking options, the FMS is good enough, but comes with several operational caveats depending on type. One has to have an idea of what goes on inside the box. If it bases its calculations on a performance model, better be sure that this model is appropriate to the present aircraft configuration if You do not want to end up like the Hapag A310 at VIE some years ago. If its calculations are based on current fuel flow and TAS, it will massively underestimate the remaining fuel at destination during climb and only slowly approach realistic values during cruise and descent.

And I also subscribe to Goldenrivetts words. A quick check to see whether fuel on board plus the amount the FMS believes the aircraft to have used is equal to the amount of fuel You found in the tank during the pre-departure fuel check is a good way to spot many malfunctions in the fuel system early (-ish).

13th Dec 2016, 22:51
Both obviously.

Use the lowest figure for any decision making required. ( leak check, continuation of flight with statutory reserves, diversion etc )

It's my understanding that the FMC used the lowest of the 2.

14th Dec 2016, 08:58
It's all in the AFM supplement for the particular box installed on the type. A typical FMC/FMS manufactured after the mid nineties is going to use totalizer quantity, current fuel flow and ground speed to predict fuel remaining for a given distance down route, then "ramp" or "blend" the expected fuel flow according to a performance model programmed in the box thereafter.

The predictive performance models in the box will consider the forecast winds & temps entered in the FPLN for each fix to calculate the fuel remaining at each fix all the way to destination. The altitude and airspeed/mach number applicable between each fix will be a default value unless a manual entry is made. This is of course most accurate when all fixes have forecast values and the forecast itself is reasonably accurate.

An older Honeywell box (FMZ-2000) on a Hawker I used fly ramped/blended the current fuel flow and speed to the values 700 NM down route. So the predicted fuel remaining at each fix on the FPLN route was a blended value between present position and 700 NM ahead, and purely according to the calculated predictive values thereafter.

It works a charm too. It was typical in the Hawker to see less than 2% difference between FMS predicted and actual fuel burn on a 6 hour flight. The greatest part of any error is generally attributable to wind/temp aloft forecast accuracy and whether the climb and descent profiles were flown as programmed in the box. ATC can't always allow that given the vagaries of traffic and weather.

In addition to supporting gross error detection, constantly comparing the box predictions with mental math derived calculations has the benefit of giving one something to do on longer sectors, thus maintaining a somewhat higher level of consciousness, especially in the wee hours.

I hope the above encourages anyone curious about the FMC they operate to crack open the supplement if available.

14th Dec 2016, 17:06

The FMC uses the lowest of the 2 ? Then you would never have a way of detecting a fuel leak ! FMC is calculated and totalizer is......totalizer. Well on a Boeing anyway "progress 2 page"

The totalizer has to be the gauge to make decisions on unless its clearly got a technical issue. Once dispatched a brand new 777 with an inop left fuel gauge. Tanks dipped and relied on the FMC only. Not a pleasant experience.