FMC Fuel factor

[JammedStab]

Does increasing drag(such as installing the big internet antenna on the roof) make the Fuel factor number increase?

What if someone sets it to a significantly incorrect number. Does that affect fuel predictions? Time predictions?

[172_driver]

Flew an old knackered 737-700 the other day. The flight plan fuelfactor was 0,0 %. The next day I had the sister ship, from the same era, with a fuel factor of 5,0 %

I take the FMC fuel prediction with a pinch of salt as, in my experience, it significantly over-estimate fuel on arrival even when the winds aloft have been loaded and the time prediction turn out to be fairly accurate.

[FlightDetent]

For SA Airbii, my understanading is that PERF only mathematically increases the estimated fuel burn as calculated by FMS.

The antenna would increase real burn, and eventually through APM it will be reflected in the revised perf factor. My experience is that the actual burn - all the usual suspects being equal - versus OFP versus FMC predictions is within 200 kgs on a typical 4 hr sector. And inside 3 minutes.

Did you see the FCOM chapters on PERF factor?

[NSEU]

Does increasing drag(such as installing the big internet antenna on the roof) make the Fuel factor number increase?

Isn't fuel factor more tied with the efficiency of the engines? The 744, for example, has both fuel flow and drag factor entries on the PERF FACTORS page. I'm sure only the drag factors would be adjusted for airframe additions. The FMC would then adjust profiles and fuel usage according to the drag factor changes.

[FlightDetent]

That makes quite a good sense. One factor for the pull, other for the push. Not on 3tw3nty, though. :)

[Skyjob]

Fuel factor is merely a number representing additional fuel flow above base line model. Thus any antennae installed or skin repairs have an effect and can be represented in this simple manner.
It is a fine art to correctly calculate and accurately enter into FMC the factor for each aircraft in a large fleet, once done it will definitely help crews with their fuel predictions.
Time predictions are not affected, as the speed will still be flown by the aircraft regardless.

[JammedStab]

Thanks,

The question then becomes, what changes the drag factor. 777 this morning had a FF of around 1.3 and a 0.0 drag factor. Our flight plan has the FF listed which is to be checked. Why not check the drag factor as well? Or do some airlines do this?

[flyhardmo]

We check both fuel and drag factor against the listed ff/df on the flight plan.

[NSEU]

And beware of reversed values.... Some time ago, one engineering department decided, one month, to issue a fleet Drag/FF value list with FF listed before Drag (normally the list showed a Drag and FF column for the fleet.. and this matched the aircraft entry sequence). At least one engineer, out of habit, entered the values as he normally would. Fortunately, this was picked up by the flight crew on the first flight.