View Full Version : DRAG/FF correction factor
gipilot
23rd Jul 2023, 21:43
Hey guys, I’m a 747 jockey and have a question about the Drag/FF correction factors in the init page on the Honeywell FMS. I know that the FF is the bias for the fuel flow but my question is the Drag factor. Is that data also used by the VNAV to calculate the correct path and speed?
Abbey Road
24th Jul 2023, 04:13
I believe it is intended to account for the addition of extra external items on the airframe that add drag to the default profile e.g. aerials for internet services.
Broomstick Flier
24th Jul 2023, 15:09
Not only that, Abbey Road, but also, with the passing of years and all the abuse that the aircraft are subjected (small indentations, skin irregularities, structural repairs, etc) the aerodynamic reality begins to differ from the projected/theoretical data used to create performance tables and graphs. The FMC factor can be used to adjust the fuel calculations to these effects.
gipilot
25th Jul 2023, 08:46
Not only that, Abbey Road, but also, with the passing of years and all the abuse that the aircraft are subjected (small indentations, skin irregularities, structural repairs, etc) the aerodynamic reality begins to differ from the projected/theoretical data used to create performance tables and graphs. The FMC factor can be used to adjust the fuel calculations to these effects.
Well, the question I have is if the FMC uses this factor only for fuel calculations or also for descent and climb profiles for Vnav.
mustafagander
25th Jul 2023, 10:20
I had it explained to me back in the dark ages by our performance chief. He said , simply put, that the "drag" factor was for airframe deterioration and "F/F" factor was for engine variations from the book.
awair
25th Jul 2023, 18:33
On the 777 FMC there are two entries, Drag & FF: “-.-/-.-“
The aircraft will have a historical FF degradation, which would be checked before each flight. In the event of an MEL item (eg leave APU running during flight), you may have two corrections: one for increased fuel-flow, and the other for drag, as the APU door is open. The effect of the (fuel) penalty would be the same, but the Drag should also influence the overall profile.
I imagine in this case the effect would be negligible, unlike adding EAI use on the VNAV descent page.
A quick review if the MEL/CDL should identify these items with FF or Drag penalties.
Good luck.
john_tullamarine
26th Jul 2023, 06:42
The ops engineering cells in the various airlines maintain continuous records of how individual airframes vary from book performance values and how these variations vary with the passage of time. It is then relatively easy to come up with correction techniques so that crews are still provided with adequately accurate performance data for flight planning. While I am not familiar with this approach it is just another way to dress the bird up so it still looks fine and dances nicely.
Been the story for many, many decades, now.
Hey guys, I’m a 747 jockey and have a question about the Drag/FF correction factors in the init page on the Honeywell FMS. I know that the FF is the bias for the fuel flow but my question is the Drag factor. Is that data also used by the VNAV to calculate the correct path and speed?
When I was flying the LCF, which is the extreme case, the FMC was loaded with a drag factor of 2.9 and a fuel flow factor of 5.0. It was otherwise a stock FMC/software load. With those inputs the FMC, including VNAV, worked. More or less....
So "yes" would appear to be the answer to your question.