View Full Version : Question about the FMC predicted fuel and CFP fuel?
cauc0001
29th Mar 2011, 02:32
Yesterday, I found a phenomenon, which the FMC predicted trip fuel is different from the CFP calculated trip fuel. I have input the same payload and total fuel as the CFP. The different between the FMC predicted trip fuel and the CFP trip fuel is about 500KG. Who can tell me the reason about it?
Tks a lot!
Where to begin.......
What wind did you enter into the FMS, how did it compare to the forecast winds for the CFP?
What ISA deviation did you enter into the FMS, how did it compare to the forecast temperature for the CFP?
What drag bias is applied to the FMS and CFP?
What fuel flow degradation is applied to the FMS and CFP?
Does the CFP have takeoff and arrival allowances built into it?
What parameters were the step climb governed by?
What buffet margin was applied to both?
Was the fuel difference noticed on the ground before the flight or after?
Is the CFP using tailnumber fuel burns or average?
Not really a strange phenomenon :)
cauc0001
30th Mar 2011, 01:03
When I was on ground, and the payload had been on board, the loading and balance manifest has been calculated. I input the ZFW as the balance manifest, the fuel on board is the same as the CFP, the cruise wind and temps is the same as the forecast as the CFP.
galaxy flyer
30th Mar 2011, 03:16
If they are within 500kg, that's close enough. Consider, mutt's list of possibly explanations and then get in the habit of watching actual fuel burns on a variety of legs. It'll make sense wit observation.
Quite common and it depends which way the '500' was! Departure/arrival routes/track miles/altitude restrictions/start-up+taxy fuel all the same?
Remember the 'authority' pre-start is the PLOG, and the 'authority' in the air is tanks reading+PLOG. FMC predictions are a 'luxury' - very useful and to be noted but not 'authority'.
MetoPower
30th Mar 2011, 16:32
To add to Mutt's list, what aircraft?? B74x, A3xx B767, B757, B737, medium turbopro, good old Dak or MS FlightSim.... track miles.... :bored:.
Computers are very good tools (and tools only!!), but a bit if common sense please...
Your profile & post don't say much.
cauc0001
1st Apr 2011, 09:21
The aircraft is B737-800.
bucket_and_spade
3rd Apr 2011, 20:22
I'd trust the CFP combined with regular fuel checks above the FMC!
B757.
FlightDetent
3rd Apr 2011, 20:32
Another one: TOC ISA dev or AVG ISA ?