View Full Version : Unreliable FMC fuel calculation with gear extended
fly744
26th Jun 2010, 13:23
Has anyone more info or an "easy technical explanation" about it?
Thanks.
Empty Cruise
26th Jun 2010, 13:36
:confused: yes :confused:
The FMC calculates fuel based on the parameters entered + the DB installed.
Since there is no "gear down" option in most DBs, it follows that the predicted fuel will be wrong when dispatching with the gear down. A quick Google of "Condor+Vienna" will lead you to a case in point :ouch:
Some FMS OEMs have included a "measured" perfromance option that will use the previously achived SAR as the base for calculations, but this will generlly need to be pilot selected, since "predicted" performance (i.e. the DB values) is default on most FMS'.
Honestly...
hetfield
26th Jun 2010, 13:40
It was Hapag Lloyd at Vienna.
hetfield
26th Jun 2010, 13:45
Airbus says flight management system not to blame in Hapag crash (http://www.flightglobal.com/articles/2007/08/10/216022/airbus-says-flight-management-system-not-to-blame-in-hapag.html)
Empty Cruise
26th Jun 2010, 18:22
hetfield,
Indeed it was - I stand corrected :ok: Classic bit of "knowing your aircraft", tho... or one should have thought that when the FMS came up with the same arrival fuel after the gear wouldn't come up as it had on the ground, a penny or two should have dropped.
But then again - there but for the grace of God...
Checkboard
26th Jun 2010, 19:54
As a rough guide, an airliner will burn about twice as much fuel per mile gear down compared to gear up.
Admiral346
26th Jun 2010, 22:51
The CRJ I fly with a Collins FMS has a prompt to switch from "predicted" to "messured" mode wich will change the fuel calculation to where it calculates with the actual fuel flow. It won't take into account descent or climb, just use the present fuel flow to predict the arrival fuel.
Not a good tool for normal ops, but having something wrong with your configuration, it would serve as a good guess on where you might be able to go.
Nic
Not again....
Look, I'm a Boeing man so know little about Airbus A310's, or any other ab,
but I CAN tell you this...after reading that report,
I can tell you it does not carry a Flight Engineer...
will they ever learn.....
ew73
blueloo
27th Jun 2010, 06:39
Having spoken to a couple of people who have done ferry flights with gear down, their opinion was that even the charts with gear extended in the performance manuals were incredibly inaccurate....ie they used significantly more fuel than they had expected.
Now thats only from a couple of people across 2 types of boeings, so it isnt a big sample, but it does make you wonder.
hetfield
27th Jun 2010, 10:18
Concerning the A310 Vienna accident, as far as I remember the FMC was really tricky there. The calculation to the next waypoint was correct, using actual FF. But afterwards the FMC assumes a retracted gear again. So from waypoint to waypoint the situation changed drastically.
really tricky???:ugh:Tricky!! Nothing tricky about "I wonder if I'm going to use more fuel with the gear down?" and "Hey Bloggs, did you see how the predicted fuel for the next waypoint suddenly changed? WTF caused that?":confused: coupled with "I wonder if we have enough?"
I just know there's a word for that - cannot recall it..............................
hetfield
27th Jun 2010, 10:41
BOAC
All I wanted to point out is the technical background of the FMC.
(Assuming a clean ship after next waypoint).
To let these guys look like idiots is your business not mine.
I think 'fly744' got the answer to the 'technical' stuff in post#2?
It does not need my help to achieve what you describe, Hetfield - and it was not "these guys" it was "that guy". Lucky indeed only 28 were injured and all were not killed. My views on Computers in the cockpit and the safety of aviation (http://www.pprune.org/Computers%20in%20the%20cockpit%20and%20the%20safety%20of%20a viation) are on record. He:
Ignored F/O's warnings
Appeared to 'trust' illogical fuel predictions
Ignored 'short of fuel' warning 1 hr before crashing
Was reported to be most senior H-L Captain - until
fly744 - you say you are a 744 Captain. I would recommend (I guess you would know anyway?)
a) listen to the F/O's views
b) when fuels start to depart from the expected, consider abandoning any gizmos like the FMC and revert to a straightforward 'how-goze-it' graph
c) if the gizmo then tells you you will run out of fuel, take note.
BTW - did we ever get a link to an English translation of the Austrian report and what happened in the court case against the Captain?
hetfield
27th Jun 2010, 18:21
BOAC
Again, this thread has a technical issue.
Nevertheless I fully agree with you about badly CRM in that particular accident.
Regards
kotakota
28th Jun 2010, 03:48
Ferried a gear-down 737 from Borneo to KL a few years back . Apart from the incredible noise and unhelpful ATC ( Why you fly so slow ? gear down ? you wanna declare mayday ? ((Mentioned on FP under Rmks , waste of time )) what struck me was how very accurate the QRH Planning data was .
I think you will find that Block18 (remarks) do not normally make their way onto controllers' flight strips, and of course the absence of the customary 'kotakota' R/T background 'clackers' would have caused confusion to them:).