View Full Version : B737 FMC Fuel prediction.
Can anyone tell me after an engine failure, is the FMC predicted arrival fuel at destination or diversion field accurate? Many Thanks.
CaptainSandL
16th Dec 2008, 17:32
It is not bad but in my experience it tends to overestimate by 100kg on NGs and up to 200kg on Classics.
FYI the fuel figure you see in the FMC is calculated by the fuel summation unit and it seems to round up the total of the figures on the individual fuel gauges by up to 100kg.
The usual FMC 'caveat' applies - deduct 200kg and add 3 mins for what will probably happen.:)
Many thanks for your replies:In the event that your LED's are unable to be retracted,why does the QRH tell us that FMC fuel predictions are inaccurate?Surely the extra drag leading to a higher fuel burn could be processed by the FMC.Many thanks in advance.
CaptainSandL
17th Dec 2008, 06:16
The FMC fuel calcs do not take account of configuration - esp non-standard ones. So the fuel predictions will be derrived from tables based upon a clean a/c at whatever alt/speed/conditions(ISA dev etc) you tell the FMC you will fly at.
Best example was the A310 a few years ago which landed short after a gear down flight that the FMC could not calculate for.
It has always baffled me that a computer cannot integrate current fuel flow using an algorithm for a 'generic' % reduction in FF with weight and cope with 'odd' configs! To have to resort to a 'back of the envelope' how-goze-it graph in the 21st Century....................
TyroPicard
17th Dec 2008, 09:29
It was much easier when we were allowed use the back of a fag packet...
Yes, but with the no-smoking rules now.......................I'm amazed more pilots are not running out of fuel.
framer
18th Dec 2008, 00:56
the no-smoking rules have a lot to answer for. Not only do they cause inaccurate fuel calculations but boredom on long haul flights is resulting in many crews falling asleep late in the cruise. I feel the safety benefits of smoking are often down played.
captakash
24th Dec 2008, 13:06
As with my limited experience on the NGs, I have noticed the Estimated Fuel Remaining Over Destination, progressively reduces by approx 500kg through the flight, without any other attributible reasons. So if you see a figure on the PROG pg at the beginning of a flight, it usually changes by approx 500kg by the time you get there. This compared to the 744s, which was definitely far more accurate than the 738s.