FMC - insufficient fuel error

[fly_ebos 1]

Hello,I'm a newbie to the PMDG 737, bought it this weekend and I'm totally excited by it ! ;-)After some short-haul en long-haul flight there's 1 thing that I've noticed and that is an error in the FMC fuel calculation :This morning I flew KPHX-KBOS with the 737-700, full fuell. During climb the FMC told me to expect 22 000 lbs of fuel left in KBOS. The moment when I am leveling of and going into cruise the FMC tells me there will be insufficient fuel. Once passed the TOD the FMC 'updates' itself and predicts 21 200 lbs of fuell to remain (sounds logic considering wind aloft).--> Some rough analysis led me to the conclusion that in the cruise portion of the flight, the FMC does not take the center tank into consideration.Next flight was a transatlantic one : KBOS-EBOS; again with full fuel. Again during climb it project 11 000 lbs left in EBOS, once in cruise insufficient fuell. But once there was only 1000 lbs left in the center tank, the FMC updated itself and was correct again.Now it seems to me there is something wrong in the FMC logic ? Or am I doing something wrong here ?BTW in both flights the reserve was set at 4000 lbs.Regards,Thomas Billiouw

[Guest jase439]

If the FMC is predicting 21,200 lbs of fuel to reach your destination, and you have specified 4,000 in reserve (which is fuel for getting you to your alternate, delay holds, unexpected deviations from the flight plan, etc) - you should have loaded out with 25,200 lbs of fuel.The FMC is warning you that based on your fuel load out, if everything goes perfectly, you will be dipping into your reserves. If everything goes badly, you will not have the 4,000 pounds of fuel you need and you will run out of fuel and fall to the ground in a smokey heap of ash and rubble :)Cheers,J

[Mike S KPDX 597]

I was not aware that the fuel reserve operation on the SU2 FMS was implemented. I now that it was not utilized on the previous version and I noted that it is not listed in the latest available fix/mod list from PMDGAre you sure this works as you state? Prior to this I belive the message was displayed assuming a 1000lbs reserve regardless of what the user placed in the CDU.

[crvm 0]

Well what was your route and CRZ ALT? Simple matter for us to replicate and see what is any issues are showing for fuel..[h4]Best Wishes,Randy J. Smithhttp://www.precisionmanuals.com/images/betaimg.jpg

[Qavion 19]

"If the FMC is predicting 21,200 lbs of fuel to reach your destination, and you have specified 4,000 in reserve (which is fuel for getting you to your alternate, delay holds, unexpected deviations from the flight plan, etc) - you should have loaded out with 25,200 lbs of fuel."The Smith's FMC doesn't look at reserve fuel entries, Jase (unlike the Honeywell FMC on other Boeings). It simply looks at the fuel prediction and if it is under 2000lbs (900Kg), it will generate the message.Cheers.Ian.

[Mike S KPDX 597]

Hi Ian,if the message is triggered at 2000lbs only, why would there be the ablity to put in a reserve fuel figure? where would this pilot input figure be utilize in the FMS? (greater than 2000 maybe??)just curious.

[fly_ebos 1]

Randy,It was a flight from KPHX to KBOS, routed with the Flight Planner from FSCOF (high altitude IFR-routing), and flew it at FL380. Some people suggested that I need only to carry 25 200 lbs of fuel, that is true but I was checking out the fuel flow with this new product (for me). Anyhow it shouldn't make a difference if I loaded full fuel or only 25 200 lbs for the FMC.Some people noted that the reserve function on the FMS isn't working? Is that true ?I still have the feeling that the FMC isn't taking all tanks into consideration when predicting the fuel left at destination.regards,Thomas Billiouw

[fly_ebos 1]

correction : I flew it at FL350 !regards,Thomas Billiouw

[Guest jase439]

If there is a problem, I don't think it's related to the center fuel tank. I flew a short hop from KPHX-KSLC, with an 8K reserve. I went up with the center tanks dry. The predicted FOB at KSLC during my climb left me with about 7K in the tanks and the FMC kicked back with the insuffient fuel alert. Once I got to my cruise altitude and was burning more efficiently, it showed me arriving with 9.2K and the "insufficient fuel" message no longer displayed. Everything seemed as expected to me.J

[fly_ebos 1]

Hi,Thanx for the info. "My" FMC looks like working the other way around : during climb it gives me a prediction which becomes reality after landing (or close to it given winds aloft).Once I level off at cruise level and the autothrust changes from the N1-mode to the FMC-mode, then I get the insufficient fuel error. That's why it looks so strange to me : during climb (with higher FF) it gives me correct info, but once in cruise with a lower FF, then I no longer have enough fuel onboard. If it were the other way around, then I could understand it.Another flight today gave me the same result : once the center tank goes beneath 2000 lbs than the FMC gives me correct info again.Nobody else is getting this error ? Does the fact that I don't have a registered FSUIPC influence the FMC ?Regards,Thomas

[Qavion 19]

Hmmm.... Don't know what happened to my earlier response, but....According to the Bulfer 737NG Users Guide, the RESERVE value has a few functions, Jase. It's used by the HOLD AVAIL field of the HOLD page and is also related to a "USING RSV FUEL" scratchpad message (rather than an INSUFFICIENT FUEL msg).Techniques for using the reserve value are mentioned in the Bulfer manual (recommended reading by the way).Hope this helps?Cheers.Ian.

[Guest jase439]

I will try my test flight again today, but this time I'll put about 4,000 lbs in the center tank and see if the FMC reports back something different.JPS. Did you turn any of the fuel pumps on or off in flight...transfer valves, etc?

[Guest jase439]

Just a follow up. Everything was as expected - ran the center tank dry from 4,000 lbs. No spurious Insufficient Fuel messages popped during the flight. There must be some other condition that's causing this to occur. Maybe you hit some erratic head wind at your cruise altitude just as the FMC was refreshing it's FOB calculation? The lack of interpolation/smoothing in the winds has caused me all kinds of squirrely behaviors.As to Ian's earlier remark. I found this in the FMC user manual (see page 8 - 21) which directly contradicts what he says about the reserve value being ignored (I can't speak as to whether the real FMS behaves in this manner, though):-=-=-=-Helpful Hint: The value entered for fuel reserves is used by the FMS to determine when there is no longer sufficient fuel remaining to reach the programmed destination with the desired amount of reserve fuel remaining. Lowering this figure will cause the warning to cease, or lowering current fuel burn rates will also cease the warning.-=-=-=-J

[Qavion 19]

"As to Ian's earlier remark. I found this in the FMC user manual (see page 8 - 21) which directly contradicts what he says about the reserve value being ignored"Which user manual are we referring to here, Jase? The Honeywell FMC manual (757, 767, 747-400, 777) or the Smiths FMC manual (737, 737NG). The two FMC's are very different in behaviour.Cheers.Ian.

[fly_ebos 1]

The problem only seems to appear on very long flighs, close to the maximum range. It puzzles me because it always tells me that I won't have enough fuel onboard once the aircraft settles in cruise and reduces the FF. But when I land I always have more than 10 000 lbs left. And the wind would have to vary immensly to have a difference of 10 000 lbs.greetz,Thomas