Accuracy of 747-400 FMC

[DF9AA 0]

After I searched in AVSIM ( PMDG support ) I found a thread about the fuel planning in and with the PMDG 747 FMC and I completely disagreed

[Nyuni 1]

I think the issue you're running into here is the lack of wind data provided to the PMDG 744FMC. Since in the real world the aircraft would be getting weather data from ACARS for each waypoint (which isn't available to us in the sim, obviously), the FMC calculations change with the current winds. So, if you have a strong headwind for a certain segment of your flight, the FMC may predict you have less fuel than the reserve value in the PERF INIT page for the entire trip, tripping the "INSUFFICIENT FUEL" message.Try this - if you use a weather addon such as ActiveSky that spits you out a navlog with the forecast winds aloft for each waypoint of your flightplan, enter this data manually into the RTE DATA page by entering the wind bearing and speed (i.e. 250/12) and pressing the LSK to the right of the waypoint to store the data. (Someone correct me if I'm wrong in this procedure, I'm not in front of the sim to check and am describing it as best I can from memory) This will help the FMC make better fuel predictions based on the forecast winds throughout your flight, not just the instantaneous winds the aircraft is experiencing at any given moment.

[DF9AA 0]

Hi Brian,pls see my 2nd paragraph - I use ASv6 and entered the winds into the FMC . . . then the FMC produces those wrong predictions !Karl, Berlin

[Qavion 19]

" I know that in RW-flying the FMC is a veryprecise instrument for the crews to monitor their flight andespecially the fuel consumption

[DF9AA 0]

Hi Q>,How should a FMC mix projected winds with real winds at ELTAX when the plane is at VALDI some 3000 nm away ?I think I did not bring the story across :Look at the total times :CALC 609 minsFMC Est. before AAL 615 mins - LESS fuelACT 622 mins - MORE fuel It is true that RW-pilots have great faith in their ground-based flightplans but they still rechk with FMC . . . and normally this fits on the dot . . .Karl, Berlin

[DF9AA 0]

Hi all,further to the comments :Look at the FF/hr figures from FMC-Est. and ACT FF/hr ! this has very little to do with winds - BTW the winds were never a problem during my flights . . .

[SierraDelta 1,337]

Karl,The way I read your story goes as follows:Both you and the FMC got it wrong, but the FMC was closer to the final result - not a real surprise... I take it that you used the ActiveSky forecast winds at the beginning of the flight and entered them into each associated line in the LEGS pages? OK, but let's be honest - these are only forecasted winds, and nowhere near the quality that you'd get from a modern dispatch tool set. So, although you say that the winds "were never a problem" I guess they were after all...One possible way of negating this factor in your research could be to disable online weather updates after the initial download, that way I think that the actual winds will be much closer to the forecast ones for the entire flight.About the DING-DONG - what value did you enter into the RESERVES line?S

[DF9AA 0]

Hi Soren,so it will be a lively discussion . . .You are right with the winds ! Now I have to be more precise :Wind to VALDI from AS was to be 200/50 which gave me a GS of 519 ( based on cost index 150 = approx M .86 or around 498 kts TAS ) and calc AND FMC forecast was 46 mins for this sector and FMC-Est for 21,7 k lbs of fuel - ACT GS was 494 ( !! ), act time was 48 mins ( PLUS 2 ) and FF was 21,2 !! So the wind was less favourable, but the FF was better than predicted ! That's what I mean - another example winds between BODRA and ELTAX were forecasted to be 270/15 , so called for a GS of 485 - 127 mins of flight and fuel to be used of 52,2 k lbs.( all FMC estimates )Actual GS for this sector turned out to be 468 and actual flight time was 132 mins ( PLUS 5 mins !! ), fuel used was only 47,7 ( nearly 5000 lbs LESS than FMC-Est., also here the wind was less favourable ) !!!!Since the OAT dropped below -63C and consequently the TAS went down I increased the Mach nbr to .875 to stay near to 498 TAS - and even so, the FF went below FMC-Est. !!Again - the winds are not a factor for wrong FMC-predictions, it is the wrong fuel-flow calculation within the FMC !Input to RES : min altn fuel of 30,1 which includes time to altn plus 30 mins holding - the 5 % cont. fuel is not considered as res fuel and normally not to be used and therefore incl in the calculation of estm. LDGW !

[DF9AA 0]

Hi all,update on FMC accuracy :I changed the cost index to 160 ( iso 150) thereby the TAS went down to 495 at Mach .855 econ speed and the FMC calculates much more precise - last tripEDDT-KSFO :Calc flight time 10:34 with 42,6 rem fuel at DESTFMC EST flight time 10:33 with 41,9 rem fuel at DESTACT flight time 10:37 with 42,5 at DEST - this mainly because of the mentionned wrong fuel prediction for the last sector between OM and touchdown

[Chaos81 110]

No offense intended here Karl, but threads like these make me glad I don't take my simming extremely seriously.

[opherben 0]

Hello,I made a few observations when the 744 came out, my background is among other areas, experimental flight test. The main points related to FMC fuel prediction and the aircraft stalling during cruise for many users. PMDG largely ignored my inputs. Let us now focus on the FMC issue:The best way to research an issue is to hold all other variable parameters constant, and if necessary, change them one at a time. I therefore set for my detailed FMC fuel prediciton evaluation, a calm atmosphere, namely zero wind, standard atmosphere with no change. I looked at one number, destination fuel quantity on the PROGRESS page. As flight progressed the subject value decreased with no change to plan and no deviation from it, VNAV and LNAV only controlling the flight. My consistent finding over many flights was that the fuel prediction value was decreasing with time, namely the FMC under-estimated actual fuel performance. I did not like this, and simply ignore FMC fuel prediction, which for a fine product is a shame.

[DF9AA 0]

Hi Mike,no offense felt . . . but this reminds of a captain we had in the sixties - he knew only one fuel indication = FULL TANKS ( or at least "fill her up to max landing" )In the sim-world the PMDG 747 is such a fine product, and if you use it you should take her seriously ....

[Guest Jpanford]

Ok there is also a very easy fix to this problem. First of all what do you put in the RESERVE line in the PERF Page. If you dont want to see the insufficient fuel message lower that number. In my company we put the reserve fuel plus the Alternate fuel in this block, so if you ever get the message you know you are in deep ####. We upload the winds via ACARS but its not updated thru-out the flight. Winds are only an estimate but I've never had a problem. Infact my company has a fuel policy which is really good. We always are with lots of fuel except LAX, SFO, westbound to HKG in the winter. Hope this helps.Joe

[DF9AA 0]

???Of course, fuel to altn plus 30 mins holding in the res bracket - to reduce this is no solution !The problem is the fuel flow calculation of the FMC and the wrong calculation of fuel at DEST at the last sector between OM and touchdown - last flight again FMC EST at ROGGE (App to R19L at KSFO ) 40,3 and TD at R19L 37,8 - ACT Rogge 40,4 and TD with 39,7 - another 2000 LBS of miscalculation !!Lots of fuel is not the solution - not in those times of extreme fuel prices ... Edit:Oh, we don't have to pay those fuel prices ....

[Guest SIX]

I read all of this...every word of this. One thing comes to mind once I have read all of this:That as much of a "realist" I think that I am, I find people who make me feel like I just started "playing" with this "game" last week.ROFLOLVery interesting. I was just laying in the bed "trying" to take a nap thinking about all of this. I couldn't sleep anymore so I got up just to check the forums and then I decided to read this thread. Interesting how things come to mind and answers pop up. I too make an Excel spreadsheet based on the manual and then I use TOPCAT for the take off and landing numbers. I have wondered for some time two fundamental things about flight planning (which to me is the best part of flying):1: Why in real life do they fill the planes full of passengers and fly from LAX to YSSY and I can't get my simultor, which is supposidly the closest thing to the real deal, to fly with 1/2 payload the same route. I have to drop my pax/cargo down to about 1/3, and then I can get within MTOW.2: In regards to the PMDG manual fuel planning method: What do you do about head/tail wind. Just thinking outloud. I will be gone for a couple of days, talk to you guys when I get back.Wilson HinesMy Blog: http://www.wilsonhines.com --------------------------------------http://www.precisionmanuals.com/images/forum/747400.jpghttp://www.precisionmanuals.com/images/forum/pmdg_744F.jpghttp://online.vatsimindicators.net/837438/3074.png