Is the PMDG 744 a fuel guzzler?

[VHOJT 177]

Hi there chapsI was lucky enough to recently get hold of a real 747-400 flight plan on a recent LHR/SIN flight plan. The plane in question had a +3.0% adjustment from book values. Using FOC2003 with the same route, the end result came out bang on the numbers in agreement with the plan I have.The strange thing is, according to the 744's FMC - it would want about another ten tonnes for the same route! I tried planning the same flight in FOC2003 without the average wind component, and got a fuel number - again it was well below what the PMDG 744 thought it would use. I have also noticed the FMC has a tendency to over-estimate the amount of fuel it needs on the ground by about 1-2 tonnes, and once airborne, the arrival estimate springs up a little.Does anyone have any experience or thoughts when it comes to 744s fuel consumption or FMC estimates?Kind regardsRudy

[rgamurot 1]

For the PMDG B744 FMC (and even the real one), if you don't put in the wind information, it will give a high fuel requirement. Especially on this route because you would tipically have tail winds blowing you over and saving you fuel. If the FMC doesn't know about these winds, it will assume no winds and tell you you need extra fuel.Same applies in the opposite direction. The FMC will tell you you need so much fuel, but in this case, you actually need more since you're flying into the wind.Ryan GamurotLucky to live Hawai'ihttp://www.virtualpilots.org/signatures/vpa296.png

[VHOJT 177]

The flight plan I have gave me an average wind direction and velocity. This should provide the FMC with the average tailwind component as once you put in the wind to the first cruise waypoint, it is automatically downselected to all subsequent waypoints. In this case, the average wind was 290/50, the average component being a 43 knot tailwind. I put 290/50 in the FMC at the first cruise waypoint.I used the 43 knot tailwind in the flight planner, and it had me bang on the numbers the same as the real flight plan (138.2 tonnes). I therefore assume it is reasonably accurate. The PMDG in the same situation demands at least ten tonnes more fuel (using the same 240500kg ZFW).When I tried planning a flight without any wind in the planner, it was the same situation with the PMDG 744 - the FMC seemed to want a lot more fuel than the planner was calculating, despite the planner matching up to the real-world flight plan.Has anyone else noticed the PMDG using more fuel than in the RW? Or do I perhaps need to reinstall or something?

[DF9AA 0]

Hi Rudy et al,this is an ever lasting story with the FMC in the PMDG . . .Here a few figures :EGLL-KPHX Jan 25 calc flt time 10:18 - TO 1115Z (ETA 2133Z)Calc burn 115.7 kgs/1000ETA accdg. FMC 2134Z with 17.7 KGS/1000 ( reading after initial level off at FL320 )- this would mean a burn of 119.5 kgs/1000Act Arr at 2147Z ( plus 14 minutes - lost 10 mins over Atlantic - )Fuel remaining at KPHX 21.6 Kgs/1000 !!Ttl burn 115.6 kgs/1000I must admit that my calculation was also a little off . . . but better than the FMC !This was discussed here a long time ago - even the boss himself ( Capt. RR ) wrote a reply, stating that the FMC is one of the most complex systems in the airplane and over 9000 pages of material must be worked into the PMDG program! He also mentionned that his team will look into this problem and hopefully improvements will come - the latest 747X still has the same problem. . .How come that I, with a rather simple spreadsheet program, be closer to the actual burn ? My program calculates the FF along the route by 15 waypoints ( all sectors more or less even ) and taken into account that the FF reduces by decreasing inflight weight and higher altitudes.The FMC does NOT calculates the step climbs, which are programmed by the FMC - after each step climb the est. fuel at destination jumps up - it should be already included in the FMC prediction.The FMC apparently calculates the time to dest with the present speed and not with the standart speed at a given mach number, altitude and temperature ISA - The last sector of the flight - OM to touchdown - the fuel calc is defenitely wrong in the FMC - average indication on all flights I did is 1200 to 1500 kgs, but in actual the aircraft needs only 400 to 500 kgs - checked in over 100 flights !!Several collegues here in the forum will now argue that this is not so important . . . but they want a simulation as close as possible to the RW - and an important part of FS is the FMC - and a relatively easy part to be checked by a normal flightsimmer - most of us cannot check if the 747 behaves like a real one ! ( Capt. RR will soon . . . )EDIT : Forecast winds are of course entered into the FMC - waypoint by waypoint, using ASX !! My calculation is based on those forecasts . . . !Karl-Heinz - EDDI/THF " Tempelhof "" The mother of all airports " - Sir Norman Fosterhttp://www.precisionmanuals.com/images/forum/747400.jpg

[rgamurot 1]

One thing about putting in average wind velocity versus the velocity at every waypoint is that instead of having an overall estimate, you have specific wind information along the entire flight path. For example, if you have high tailwinds at the beginning of the flight then it drops off during say the last quarter of the flight, you can take off with less fuel thus at a lower weight which in itself will save fuel. Also, by the time the wind drops off, you won't need that much fuel to complete the remainder of the flight. If you only put in a average, the FMC will assume you'll have the same wind speed at the same direction at every point in the flight. Like Karl said above, the FMC is very complicated and even in the PMDG simulation, not complete (as hard as that may be to believe). This entire topic in itself is complicated and as it's currently rather late where I am right now, I'm having some trouble elaborating on my example. But also remember that in real life, the FMC downloads the wind information at every waypoint and also at different altitudes. It even downloads the temperature which also plays a major role in fuel predictions. Ryan GamurotLucky to live Hawai'ihttp://www.virtualpilots.org/signatures/vpa296.png

[Guest tompeel]

I'm using Active Sky, which for a given flight plan will calculate all the wind predictions in advance, and give you the estimated flight time and the effective distance flown, which you can then put into the PMDG tables. The effective distance is slightly unintuitive, but quite logical - if you fly 10 hours into a 100knot headwind, your distance flown has effectively increased by 1000 NM.T.