Fuel consumption

[Guest wisborg]

Hi all,I'm looking at some fuel planning, and I'm currently using the table on page 64.If you look in the column with 600k lbs gross weight the optimal altitude is listed to FL370 with a fuel consumption of 22.0k lbs/h. However if one then looks for the same weight at FL360 then, the fuel consumption is 20.0k lbs/h and for FL350 it's 20.4k lbs/h.Is this a typo? If it's not, how come that the optimal altitude has a fuel burn around 10% higher than for the two lower flight level? (Actually the figure for FL370 is the highest for all flight level above FL300).There are several other cases with the same trend, which is the reason I don't just consider it a typo./ Jesper

[michal 44]

I don't think it is a typo but I also don't think there is any error here either. Remember this table is for 0.86 Mach only and your range (fuel burned per 1 NM) is still going to be lower at FL370 than at FL360 simly because 0.86 Mach at FL370 translates into higher ground speed (no wind) than at FL360. Of course it is all conjecture on my part, one would have to do some calculations to prove it. These tables most likely came from Boeing so I would be careful in proclaiming them in error.Michael J.http://www.precisionmanuals.com/images/forum/pmdg_744F.jpghttp://sales.hifisim.com/pub-download/asv6-banner-beta.jpg

[Guest wisborg]

I'am not claiming that the numbers are wrong, I was just a bit puzzled by them and wanted to get a better understanding of the logic behind them.Tonight I had a case where restricting the plane to FL350 according to my fuel planner would require 86200 lbs fuel (from start of taxi to touch down) whereas allowing a step climb to FL370 would require 94400 lbs fuel.On this particular flight I happened to use 88600 lbs fuel during the cruise (including a step climb), however usual I'm within a couple of percent compared to the calculated number (I think the wind was a bit more kind to me than estimated, but that's just a hunch, but the flight was also 12 minutes faster than expected). Most of my flights are flown for a large part at FL370 (I usually aim at flights between 1000-2000 nm), so it might very well be that the number is correct./ Jesper

[michal 44]

>to get a better understanding>of the logic behind them.Not sure if logic is the right word. It is all very mathematical - altitude, air density, Mach speed, temperature, engine operating characteristics, etc - the number of factors is so huge that it would be impossible using 'common sense' to explain why this particular difference between FL370 and FL360. On the more 'crude' scale everybody of course knows that jet engines operate more ecnomically at higher altitude.Michael J.http://www.precisionmanuals.com/images/forum/pmdg_744F.jpghttp://sales.hifisim.com/pub-download/asv6-banner-beta.jpg

[adamv66 0]

Hi,I'm dragging this thread up as I was just going to start a new post questioning the same thing.... (referring to page 10 of the "cruise and fuel planning" manual):How is it that in the example mentioned above fuel burn of 22000lb/hr is shown as the optimum over 20000lb/hr? It is not as a result of speed as another user posted, because M0.86 at FL360 provides exactly the same TAS as M0.86 at FL370. There might be 1 knot or so in it.... Theres no way a 2000lb/hr burn is offset by an extra knot of airpseed!Interestingly, I tested this out (600000lbs gross weight) and at FL370 and FL360 found that fuel burn was almost exactly the same (22000lbs/hr) Which leads me to question the accuracy of the figures in the table...Also the IAS/TAS column has some major discrepancies... It does not correlate to M0.86 (for example at FL360 M0.86 equates to 493/288 TAS/IAS, not 475/264) There's quite a discrepancy.... And I am using standard TAT. Which of these figures takes precedence?Does anyone know where this chart came from? Are these confirmed typo's?The only reason I'm being so particular is that after a few years of flying the queen I'm now trying to get my fuel planning figures SPOT ON. I feel that in FS we have the ability to MORE accurately plan our fuel burn than in the real world. We have no variables such as different burn rates for different aircraft or unexpected/unpredictable winds or having to fly different routes due to ATC/weather etc etc. The simulation performs exactly the same every time, so given the correct figures in the burn table theres no reason why we should not be able to predict our fuel burn precisely...Adam

[phil747fan 14]

Adam,from what i remember years ago those datas are taken from first commercial product delivered by rob the PS1 Manual (yes i was in the wagon at that time only dos to let work a nice cbt and this manual was helpful ...) and in fact this is a mix between from UAL PW and GE from DLH and including some mystakes due to wrong calculation with conversion from LBS to kilos ... im not in the secret this is what we found in that old time of simulation ...!!! and for the TAS this is exactly the UAL chart but at the beginning of 744 ops it wasnt .86 that was used so maybe a start for an answer ...!!! i definitly wanted to see a full FFPM manuals for planning those long range aircraft or use the FOC 2003 there is some nice profiles inside ... the one for the 744 is done with .86 .87 .85 and LRC cruise not that bad ...!!!see youPhil

[adamv66 0]

Hi Phil,So you're saying that the chart is basically wrong?I wonder if one of the PMDG developers would be able to provide the actual chart/algorithm that the aircraft was programmed with... I'm not a computer programmer but I'm guessing the queen must have been programmed to calculate how much fuel it would be using at a given configuration (weight/altitude/drag etc). If this algorithm/chart were provided we could prioduce our own, completely accurate, fuel burn tables....Adam

[phil747fan 14]

Adam,the FMS datas are taken from GE engines and then adapted for any engines combo used (i m speaking for FS9 ...) and nothing is different on fuel burn for PW or GE and not for F version of course. but for simmers this is very good ... for ops a little different.the datas are not wrong but this is adaptation, conversion from different datas and a compilation is always with some lost ...and im not into PMDG at all but i dont think they ll release any algorythm or charts better than the one you have because basically you can do a very good planning with that ...!!! philedit : i forgot to write down "Phil"