At what turbine RPM are IF fuel flows calculated?

Flew a Cargolux B748 from KIAH to EGPK (Houston to Prestwick, RL flight) last night, but only made it to 40 NM out of EGPK before I woke up finding myself stalling out of 10k on empty.

I took off with my own calculation of 9:25 in fuel, and the trip was to last 7:50. It turns out I was closer than expected when I woke up.

My weights:

BOEW: 465.4
Cargo: 119.2
ZFW: 584.6
Fuel: 175.2
TOGW: 759.8
LGW: 616.7

Judging from IF’s fuel figure for the plane, I should’ve had 9-10 hours on board. I did cruise at FL370, and my turbine RPM was hanging around 95-98% as I recall. If it’s calculated @90%, then in the future, I’ll plan lower to accommodate that at this weight.

Someone have info? Thank you!


I am assuming that 175.2 is thousands and of pounds or is that just hundreds if it’s hindreds that’s wat below reserves and that’s your r reason.

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Here are some tips to not run out of fuel:

  1. Step climb. Gain altitude as you burn fuel. Here’s a guide to that:
    A Guide to Step Climbing
  2. Always make sure you’re using the correct fuel planning. Here’s an awesome tutorial by our very own, @Brandon_Sandstrom:
    Flight planning series | Part 4 Flight plan & fuel planning
  3. Use trim while flying. You want your nose to have a very slight pitch while flying.
  4. Don’t make it so that you are extremely overweight. You can use @Chris_S’s website:

Your N1 shouldn’t be above 100%, under any circumstances (unless it’s an emergency). If your throttle was at 98%, then check all the above tips.


How fast were you flying? Did you do a step climb?

The 747-8 does this a lot. Just to test, I loaded one up (IF says almost 24 hours flight time), took it up to 28000, and it was getting like 13 hours trimmed.

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That is a terrible site to get fuel calculations from.


Simbrief does proper flight plans and for KIAH-EGPK with 747-8 selected at the weights you list it shows block fuel around 205,000 lbs. This is based on the calculated optimal cruise speed of M0.85.

When you say your own calculations, how did you arrive at a figure of 175200 lbs? This seems too low.

I took my FOB time from simbrief, and multiplied that by the fuel flow rate provided in IF. Has worked like a charm with about 1.5-2 hours remaining at the termination of every flight I’ve done (in other planes, of course).

That is based on their default fuel flow for the 748 provided in the aircraft list. It can be corrected for IF based on what you enter in for the fuel factor. I’ve tried using the simbrief planner for other planes and I end up either seriously overloading or underloadinf fuel with the default flows. Maybe the 748 is a different case - at 14-15k PPH, an extra 30k pounds isn’t much.

I should also add that I followed the cruise figure provided in my plan as well - M 0.85

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I don’t understand, why didn’t you just take the block fuel figure if you did a plan in simbrief?

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VI like doing things myself :) haha. To each their own.

Anyway, just did an experiment. As it turns out, at my cruise altitude and power to hold M 0.85, I would be burning 29k PPH. Awful close to the red area in terms of thrust too. But that is almost required, as to not hit Vs (min maneuvering speed). In this scenario, I would need 273k for block fuel to last 9:25, and would expect to burn 227.2k pounds. So even the simbrief number wouldn’t be sufficient - I would still run out.

I set the thrust to the IF FF Figure, and the plane started slowing through 180. Descended to FL310 and it still wasn’t holding speed nor altitude.

Yes, the weight has stuff to do with, too, but I would think with a 14k pound difference in fuel flows, you can’t really fix the issue by making the plane lighter. That’s too big a difference. Also I should make mention of the point that lower altitudes = denser air = more fuel required to hold an N1 setting. And during climb, we climb into thinner air, requiring more thrust, which is more or less like holding a set fuel flow. (Fortunately makes climb planning easier…). I’ll try a lighter load at some point today as well.

Your fuel flow rate during your flight is not the same all the way through. It is significantly less on the descent for example.

The simbrief calculated value would have been fine. Trust me when I say simbrief knows more about fuel calculations then you do, and having used simbrief for many years it has never left me short of fuel.

EDIT: rule of thumb fuel flow for a 747-8 is around 21000lbs ph (found this out online) so for a 9.5hr flight a back of the envelope calculation would be about 200,000lbs which is more or less exactly what i mentioned simbrief actually calculated in the flight plan I did earlier and mentioned up thread,

I really do not try and manually do fuel calculations. To the extent I do a check it is to use the rule of thumb fuel burn figure for the 737-800 which is what I typically fly (around 2.7T ph) multiplied by my expected flight time to see that the figure I am given my simbrief is roughly what I expect, so if I am doing a 3hr flight I am expecting to see a figure of around 8.1T block fuel, if it’s significantly off that then I will recheck as something is wrong.

Does N1 measure the RPM’s of the turbine fan? Which fan? Does it average all the rpm’s of the compressors?

N1 is related fan speed, i.e. the big one at the front of the engine and the low pressure compressors which are all on one spool.

The high pressure compressors and turbine are on a different spool and that is expressed as N2.

The two spools (there are three spools on a RR engine but let’s not complicate things for now) operate separately.


Interesting information! I may ask more in a PM. Thanks!

Ok, did some more investigation. I was in the wrong. Next time I’ll have to use a cruise table. Turns out, I should’ve been at FL310/330… FL370 is above the max cruise alt for what my plane weighed in at. I set my TOGW to about 680k lbs and flew according to the attached cruise altitude vs. weight chart. Set my FF to about 21k PPH and ended up managing a stable a cruise speed of M 0.86.

However, IF still needs to enter the proper FF figure into the AC menu… setting that FF still stalled out the plane.


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I am still confused about what you are doing? Why are you trying to set fuel flow rate?

You do not set a fuel flow rate on a jet engine when flying, you set (or the auto throttle sets) a required thrust level to maintain, accelerate, or decelerate to your target speed, and the fuel flow rate results from that.

If the fuel flow rate is higher or lower than what you expect it to be for that thrust level at that point you might start getting concerned since it will mean something may be wrong with the engine or you have a leak. ON any flight plan you will see there is an field showing expected fuel level at every waypoint and you are supposed to check that against what you actually have left to see whether actual fuel use matches what the flight plan states it should be - that is one of the ways you can find out if you have a leak.

If you find that the thrust being commanded by the auto throttle during the cruise is very high then it will be because you are too high, too heavy, and/or have very strong headwinds.

I know it can’t be set exactly. I wouldn’t fly like that ever. But for the sake of trying to figure this whole thing out, it is still a factor in the equation. Currently trying another transatlantic from IAH to AMS with the simbrief fuel load that was provided. I was 700k or so TOGW, and so flying at FL310. M 0.84 was the cruise Mach, so that’s what my speed is set to. Last time I saw my ipad, I was burning 23k PPH.

As noted above, at every waypoint there are estimated fuel on board firgures on the flight plan with space below to record your actual fuel on board. Perhaps you could record them. It would be more meaningful than focusing on fuel flow as this is not a parameter listed in the flight plan.

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