There have been some major fuel burn issues/irregularites on some aircraft for a long time. For example:
On the CRJ and A350 the most efficient altitude is very low (below FL350). This is especially inaccurate for the A350 because FL350/360 is often the initial cruising altitude of long flights.
The 757 has a very big jump in optimal altitude from 320 straight to 370.
The 747’s fuel burn increases from 300 to 340 (or somewhere around there) but then decreases again at higher altitudes at lower weights, and as such FL300 is always most efficient.
There is also the lesser problem of many aircraft favoring lower altitudes than IRL (The only aircraft types that I know don’t have this issue are the 767, A380, and to some extent 788*). Also most aircraft favor higher speeds.
Will the devs address these issues at some point and give us a more realistic flight experience? Especially, if the numbers in IF matched closer with those in simbrief it would be much easier to do fuel planning.
The 788 fuel burn seems to be constant above a certain altitude, and that makes it kind of favor the lower altitude since speed of sound decreases with altitude up to around FL300-400 depending on weather.
PS. It has long been a “feature” of IF that some aircraft have lower fuel burn than IRL and thus can fly for much longer (eg. 767, A350). I think it could be a good idea to allow these to stay by giving a larger fuel tank than IRL like on the B737 so that liveries that are only available on a shorthaul aircraft can fly longer routes non-stop.
Do you have data that you’re using to base these observations off of? The developers would greatly appreciated additional data if you’ve got it available. All of the aircraft you have mentioned had their fuel calculations pulled from real world data so anything additional that you could help provide may be of use.
That said, I can tell you that the CRJ numbers are near spot on. 😉 take it from personal experiences
I don’t have any IRL data, no. I based these off of what one would expect based on looking at flight tracks. For example, most A359 flights stay at FL350 or higher, so it is unlikely that the optimal altitude is FL340.
The lower optimal altitude observation is mostly from simbrief.
Is it possible that IF is using IRL data for engine fuel burn by N1, but the drag calculation is off? That would explain why some planes are more efficient in IF at a higher speed than IRL (eg. M.85 in IF vs M.8 IRL on the 757).
BTW is the CRJ supposed to have similar fuel burn at many flight levels? Because that’s what I observed when testing a while ago.
Can i get the source where you guys are using to set the fuel burn numbers for the A350 because the A350 is supposed to be more fuel efficient the higher u go but i do noticed that the most fuel efficient altitude tops at FL340-350ish and whenever i tried to follow my OFP the plane tends to burn more fuel than what is planned so i am curious from which source you guys get the A350 fuel burn data from
The best option here is what @DeerCrusher mentioned.
Get us some real world data to work with if you feel that we’re off :)
We’re usually more than happy to correct any errors that might occur, and these ones are usually quite straight forward to change.
We can’t share our sources for various reasons. But whatever they may be, is quite irrelevant if you don’t have anything to “counter-act” with :)
It doesn’t take real world data to see something is off though. Anyways this feels much more like the data is correctly, but the aerodynamic drag is off.
There is a specific problem on the A350 that is definitely a mistake in the data though, and I have pointed this out several times already. Above around FL360, there is a problem where in a range of N1 values, N1 increases while fuel burn decreases. This could be what’s actually causing the problems with the A350 specifically.
Fuel burn vs thrust data at FL300, 360 and 400. The linear trend clearly break between 80-90%, and at FL400 there is a clear error because the fuel burn goes down.
The issue with it not being more efficient at high altitude even doesn’t seem related to this. I decided to test agian just to check. Cruise at M.85 with 180tonne weight, FL340 and 360 uses just under 5000kg/h, FL380 uses 5060kg/h, FL400 uses 5130kg/h. IRL this weight wpuld be the last2-3 hours of a flight, and usually at this point you see A350s at FL380-430.
I was actually able to check some numbers while flying the B738 and A333. There is a LRC table for the 738 on stackexchange, and someone posted an A333 table on the forums. For both aircraft, I noticed that the N1 to fuel flow numbers were very accurate. However, I was using a lower N1 for the speed indicated on the charts. So what I assume is wrong is that the drag at higher speeds is a bit too low.
TBH this is kinda what I expected. The B78X and the very old A333 (when it’s max N1 was high) are literal concordes on casual after all.
