We start out our MSFS 2024 flight in the LearJet 35a going from KFNT to KAMN. Since it's a short flight (47 nm) we'll cruise at 8000, averaging 200-250 knots.
View the spreadsheet calculations for this analysis.
First, we load the jet in FSE, 50% fuel, 467 gal.
Next, we start the sim, load cold and dark get in the jet and then start the FSE client and start. This automatically sets the fuel by percentage in the aircraft. We cross-check by opening the MSFS 2024 EFB and checking fuel state (50%, 466 gal, total mass 13481 lbs)
Next, we cross-check that with the fuel state in the FlySimware EFB (fuel 3080 lbs (~50%) / max fuel 6240 lbs total weight 13481 lbs)
However, when we started the FSE client, it shows this screen right after setting the fuel percentage:
We can clearly see the discrepancy. the current weight (13481 lbs) was verified by both the MSFS and FlySimware EFBs, it represents what the sim really thinks 50% fuel looks like. even the numbers for the tank max fuel match. The FSE client agrees with this weight, but then disagrees about the required weight: 12938 lbs. If ZFW is 10361 lbs, then this represents an expected fuel weight of 2577 lbs (-503 lbs)!
So FSE thinks that the LearJet 35a should weigh only 12938 lbs at 50% fuel, hence the discrepancy of 540 lbs (+/-8.0% error) over the FSE expected weight. So let's see if that math checks out. We know from the FSE screens and the fuel EFBs that the gallons of fuel are all in agreement. Using an E6B fuel weight calculator we check 466 US gallons of JetA: 3,122 lbs, which is really close to the 3080 lbs (+42 lbs, +/-0.6% error). So FSE is wrong.
There is another hidden value we can't see. ZFW in FSE. In the FlySimware EFB, we see empty weight as 10361 lbs. FSE isn't so direct, but we can look up the aircraft details here
This chart has 1016 kg passengers and cargo across the board, so it's hard to tell what this means. But lets take the columns for 50% fuel and 100% fuel. Capacity changed from 2608 kg to 1359 kg, implying that we took on 1249 kg of fuel between 50% and 100%. Using the E6B calc again to convert 1249 kg of JetA to gallons and we see 447 US gal. So at least as far as FSE's web site is concerned, we are in agreement about what 50% fuel means and weighs.
So why does the client still report an 8% error in weight and expect 540 lbs less than is correct by multiple sources?
Is the FSE client is using the wrong fuel density for the weight calculation and not using the website numbers. Let's check if they got it wrong. We seem to all agree on 446 gal roughly, so what if that was a different density? could we account for a 540 lbs difference? Even if we use Avgas, we get 2889 lbs (-191 lbs) which isn't close to -540 lbs. So this seems unlikely.
Another reason on the forums is something about multitank aircraft causing problems for FSE, since it can only look at one tank? However, if we look back at the FSE initially loaded tanks:
it doesn't really make any sense. none of the weights of the separate parts add up to 540 lbs.
Well, we know that the calculation is off, but not why. In the meantime, we need to fool FSE into thinking that it was correct, so the easiest way to do this is to meet the required weight by adjusting the fuel down manually until we reach a percentage that matches the expected (we're ignoring passengers and cargo for now).
As we said before, we need to look at the Total mass in the MSFS efb and move the quick fill slider until the numerator is as close as possible to our target weight of 12938 lbs!
pretty close: 41% fuel, 382 gal, 12920 lbs.
we take care of the remaining weight by adding a suitcase or two.
Now, it's time to test and see if our work pays off.
Ok, flight was good, we're down and parked, let's check our MSFS fuel state before setting the parking brake (which tells the FSE client to either accept or reject our flight before we can do anything grrrrrr!)
Our numbers for the flight:
- takeoff: 41% fuel, 382 gal, 12920 lbs.
- landing: 36% (-5%) fuel, 339 (-43) gal, 12634 (-286) lbs.
And finally the moment of truth. Set the parking brake.
It works and was logged!
In short, if you see a required weight mismatch, don't ignore the loading in the Lear 35A, instead adjust the fuel down to match the required weight regardless of what the percentage is.