This looks like an error, as the engine should stall at the zero readings of the instrument.
FAR 23.1337:
Each fuel quantity indicator must be calibrated to read zero during level flight when the quantity of fuel remaining in the tank is equal to the unusable fuel supply
Does this behavior of the engine and the fuel indicator in the simulator correspond to reality?
It is a real world issue that GA fuel gauges are highly inaccurate, often giving indications up to a quarter tank misreading.
14 CFR 23 describes the certification standards of airplanes. Sounds like you’re aware of this already. However, the “long-historied” airplanes in the GA world are governed by a mix of CAR 3 and 14 CFR Part 23 (as components get modernized).
From a manufacturing standpoint, these instruments are indeed calibrated during completions. However, once they get into real world, deviations happen.
This is why we have procedures as specified in 14 CFR Part 91 that requires that we do flight planning and validate fill-up in Preflight - External checklist with visual inspection prior to engine start. From there, we need to make sure that we’re rotating tanks (single feed planes) and that our indicator is “roughly” in line with what we’re expecting. Something wildly off can be an indication of another problem.
The indication is a mechanical tool that has its own faults. The mechanical bob within the tank on a GA airplane is not precise enough for your engine to run out of fuel at exactly the 0 indication.
As an aside, in real-world no pilot would ever operate in this bingo-fuel scenario. My personal minima is to land if i’m indicating 15-20% fuel on board, as there are dregs in the tank that are unusable (AFM weight/balance should have an account for this). Then I would expect a slap from the FAA as to why I was operating in the national airspace with fuel insufficient for the flight. Under instrument flight rules, I might be running afoul of requirements for diversion and IFR reserves.
Yes it does correspond with reality. I am an instrument rated pilot and an aerospace engineer who formerly worked on several of the airplanes that are included in MSFS.
But, FS is not real world and gives us the opportunity to play with or experience things we are unable to do in real world. It is realistic to be playing a guessing game as to when you will actually experience fuel starvation.
IIRC the 172 has a 56 gallon tank, 53 gallons usable; this sounds about right for 5% fuel leading to cutoff.
I checked a copy of the PIH for the 172S I have floating around and I think you’re right though that the readout is wrong – it should be reading only the usable fuel, I think, rather than total fuel, and so 5% should read at the red line at 0:
@trex5365, thanks for the detailed answer! It was interesting to read it.
I am not a real-life pilot, but I think it is hazardous in real life if an engine stalls when the fuel indicator is not zero. I guess that is why my motorbike can work several decades of minutes at the instrument’s zero readings. I understand that I do not have any fuel for this reading, and I should do something as soon as possible.
The story that led to the writing of this post: I got lost and made a short detour to the airfield, spent more fuel than I expected. Having found the airfield, I checked the fuel gauge readings, and I made sure that I had fuel for 30-45 minutes of flight (5-6%). I did not take any action to save fuel and las as soon as possible. On the contrary, I began to make a U-turn to enter the runway against the wind. Suddenly the engine stopped when I was far away from the airfield…
I would have acted entirely differently if I had seen zero on the fuel gauge. It would be more helpful “fortunately” the engine was running at zero readings than stalling at the moment when I was sure that I had at least 30 minutes of engine operation left.
Interesting story. Fuel endurance and range assumptions are based on straight/level.
If I were showing even 5% higher, I’d be trying to get on the ground ASAP. When your tanks are that low, all it might take is turbulence (or a banked turn perhaps?) to disrupt the flow of fuel into the engine, causing fuel starvation.
During a turn, gravity might pull fuel away from engine inlet within the tank. If this happens, results might be catastrophic. Very happy to see that this airman was able to tell the story.
