[FUEL SYSTEM] line flow doesn't match engine fuel flow A:var

[MSFS 2024] Bug Reports Aircraft
, , ,
1

Version: FS2024 SU5

Frequency: Consistently

Severity: High

Marketplace package name: sws-aircraft-airvan

Context: Devmode and mounted

Similar MSFS 2020 issue: [FUEL SYSTEM] Fuel line flow always feeds engine at required flow - #4 by SWS-AlexVletsas

Bug description:

Continuing from the above topic, when using the modular fuel system, the ENG FUEL FLOW A:vars don’t match the flow of the feeder line. I am attaching a couple of shots to see for yourselves.

Line 7 is before the pump, Line 8 is after the pump and feeding into the engine.

Both screenshots are in full throttle setting, you can notice how the engine fuel flow stays the same, regardless of the feed line having dropped.
Before pump failure:

image
image508×108 2.64 KB

After pump failure:

Feeder line provides 14gph of fuel flow to the engine. the ENG FUEL FLOW GPH variable still reads 26gph.

image
image516×110 3.2 KB

In this case I would expect the engine to drop RPM and gradually stall, which doesn’t happen.

This is also a problem with priming the engine. When you open throttle, mixture and feed fuel into the engine using the electrical fuel pump, the engine fuel flow A:vars still indicate 0gph, despite the modular fuel system having correct flow values.

Lastly, this raises a question for flight planning: what is the correct fuel burn? The ENG FUEL FLOW A:var, or the FUELSYSTEM LINE FLOW:index?

Repro steps:

Start the Airvan (non-systemspulse version) and poll the variables shown in the screenshots.

State: Apron

  1. Fuel shutoff valve: open
  2. Fuel pump (overhead): on
  3. Mixture: rich
  4. Throttle: max
  5. Monitor fuel line and engine fuel flow A:vars and note the difference

State: In-flight

  1. Max throttle, max mixture
  2. Turn on the fuel pump
  3. Turn off the engine-driven pump
  4. Notice the mismatch in fuel flow. Fuel lines will have less flow than required, but the engine still burns at full flow.
[FUEL_SYSTEM] does not generate fuel flow when aux fuel pump is on
2

Hello @SWS-AlexVletsas

I must be missing something, when I trigger the main fuel pump failure from your EFB, the pressure drops from 53 to 25 psi but the fuel flow does not change:

image
image1706×783 377 KB

I am testing with sws-aircraft-airvan and sws-aircraft-airvan-systemspulse from your draft repository.

However I can already answer part of your question: in its current state, the modern fuel system interaction with the engine is very limited.
The engine will essentially just check that there is fuel flow, and that the pressure coming from the fuel system is above the combustion threshold defined with min_fuel_press_for_combustion_psf.

So even if the fuel flow from your modern fuel system drops below nominal value, this will not impact engine side until it drops below the critical pressure threshold.
If you want to go further, you will have to take control of some of the engine input and implement your own logic. In your case, you could write the RECIP ENG MANIFOLD PRESSURE.

That said, I agree this needs to be reviewed design wise, and we will log something in our bug tracking system. This would require a significant rewrite of the piston and turbine engine code so even if we decide to move forward, this is a long term subject.

Regards,
Sylvain

2 Likes
3

That’s the problem! Thanks for logging that.

4

Wasn’t the problem the fact that the fuel lines flow drops but not the engine one?

In my case, the fuel lines flow does not drop either. I do not end up in the situation demonstrated in your second SimVarWatcher screenshot.

1 Like