Fuel Flow Simvars not updating consistently

[MSFS 2024] Bug Reports Aircraft
1

Version: 1.7.27.0

Frequency: /Consistently

Severity: High
(Low - quality of life, workflow optimization, rare enough to not impact production, etc…
High - critical but workarounds are available, important feature not working as expected, frequent enough to impact production
Blocker - prevents from working on the project, prevents from releasing the product)

Marketplace package name: if applicable

Context: Aircraft package in devmode

Similar MSFS 2020 issue: insert url here if applicable

Bug description: The Simvar ‘ENG FUEL FLOW PPH’ sometimes pauses randomly for ~0.2 seconds, causing anything it’s being used to drive to also pause

Repro steps:

Attachments:

Private attachments: Send a PM to @PrivateContent with the link to this topic and the link to download your content

2

Hello @kcgb

I did not witness anything similar on our own aircraft so far.

  • “Aircraft package in devmode” → Does this happen specifically in edition context? Or this also happen when your package is loaded from Community?
  • Did you witness anything similar on any other aircraft?
  • Do you see anything suspicious in the Fuel System debug? Lines blinking, valves closing when they shouldn’t, etc…

If this is specific to your aircraft, we would be interested in a repro package.

Regards,
Sylvain

3

It happens in both community and devmode.

I haven’t had time to look at many other aircraft, but i at least dont see it in the default 737 Max

Almost all lines are blinking rapidly

Will send the package

4

image
image1569×797 107 KB

this is the intended fuel system setup

5

Hello @kcgb

This happens because your turbine is hitting a N1 rate limit.
There is a rate limit on N1 that is interpolated from a hardcoded table with these values:

0 % N1 -> 8.0 %/s
40 % N1 -> 15.0 %/s
60 % N1 -> 20 %/s
100 % N1 -> 20 %/s

When this limit is hit, the fuel flow PID is “paused” until N1 rate goes below the threshold again.
Pretty much every jet is hitting this limit at some point but usually this is on a very short period of time and isn’t noticeable.
This becomes more of a problem when use_commanded_Ne_table is used as this can lead to a steeper N1 rate, compared to the default implementation.
I’ve witnessed other aircraft with similar configuration demonstrate the same behavior, but they had glass cockpit with low FF update frequency so this was not as obvious as in your case with analog FF gauges.

I’ll have this case reviewed by our core sim engineers.

In the meantime, you need to adjust your turbine config so that N1 does not hit that rate limit, by adjusting the FF PID, adjusting your commanded Ne tables, or any other change that would slow down N1 on throttle increase.

I hope this helps.
Regards,
Sylvain

1 Like
6

Thanks for that, ill play with the PID to get the 60-100N1 range slowed down. I haven’t gotten to tuning the engine response yet anyway, so good to know if it keeps being a problem after that.

1 Like