Version: 1.5.27.0
Frequency: Consistently
Severity: Blocker
Bug description: Our 2020 modern fuel system is very complex and works perfectly in 2020 and was working perfect in SU2 for 2024. Now it seems to be having flow issues and flickering in the debugger. Flipping the switch to fill for the fuselage is not getting fuel from both tanks anymore. When I move the switch in the video only a single line was flickering but i get different results each time and sometimes many lines flicker so hard to tell what it going.
You can see our logic from our Learjet35A 2020 product.
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The first thing you see in the video is I left the igniter on as this keeps the engine running during the bug. If i turn it off the engine will flame out.
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Next I show the transfer from the fuselage to the wings and all works fine. Then I reverse the directions and then you see all the flickering on some of the lines. Which will cause a flame out.
This is a issue we found a few years back when you have pressure coming from two directions and then trying to lead to a single tee. See #1 details on the left in the image.
Typo in image…Line #3 is actually #13.
So our issue is we have a crossflow valve on the left side of the FuselageXferTee. And because of this it causes pressure issues. We found a work around and has been working for a long time until now. Be great to figure out how to get this to work properly.
You’re using a set fuel system version number that hasn’t changed, i.e., not using “latest” in the fuel system inputs?
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I am using latest. I tested my 2020 version today in 2024 and it also fails. Maybe I changed to latest in the last update. I will look into this.
I now see that Latest might be using the new improved realism for fuel pressure. Here is a summary that AI explains the difference.
Summary of what’s happening:
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In your older Microsoft Flight Simulator 2020 setup, you had T3 (FuselageXferTee) feeding downward to the fuselage tank and also connected sideways through V4 (CrossFlowValve) toward the opposite wing.
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That created a loop: pumps on either wing could both push toward T3, but one pump’s output path ran past T3 and directly toward the other side.
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In Microsoft Flight Simulator 2020 this often still “worked” because the simulation didn’t model pressure conflict realistically. Removing the T3→V4 leg broke the loop, and the system behaved.
What’s different now in Microsoft Flight Simulator 2024 (post-SU3):
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The fuel system now actually enforces pressure-based directional flow at junctions and valves.
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With equal pressure from both sides, flow will stall (no net movement).
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The left side gets isolated because:
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Any attempt to send fuel from left wing → T3 hits pressure from the opposite side (via V4).
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With your current layout, there’s no path for left fuel to get pulled unless crossflow is open and the pressure differential is correct — which rarely happens when both sides are pumping.
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In short, the sim now respects the “rule” you wrote in your own note:
“Valves when open can only flow the direction that has higher pressure… Equal pressure will not flow.”
Putting the version to 1 fixed the bug. I am aware that versions and major’s make a difference especially for the electrical. For some reason this did not register in my brain for our upgraded project since we designed the modern fuel system a while back for 2020 version. So, thanks as this solves the issue. I am planning to see if I can get my current 2024 project to use Latest by redesigning the shared crossflow tee.
I moved the topic to questions & community discussion.
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Keep in mind that setting your version to “latest” means your plane will likely break in a future update when there is a new system version for latest. It’s better to use a specific version number.
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