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Fuel X-Flow and Fuel Imbalance

(Original thread started on 07-22-09 by Shane Barnes)

Simulator: FSX

Issue Brief: No working Fuel Cross Flow

3rd Party Mods/Name?: Yes-Friendly Panels and 3d Lights Redux

Airfile or Model Mods?: Yes, various performance tweaks.



I was seeing whether the fuel X-flow worked in FSX and so far this is what I find. I set up the fuel payload with about 40 gals fuel in left tank and 260 in right tank.  According to the pilots manual I should get an amber CAS warning "FUEL IMBALANCE". This works as I did get this warning.


Now to the question: I know that the fuel X-flow switch is used to pull fuel from both wing tanks and according to the pilots manual can be used to correct an out of balance situation by selecting the heavy side (L or R STBY pump switch) ie: wing tank with most fuel in it.


I tried this in FSX and fuel xflow CAS will illuminate, STBY CAS illuminates but no fuel appears to be moving from one tank to the other.


Either this is not modeled in FSX or I am doing something incorrectly or misunderstanding how this works. Anybody have any input on this? It would be nice if this would work and add a little more functionality to our sims.


(Posted by Eric Tomlin on 07-22-09)

As far as I know, FSX only pulls fuel from all tanks at once. This in and of itself will prevent any fuel imbalance since it is from the time the engines start, pulling from all the tanks at once. The only time in our sims that it would be an issue would be if we did as you did and intentionally make one side more full than the other. I do know for a fact that without any interaction/intervention it will pull equal from all 3 tanks at once. It just happens to empty the center tank first because it holds much less fuel.


(Posted by Shane Barnes on 07-22-09)

Interesting, I wonder why MS modeled the "Fuel Imbalance" CAS alert in the first place. Well, until someone models the 40/45 in more detail we will make do with what we have. At least the switches do work and show up in the CAS and the STBY pumps have an audible sound.


I had some downtime between working on the shell and have been reading the Pilot's manual, probably will lead to more questions than answers.


(Posted by Ron Rollo on 07-22-09)

It's been a while since I have played with the left and right "STBY" switches on my flight box using FS9, but I seem to remember that if I select one of the STBY switches, it stops pulling fuel from that tank. I will test this and see if I can do it.


I just tested it and it does work in FS9. If I select one STBY, it pulls fuel from the opposite tank to feed both engines. I'm not sure if this function is available in FSX. You'll notice it if it is working. It's pulling a pound of fuel at least every five seconds one way or the other, depending if your climbing or not of course.


(Posted by Shane Barnes on 07-22-09)

Seems when I was testing, I noticed it pulling from all three tanks as well. I did not notice a difference when I had one of the STBY activated. I will do some more testing to see whether it stops pulling from one tank as Ron is seeing in FS9, it may be and I did not observe long enough to notice.


After additional testing, I don't think this is working in FSX. I tried opening x-flow then activating one of the STBY switches and I did not see any difference. The same amount was being pulled from both left and right tanks. Must just work in FS9.


(Posted by Mark L. on 07-26-09)

I found the same thing or we're both doing it wrong? However in the ERJ-145 for FSX, it works as expected.


(Posted by Shane Barnes on 07-26-09)

If it works in FS9 it has to be programmed into the Lear FS9 file somewhere. Is this file accessible to us? If so what is it called? And does the possibility exist of kinda kit bashing the FS9 and FSX file. I was reading somewhere last night on the Internet that FS9 aircraft files are usually compatible with FSX.


I think that it can be made functional by programming thru FSUIPC. I played around with it some last night and by using different commands gained some functionality. I think with the correct I/0 and programming the switches for multiple commands it can be made functional.


(Posted by Eric Tomlin on 07-27-09)

I got to test this and it is indeed working fine in FS9 and not (per the mini panel anyhow) in FSX. Not sure what's going on here, but I still have to ask the question of 'Why?' Why we would end up with a fuel imbalance is my question, seeing as the sim world is the 'perfect' world and unless you have someone working at an Instructor's station punching holes in your tanks, then you likely wont have a use for this. I do admit though, it was fun watching it work in FS9.


(Posted by Ron Rollo on 07-28-09)

To answer you question of why:

Right off the bat, what if you have an engine out in simulated flight and we have a long way to go to get back to an air field? It seems to me that we would need to transfer fuel because it wouldn't take long before the fuel would be imbalanced in the two wings. (I tested this in FS9 and the X FLOW is not working properly with one engine out as best as I can tell.)


Also, The APU pulls fuel from the right tank. I'm not sure if this is modeled in FSX, but if it is modeled correctly, we should be able to see this after we select R STBY according to chapter 5 in the Pilot's Manual.


There are other scenarios like fuels pump failure and accidentally selecting the X FLOW or STBY switches causing an imbalance. If it worked properly, we should get a CAS message.


In the end, this is a "bell and whistle" item not needed to get up in the air. But it sure would be cool if we could model it and get it working correctly!


(Posted by Eric Tomlin on 07-28-09)

That's all true in theory, but the issue is that I don't believe that you have to actually activate the standbys to get the fuel to pull from both even with an engine shut down and also there is no APU for FS9, hence no pulling of fuel and the APU in FSX doesn't pull fuel correctly at all I believe. However, if someone were to create a 3rd party LJ45, then it could be made to do so correctly. I will check to see about fuel imbalance due to 1 engine only running in FS9 and then APU consumption in FSX.


BTW, I'm not poo-pooing the idea of interfacing anything IF it works, but it makes little sense to chase something that either is not working correctly, or not modeled at all. Here's to hoping that someone, or some vendor, will come along and make a quality LJ45.


(Posted by Shane Barnes on 07-28-09)

The APU in FSX does not pull fuel from the right tank as it should, tried that one too. It really doesn't matter if we can't get it to work. I've just been trying to see how many things will work correctly whether we need it or not.


As for myself, I would like to be able to get as many systems operable as I can, the closer to realistic the better for me. May not need it but I can show that it works as in the real Lear. I guess it is all in how you look at it. We don't need realistic looking seats but we will go the extra distance to model them to look correct, just because in our minds it adds to the realism.


(Posted by Sheppard on 07-29-09)

This is exactly what happens if you have the Just Flight Rescue Pilot mission pack. You are flying back from Manchester to Luton and the left engine quits (it does this in another mission too). At this point you are still a good 100 miles away from the destination. I tried restarting the failed engine (which would not restart) and so turned the cross feed on so as not to get an imbalance.


Both tanks came down at the same rate. Interestingly with the cross feed turned off, they still seemed to come down at the same rate - ie twice as much fuel as you would use....I need to double check this tonight as the mission went drastically wrong shortly after (the plane suffered a "fatal facebook error" wherein the wife closed it to look at facebook) and I haven't had time to recheck.


Maybe that's down to my lack of tech knowledge but that scenario doesn't seem right?


(Posted by Rand Mathews on 09-03-09)

I have an issue in FSX with the fuel only draining from the right wing tank on the Lear. It will sometimes drain evenly after I make a stop at a fuel depot. But most of the time it will just drain from the right tank. No matter what switch combination I use on the fuel panel it doesn't help. It used to be that after I would refuel at an airport the tanks will drain evenly, but now that seems not to work either.


Has anyone else seen this problem? It's just not an issue with my FSX, I have a friend across town and his does the exact same thing.


(Posted by Eric Tomlin on 09-04-09)

So it only did it AFTER you refueled at a FSX fuel depot? I would say go to the Peter Dowson forums and ask him, as this sounds like an offset/internal flagging issue. Have you deleted your startup situation and created a new one? If not, try that. Also, try starting a flight in the C172 and then switching over to the LJ45 and see if it persists.


(Posted by Shane Barnes on 01-02-12)

I revisited this problem kinda by accident. I've been playing around with some GA aircraft and found out a little about the Lear crossflow.


First off, FSX model of the x-flow is broken (already knew that) but here is the offset that will give you the ability to select the fuel tanks on the Lear in FSX and have fuel flow only from the left or right tank or all three tanks at the same time.


The offset is 3880. The parameters are left fuel tank (2) right fuel tank (3) all fuel tanks (1)


I have this assigned to a rotary switch in my generic sim that I use for testing purposes. I can switch to left fuel and fuel will only drain from the left tank, switch to right fuel tank and fuel will only drain from the right, or select the middle position (all) and fuel will drain from all fuel tanks evenly.


As our flightdecks start taking shape you should be able to implement the above offset to closely simulate a crossflow. Eric T. and I have discussed this and he is going to try it with some different hardware that I currently do not have to see what results he gets. So maybe a workable solution has been found for this system. We may never need it but at least if we have a visitor to our sims and they ask "what does this switch do?" we can show them instead of saying this is what it is supposed to do but it does not work.

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