Good evening, fellow pilots. I just ran into a bit of a head-scratcher. It could very well be something I did, since I have not had the time to do much flying lately and may have missed something in my cockpit flows. I was attempting a flight from GCLP to LEBL, and everything was fine until climbing through FL130 when a warning horn began to sound and a Fuel Imbalance was indicated in the #2 tank (it was already almost 1,000 lbs less than the #1 tank). After landing back at GCLP, with almost a 2,000 lb difference between the wing tanks and a little over 11,000 lbs in the center tank, I checked the failures to clear them, only to find that there were no failures active. So, now I'm wondering what happened to that missing 2,000 lbs of fuel in the #2 tank, and what (if anything) I may have done to make it go somewhere else. The crossfeed valve was closed, and all fuel pumps were on at the time the warning horn started to sound (I turned off the #2 pumps just to make sure I wasn't pumping fuel overboard, and the fuel level continued to decrease). The lack of failures makes me suspect that I may have accidentally done something (or not done something) to cause this to happen. Any thoughts or suggestions would be greatly appreciated.

That's a strange one if you had failures turned off. With all fuel pumps on, you should not have been using any fuel from either wing. The center tank pumps will override the wing tank pumps, so that wing fuel levels should not decrease as long as the center tank pumps are on. Perhaps a bad panel state file, or something went wrong during initialization when you first loaded the NGX. When you were on the ramp, did you load fuel using the menu in the FMS? In the NGX, one should never try to load fuel or passengers/cargo using the standard FSX loading menu.

I use service based failures for all of my NGX flights, but what I meant in my OP was that when I checked the failures in the FMS after shutting down, there were no active failures that needed to be cleared. If there was a fuel leak, I would have expected to have seen at least one active failure in the fuel system. I loaded the default cold and dark panel state before the flight, and did use the NGX fuel and payload loading system instead of the FS system.

You got a warning horn out of 13,000? Sounds like a pressurization issue, too. There's no horn for fuel imbalance.

Yes, I wasn't very clear. The warning horn drew my attention to the fuel imbalance, but the horn was because I hadn't turned the packs and bleed air back on after starting the engines. I did mention I was a bit rusty. :(

Is it due to APU fuel burn?

No, I shut down the APU once both engines were started and running. I actually feel quite stupid for switching the electrical system over to the engines and forgetting about the pneumatic system.Anyway, I'm not sure the APU would burn off 2,000 lbs of fuel in half an hour.

Good evening, fellow pilots. I just ran into a bit of a head-scratcher. It could very well be something I did, since I have not had the time to do much flying lately and may have missed something in my cockpit flows. I was attempting a flight from GCLP to LEBL, and everything was fine until climbing through FL130 when a warning horn began to sound and a Fuel Imbalance was indicated in the #2 tank (it was already almost 1,000 lbs less than the #1 tank). After landing back at GCLP, with almost a 2,000 lb difference between the wing tanks and a little over 11,000 lbs in the center tank, I checked the failures to clear them, only to find that there were no failures active. So, now I'm wondering what happened to that missing 2,000 lbs of fuel in the #2 tank, and what (if anything) I may have done to make it go somewhere else. The crossfeed valve was closed, and all fuel pumps were on at the time the warning horn started to sound (I turned off the #2 pumps just to make sure I wasn't pumping fuel overboard, and the fuel level continued to decrease). The lack of failures makes me suspect that I may have accidentally done something (or not done something) to cause this to happen. Any thoughts or suggestions would be greatly appreciated.

Hello Tim,If i remember correctly, even you turn off the fuel pumps, engines can be fed by suction feed in NGX. Therefore, you should turn crossfeed valve on while turning off the low side pumps to overcome the fuel imbalance. This might be the explanation of fuel level decrease after turning fuel pumps off.EDIT : Sorry i missed that you still have fuel in the center tank and center pumps are on. In this case my scenario is not valid of course.Regards,Suavi

I have this happening to me my last 2 flights now. The fuel in tank 2 stays where it was set at during preflight and the center tank os using fuel like normal. Tank 1 fuel is gaining/climbing. I start the plane with the stock cocpit/FMC set up with engines running and do not have failures turned on. The fuel gauges read imbalance on tank 2.I'm going to fly the tutorial flight and see if it still happens.

I found that if I selected my fuel load at the preset 1/3, 2/3, full settings it did not have a problem. but if I set my fuel for each tank then I would have the issue.So what I did to make it stop in flight, I switched the #1 FWD fuel pump off and it made the #1 fuel gauge stop climbing/ gaining fuel. I have no idea why this just started this weekend and did not occur ever in the past.

The problem is called fuel scavenge system.The system is installed also on the real plane but in NGX doesn't work properly creating a dangerous unbalance. Few things:APU suction is from tank 1, if crossfeed is closed te left tank will feed the APU when it's running.So the APU will not create more fuel on the left tank.To crossfeed an engine you must open the crossfeed valve and turn off the pumps of the tank with less fuel, this will let the pumps of the opposite side to feed both engines (There will be no direct suction from the engine pump, the boost pumps of a single wing are sufficient to feed both engines. Now back on th e scavenge system:The scavenge system allows fuel to be transferred from center tank to the left tnk only. The transfer is fully automatic and cannot be directly controlled except by turning off the forfard left boost pump.This transfer will start for fuel qty of 1000kgs or less in the center tank if the left wing fwd pump is on.On the pmdg the errors are 2:(pmdg) The fuel transfer starts only in flight (real) the fuel transfer works either on ground or inflight as the valve is fully mechanical and opened by the fuel liquid.(PMDG) fuel flowing in the wing is directly related to the engine thrust, this means that at higher thrust we will have more fuel flowing from center tank to the wing, this will let the wing tank qty to increase also if engine is burning fuel. (real) The fuel rate is less than 300kgs per hour and the rate is almost fixed.I must check the hotfix if solved the problem.

thanks for the explanation Andrea, that makes sense.

Why other users probably don't have that problem? Because the scavenge system requires that there is "space" in the wing to fill it, so if you have full wings, and still fuel on the center tank (with pumps on) the fuel in center tank will be burned first. As there is no simulation of low pressure before reaching 0kgs only few users can find this feature/issue.You had this problem because you turned off the pumps of the center tank before it was empty. I tried to see if it was corrected after HF4, for my system the answer is yes and no."Yes" because there is no change in the rate during flight or ground or when changing thrust, "no" because the actual scavenge rate on my system is 0, it appears pmdg deactivated that feature or is not working on my system. Can PMDG confirm what they did on the latest hotfix?

OOOPS, correction, I was forgotting somethings...Ok, the valve that let the center tank scavenge start opens at a determined fuel level on the left wing tank, not ctr tank.Maybe that te value is around 2000Kgs or something less.So:If you have remaining fuel in the center tank (in the real one the reason is that fuel pumps are not able to drain all the tank) it will be transferred by the scavenge system to the left wing tank ONLY if the quantity on the left wing tank is less than 2000 or 1900Kgs. The transfer rate is as stated above, always less than 300kgs/hour.I'm going to test again with lower wing fuel.

I had an enexplained massive fuel imbalance also - have done 100 odd flights and never experienced this and out of the blue it ocurred. Corrected it by crossflow etc and all worked nicely following the procedure it was just bizzarre why it occured.