Fuel transferring from AUX tank to main left??? That can't be right - [Yep it is right!]

[louism]

Ok, so I ended up completely fuel imbalanced on the ground and I tried to save the day.

I managed to pump fuel into the tail tank to remove a AUX fuel heavy message.

But I still had way to much fuel in the left main because I had been running the APU for too long.

So I turned on the L to Aux pump and noticed in the study menu the fuel was going from AUX to main left. What? I thought it was supposed to be the other way around?

So I switched to R to Aux pump and sure enough it was pumping fuel from the AUX to main right. (See attached screenshot if you are skeptical! You can see the Right-to-aux transfer Q value is negative) and sure enough fuel quantity was increasing in the main right tank. In the screenshot here it was almost full so the GPH velocity is very small but it started much higher.

 

When I was fueling the tail tanks to resolve the imbalance, I realized I did so with the fuel/defuel panel in the defuel mode (because I wanted to defuel before I realized it is not an option and I forgot the switch in that position). When I was refueling, I only opened the tail tanks valve, not the aux tank valve. Yes, I know I'm not supposed to do that, but that's the only way I could get fuel going in the rear tanks. Maybe that's what triggered weirdness?

Further testing: created a new airframe while still in this plane, went in the FBO, loaded the ne airframe and I have the same behavior. Here you can see the very high negative fuel flow and I took 2 screenshots so you can see the main right tank filling up. I also show the pump buttons.

More fuel in the main right after a little while:

The only I could resolve the problem was to create a new frame in a new x-plane session. 

Here it is working ok:

I am keeping a saved state for a while in case it is useful for debugging.

Edited January 14, 2022 by louism
Remove [bug] tag - edit title

[louism]

@Graeme_77, just want to make sure you see this.

THIS time, I'm sure it's a bug and not me! (famous last words... )

[Graeme_77]

That’s a very interesting question, and one that I had during the test phase. And no, it does not appear to be a bug, as crazy as that sounds. Will verify with the consultant pilots, but at the moment the understanding is as follows.

Firstly, there are no check valves shown in the system, so if the Aux level is physically higher than the wing level (which it can be with a full Aux tank) then fuel can indeed flow from Aux to the Wings when you open the L(R)-to-Aux valve hoping for fuel to flow into the Aux. It is purely a gravity system.

Let me explain why it shouldn’t ever matter. Fuel only goes to the engines from the wings (via the small collector tanks), so Aux fuel is pumped to the wings. The system needs a bit of space in the wings for that Aux fuel, so the automatic Aux to wing feed happens when the wing tank is 93% full.  Should you end up with one wing tank at 100%, and the other at 93%, that isn’t enough to trigger an imbalance warning anyway, and then the automatic aux to wing transfer will begin on the lower tank keeping it at 93% while the other wing fuel is used by the engine with no “top up” from the aux.

The aux fuel is effectively pumped into the wings keeping them both balanced in normal operation, so there should be no need to use L-to-Aux or R-to-Aux for balancing when there is any fuel in the aux at all. As a result, this reverse flow situation, while technically possible, should not be encountered.

if you do have a wing imbalance with fuel in the Aux, consider the gravity x-flow to balance them instead. You can fly with a tiny bit of rudder trim keep the light tank on the low side to make sure the gravity flow goes the correct way.

Edited January 14, 2022 by Graeme_77

[louism]

I was convinced it was a pump (even though I was staring at Toto's diagram), but now I realize it's because I was confusing that valve with the transfer ejectors that supply the mains from the AUX.

I never realized the AUX tank's top went higher than the wings. Given that's the case and it's a valve, it is logical that the flow goes to the main if the AUX is full. This matches with what is happening in my diagrams. You can see that in shots 1, 2 and 3, the AUX is 94%, 93% and 92% full and the flow is going to the mains. The AUX is empty in my last screenshot and the flow is going to it.

It is very confusing because the wording in the 605 manual available online is "Pushing the L (R) TO AUX XFLOW switch/light opens a valve which allows fuel flow from the selected wing tank into the auxiliary tank. Fuel is then evenly distributed by the transfer ejectors to both LEFT and RIGHT main tanks." and it's exactly what Toto said in one of his streams as he was rebalancing his fuel in flight.

It mattered at the time because as I played with the systems for hours, I kept the APU running and I ended up in a situation where I had a huge imbalance between right and left which I had to fix before T/O. I still have a problem because I planned a short flight and now I am 6 tons above max landing weight.  I guess I'll fly close to VNE at 12,000 ft to burn fuel like crazy! 

Toto's systems make you learn the hard way and I love it. 

I'm looking forward to the consultant's confirmation. Please, send him all my thanks for his awesome dedication to the project as you mentioned in your marathon stream.

3 hours ago, louism said:

THIS time, I'm sure it's a bug and not me! (famous last words... )

Famous last words indeed... I'll remove the BUG tag for now.