[No Bug] External fuel tank issue?

[Mud]

I noticed that the centerline (did not test with wing tanks) is empty almost immediately after enabling the fuel transfer.

Test scenario:  Take-off from runway with full 600 gallon centerline tank. Once airborne switch on the fuel transfer to CENTER and you can see the fuel increase for a bit on the fuel quantity indicator.  Within a matter of seconds the warning light comes on that the external tank is empty.

I'm not positive if this is a bug or something I am missing.  Track has been attached.

Br,

Mud

Ext_Fuel.trk

Edited May 31, 2024 by HB_Painter
Classification of Bugs and User reports

[Zabuzard]

This is correct behavior. The tank is not empty yet. Kindly see the Fuel chapter in our manual. Thank you.

The tanks only transfer for a short period to fill up the infernal thanks, then they stop again and will start transferring later again when the internal tanks lost sind fuel again. Each time this happens, the lamp will go on, indicating that nothing is currently transferring.

To confirm that the tanks are empty, the lamp has to stay on for around two minutes without interruption. As well as your fuel totalizer will drop below a certain threshold.

[Mud]

Ah splendid!  Thank you, Zabuzard!
I had read that section of the manual, but clearly not well enough

[Zabuzard]

No worries. The fuel system is complex and this part specificially is confusing. You are not the first and wont be the last to step into these pitfalls :)

[markturner1960]

On 5/26/2024 at 3:41 PM, Zabuzard said:

The tanks only transfer for a short period to fill up the infernal thanks, then they stop again and will start transferring later again when the internal tanks lost sind fuel again. Each time this happens, the lamp will go on, indicating that nothing is currently transferring.

To confirm that the tanks are empty, the lamp has to stay on for around two minutes without interruption. As well as your fuel totalizer will drop below a certain threshold.


 

Sorry Zabuzard, I am also having trouble fully understanding....can you clarifythe above for me? So the external tanks will automatically transfer their fuel several times until empty. Got that. Do the lights come on when transferring ? ( above,  you say the lamp "on" shows nothing transferring...this does not sound right, would mean they are on all the time except when transferring..?In the manual it says the opposite:  Automatic fuel transfer, once activated, can only be deactivated again by cycling the Air Refuel Switch. This is important, as most switches on the fuel system panel are ignored while activated. An indication for the automatic transfer being active are all three external tank fuel lights being lit regardless of their actual status.) How much do they transfer each cycle and at what level in the internal system is this initiated? 

Regarding the manual passage I quoted above, thats confusing as well...it says auto transfer can only be de activated  by cycling the air refuel switch - does that mean simply that when the door is open no auto transfer happens, then when shut, it will carry on doing it cycle of partial transfers until the tank is empty? IE a temporary interuption of the cycle rather than a permanent one? 

And what is the threshold the totalizer will drop below to show tanks are empty? 

Apologies, its all rather confusing as you say....I have been reading the manual section and to be frank, its scrambling my brain...What would be very useful would be a summary of a hypothetical flight, with the jet loaded with 3 external tanks all full. The summary would explain what happens ( which warning lights come on and when, which switches are activated etc) at what point in the flight as the fuel is used, right down to say 2000lbs , then the jet refuels completely again. That would be really helpful.....I am trying to write it myself, but there are too many bits I dont understand or am unclear about to do this......

Edited May 31, 2024 by markturner1960

[Zabuzard]

Okay, so first of all, the ext tank lamps on the warning panels indicate that the fuel valve to the external tank is open while nofuel flow is sensed.

Whenever something opens the valve but nothing flows, the lamp goes on. The question now is, when is thr valve opened and when closed.

Let's assume normal operation. The corresponding valves are opened the moment you put the triangle switch to outboard or center, commanding fuel from the external tanks. So lets say you put it to CENTER.
Now, imagine that the tanks (1, 3 and 5) your external center tank is connected to are currently full. Obviously in this case nothing will flow from your center tank, as there is nothing to fill up.
The valve is open, but no fuel flow, lamp goes on.
After some time, tank 1, 3 and 5 arent full anymore and the center tank starts transferring. The lamp goes off.
However, the ext tank transfers so fast that after just a few seconds, tank 1, 3 and 5 are full again. Transfer stops, lamp goes on again.

Under normal conditions, the indications can only show you ext tanks empty if the lamp stays on without interruption for around 2mins and the fuel totalizer is going down without being filled up. (The fuel that matters is tank 1, 3 and 5, but u cant see them directly)

Let's now talk about non standard situations. Even if you do not have the triangle switch set to any external tanks (or perhaps don't even have ext tanks loaded), some situations will cause the valves to open.

Whenever you have the AAR door open, all ext tank valves are opened. The same happens when the fuel in tanks 1+2 go below a certain threshold (iirc 2300 lb), which happens somewhere around 3-4k lbs total indicated (unless u trapped fuel in a wing tank, then more). In this situation, the plane activates an emergency mode called "automatic fuel transfer" which overrides your switch settings and force-opens all valves (including the ext tank valves) to force any trapped fuel to transfer.

Recall the the lamps go on whenever the valves are open but no flow is senses. That means that when you do AAR or run ibto this low fuel situation, you are likely to see all 3 lamps to illuminate.

To sum it up, the lamp indicates someone wants fuel from the ext tank but is currently not getting any fuel from the ext tank. But you can't differentiate that directly from the normal situation that the internal tanks (1, 3, 5) are currently all full enough and hence don't accept fuel.

I might also add that unless you ran into an abnormal situation or are flying neg G full AB, tank 1 doesn't play that much of a role in the above, since it is directly fed from tank 2, 4 and 6 as well. So its usually always full.
Tank 3 and 5 however only get refilled from external or wing tanks. So their status primarily dictates when your ext tanks can actually fill up something in practice.

[felixx75]

https://youtu.be/2u2yLEM1iHM?si=6wzagwLsTTHs5tZn

[markturner1960]

Thanks both, video was very helpful ...