Loosing Fuel On The Ground

[JJ Campbell]

I appears that whenever I forget to turn the fuel valves off on my P92 Eaglet when I park it, a lot of fuel goes somewhere. There is a stain on the ground (new I think) under and mostly behind the right wing. I don't know if it is related or not but the plane very highly favors drinking from the right wing and I spend a lot of time with that valve closed. Oddly, when I close it, within a minute the fuel gauge reading will go up a couple of tenths of a gallon.

Any suggestions on what I need to check/fix?

[JimParker256]

I don't know the specifics of your fuel system, but there are several planes where if you park with the tanks full, and the fuel selector on "Both," fuel can transfer to the "lower" wing. The plane may appear to be level, but even a small disparity can lead to fuel transferring, then flowing overboard via the wing tank vent. That could account for both the fuel on the ground, and the slightly higher fuel level after sitting for a while...

The Commander 114 was notorious for this, made even worse by the long-travel gear suspension that would allow the "low" strut to compress and the "high" strut to extend as the fuel transferred and was vented overboard... One buddy of mine returned to his hangar and found almost 20 gallons had transferred overboard! (And that was when AvGas was around $6.00 a gallon!) I put a sticker on my panel that said "Fuel Selector OFF When Parked" to remind myself.

[JJ Campbell]

I don't know the specifics of your fuel system, but there are several planes where if you park with the tanks full, and the fuel selector on "Both," fuel can transfer to the "lower" wing. The plane may appear to be level, but even a small disparity can lead to fuel transferring, then flowing overboard via the wing tank vent. That could account for both the fuel on the ground, and the slightly higher fuel level after sitting for a while...

That sounds like a real possibility! I've been telling the fuel guys to fill her up to an inch and a half from the top. Which is going to present problems now that I might actually get to take a passenger in the near future. I'm thinking of starting to tell the fuel guys how many gallons to put in each wing to keep the total at 18 gallons instead of 23 I have been fueling to. This might help if I forget again which I will endeavor not to let happen again.

I like your reminder. I am going to put a note on my knee pad so I see it when I write down the Hobbs.

Thank you.

[3Dreaming]

With any carbureted Rotax powered airplane with the fuel tank above the engine you need to shut off the fuel. Most of the time everything will be okay, but if you have a leaking float valve in the carburetor you can fill your engine and oil tank with fuel.

I am not familiar with your fuel system either, but on the Flight Design airplanes with the 912ULS the fuel can still transfer from one wing to the other when the fuel valve is closed.

[JimParker256]

Yeah, as 3Dreaming points out, aircraft with dual tanks, but only an "ON/OFF" fuel valve might well cross-feed even with the fuel valve turned off. My RANS S-6 would almost certainly do that if I filled the tanks to to top and parked the least bit out of level, since the lines from the two wing tanks are joined together at the header tank, well upstream of the fuel shutoff valve....

Planes with a "Left/Right/Off" or "Left/Both/Right/Off" valve are less likely to siphon fuel between tanks if you're in the OFF position, as the fuel tank lines are usually connected directly to the fuel valve itself, and most of those valves will not allow fuel to siphon.

[Type47]

My P92 acts similar. I’ve taken to closing the left tank valve whenever I park to prevent the shifting to the low tank.
One time, I parked for the night at a turf field with 1/2 tank in both and left the valves on. The next day when I got in to leave, the right tank was full and the left empty. I started and took off on the right tank and landed at the first airport with fuel to fill up.
I wasn’t really sure why. The right tank may have been a little lower. Also, there was a sustained wind from the left. Perhaps it caused just enough pressure to push the fuel to the right tank over night. Maybe both.
Another thing I’ve noticed is that when both tanks are full, with both valves open, the tank that starts being drawn from first will continue to draw down until I shut it off for a few minutes to “encourage” it to draw from both.
“Break the suction” if you will.