Fuel leaking BETWEEN main tanks?

[Skymaster337B]

My simple advice is to move the fuel selector valves to the "OFF" position when the aircraft is not in operation. I've noticed "seepage" thru the 40+ year old fuel selector valve, when the aircraft is parked and fuel selector valves are on.

I think it has something to do with the combination of ground level, and the fuel trying to seek gravity, and possibly the vent system. Do you know if the fuel caps are original or are in compliance with AD 79-10-14R1. Also there's a check valve in the wing tips that will help vent the tanks, but may also become clogged.

Do you keep the aircraft registered under the FAA or with Australia's version?

Keep us posted.

[ConwayB]

G'day 337B,

Thanks for the response. Your answer made perfect sense and I'll pull out the manuals and have a closer look at the fuel layout. The machine was parked on a bit of a slope at the time.

If you want to check out some pics, go to my website at www.ipas.com.au and go to the aviation services page. The machine was de-registered in the States prior to shipping to Australia and registered over here.

Sorry my response is brief, for I'm writing this on my iPad.

Cheers

Conway

[Roger]

I suppose this goes witout saying, but none-the-less I will say it: Make sure that you are talking about fuel and not water. It is possible that your "downhill" tank has filled with water after 8 weeks of heavy rain running down the top of the wing into the cap.

[hharney]

While looking at the attached fuel system schematic, it looks like the most likely culprit would be the selector valve. I don't think the check valves would be a suspect. One set of check valves on the return lines from the fuel pump/throttle body and the other check valves are on the vent for the main tanks. Are the bypass valves, near the aux fuel pumps, check valves?

To have fuel transfer from one main to the other would almost have to go through the selector valve but if the bypass valve is one way only it would have to pass through this also.

I would inspect the fuel that you have in the tanks to be sure that you are not taking on H2O as Rodger has described. Either leaky fuel caps or corroded service bay drains can allow water into the tank. Plug the bottom of the service bay drain and allow some fuel to spill over into the service bay to be sure the tube is not compromised.

From reading your scenario it sounds like the fuel transferred but it is best to make sure.

Cheers mate

[skymstr02]

Please see the attached fuel system schematic for an O-2A. Check to ensure that the "EMERGENCY CROSS-FEED SELECTOR VALVE" is fully rotated to the "CLOSED" position. This is the only way that you can cross feed the AUX tanks. It is also a sole source of uncommanded fuel transfer in an unlevel condition.

It could also be leaking internally.

The only check valves in the fuel system are used in the return fuel from the fuel pumps to the main tanks, and the maint tank vents, so don't chase ghosts looking for leaking check valves.

Myself, I turn both fuel selector valves to the "CLOSED" position in my shut down checklist. This also helps to keep the valves from binding.