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Unread 03-08-11, 11:33 PM
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ConwayB ConwayB is offline
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Fuel leaking BETWEEN main tanks?

G'day All,

I have read a similar post regarding leaking fuel tanks, but so far have not found a similar problem to one I have recently discovered.

I have an O-2A(337B) with two main tanks and two aux tanks. The aircraft has not flown for about 8 weeks but I go and run the engines at least once a week for about 10 minutes. (We've been having terrible weather here in Queensland, Australia with hurricanes and massive floods and deluges). When I left the aircraft last week, it had 1/3 in each main tank and about 1/4 in each aux. (My left aux fuel sensor has not worked for a long while and has been a problem since importing the aircraft). This week, when I returned to the aircraft, I was shocked to find that my left main tank read almost empty, but my right main tank read 2/3 full! The aircraft was parked on a slight slope with the right tank lower than the left. I have not changed the position of the fuel cross feed valve.

Is it possible for fuel to leak from one main tank to the other through a closed (or faulty) crossfieed valved? I haven't had a chance to go through the manuals to check on the locations of any check valves.

And if that wasn't enough, now my left main fuel tank gauge/sensor is faulty. Augh!

Any advice would be appreciated. Bear in mind, the aircraft - at this stage - is sitting in the open due to lack of hangarage... and we've been having torrential rain here.

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Unread 03-09-11, 01:53 AM
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Skymaster337B Skymaster337B is offline
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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.
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Unread 03-09-11, 08:28 AM
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ConwayB ConwayB is offline
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Thanks for the reply

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 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.


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Unread 03-09-11, 09:16 AM
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Roger Roger is offline
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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.
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Unread 03-10-11, 05:32 PM
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hharney hharney is offline
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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
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Herb R Harney
1968 337C

Flying the same Skymaster for 45 years
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Unread 03-11-11, 07:50 AM
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skymstr02 skymstr02 is offline
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O-2A Fuel System

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.
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