++safety Alert++

[Jim Smith]

I forgot to mention the plane is a 1969 T337D (Turbo).

[Skymaster337B]

Couple of possibilities come to mind. Verify the fuel selectors in the wings are rigged correctly, and have not slipped. Verify the fuel selector valve in the wings works correctly by changing tanks while running on the ground, including the shut off position...the ball and detent should be well greased. Also, check the fuel vent valve in the wing tip...in might be clogged, as well as the fuel cap vent for that matter. Finally, check your tank sump for blockage.

[B2C2]

Probably a dumb question but did you actuate the fuel selector handles in the cabin and verify that the fuel selector valve in the wing is actuating properly?

[Jim Smith]

The fuel vents are clear. I have been flying with the valve selectors in their respective normal positions without changing them for quite some time, so there should be no reason for the valves themselves to be out of correct position. From what I understand about their design, they can't accidently change positions unless manually operated and then a cable breaks. We are not going to change any position of the valves until we do a one hour test flight and top it back off to see if the condition persists. Also, from what I understand a blocked fuel vent or fuel cap would just cause starvation to the engine and not create a forced crossfeed situation. We have not checked for a complete blockage from that tank as of yet, but I don't know that if there was a complete blockage if it could cause a forcible crossfeed situation. It would seem that if this is possible, it would have been noted from past occurrences somewhere or somebody would be aware the possibility of unknowingly sucking one tank with both engines exists. I would imagine over a span of 50 years there would be numerous crashes due to this or some type of AD out on this. Understanding the plane and it's fuel system this is just quite baffling.

[hharney]

Could the fuel have been returned to the AUX tank? The Turbo models are different from the Normal. The return fuel from the Turbo model has a path to the AUX tank. I am not sure if the T connection is plugged in the return system?

When you checked the vent lines did you use the procedure in the SM?

[Jim Smith]

Based on my math all of the fuel used from my last flight came out of only one tank. About 21 gallons for an hour of flight. Both auxiliary tanks were about half-full so I don't think there was any fuel returned. I have externally inspected the fuel valves, went through all positions, and everything appears to be operating properly and stopping in their correct detents. I ran it on the ground for nearly 30 minutes at different power settings and everything is functioning properly, but the run wasn't long enough to determine any uneven fuel burn. I will do a one hour test flight tomorrow and then top it off to see where the fuel is burning from.

It's my understanding the number one cause of Skymaster crashes has been fuel exhaustion. It makes me wonder if there's a possible hidden defect in the system that only materializes under certain conditions. Such condition would most likely be undetectable by the NTSB, especially with all fuel selectors in their correct positions, it would appear that the pilot just accidently exhausted the fuel supply.

Herb (and others) thanks for your replies. Quite frankly, due to the serious nature of this problem I'm quite surprised I've had such little responses.

[edasmus]

Jim,

As for having little response to your post, I can't say for sure why that might be but as for myself, I visit this site everyday and read the posts with great interest. I wish I had some ideas for your issue, but I don't. I certainly hope you resolve it and post the results. Just know that lack of response doesn't mean people aren't following your posts. It may simply mean they are as baffled as you are. I know I am. Good Luck!

Ed