++safety Alert++

[Jim Smith]

First of all I want to say the subject plane is a one owner for over 40 years, extremely well-maintained and always hangered. I have owned it for about two years and continued with hangering and tip top maintenance. I have a very good understanding of Skymasters and their systems having owned six of them. The plane is flown regularly. It has a Shadin Fuel Flow system that is very accurate. I rarely use my Aux tanks and have never had the need to cross-feed. The plane has had no unusual alterations, modifications, or damage history.

Now here's the story:

Return from a trip with my usual 45 gallons of fuel burned off the main tanks. A visual inspection indicated the burn appeared to be even with 22-23 used per side. Plane sat in my hanger for approximately a week and a half. Last weekend, decided to fly to an airport only 55 miles away. Did a visual inspection and both tanks still showed the same approximate amount (22-23 gals) per side burned from my previous trip so I decided not to top off. I knew my trip for the day will burn 20-23 gallons since I was going to shoot a few approaches on the way over and way back. Upon my return home I did notice the left main fuel gauge was lower than normal. Upon landing I taxied directly to the fuel farm to top off. Ready for this? The left main took 44 gallons (2 short of its 46 gal capacity). 22ish from my previous trip and 22ish from the trip I just took. The right took about 23, the correct amount I burned on that side from my previous trip. The total fill up was about 67 gals, exactly what I burned since last fill up according to the Shadin. What this all means? ALL of the full used from my short trip that day came from the left main. Nothing used from the right. I landed 2 gallons short of a double engine failure, as both engines were obviously using that tank. BOTH fuel selectors had been in their 'normal' respective positions, confirmed during my preflight, my GUMPS checks, and after landing. I know this plane well and have three mechanics very experienced with Skymasters. We are all baffled by this scenario. The plane is grounded and the moment, as we haven't had a chance to tear into it. I would like to get some thoughts and opinions before doing so. I did check the fuel selector handles to ensure they couldn't be installed on the shaft improperly. They are slotted to fit only one way. I pulled the panels to the fuel valves. No leaks or apparent issues and the cables from the selector handles appeared to be intact. This is obviously of great concern and should be to all owners from '65 thru the F ('71) models. With both engines failing simultaneously and both fuel handles in the normal position, I would've had no clue as to why they were really failing and would've made the logical assumption that both tanks were dry. At 2000 feet we all know what the options are, as things start happening really fast.

[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?