++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?

[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

[FMild]

Jim,
I'm by no means an expert but I've learned alot about my "D" model (non-turbo'ed) in the last two years with malfunctions due to the a/c sitting awhile. The most relevant one to your situation was after doing some work with the fuel selectors moved to "Off" I couldn't get the front engine to start. I'd prime it & it would pull away from the starter but then die. Turns out the fuel selector didn't make it all the way from "Off" to "Left." The little ball/spring deal which gives the feedback of the detent was gummed up and I truly wasn't getting it into the detent. They need to be lubed well & watch for track wear on the plate if they're not which will cause more problems.
That being said if you didn't move it at all then the only apparent culprit is the valve itself. I understand from the manual that you have an extra return line to the sump but that wouldn't cause it all to burn from one tank. I would suspect you have an internal failure in the valve itself which allowed the valve to port fuel to the front engine. I'm interested to see what you find.

[B2C2]

Having studied the fuel system in the parts manual, I would have to agree with that. The only path I can see for the fuel to be cross fed is via the valve assembly. I am not familiar with the internal construction of the valve. I looked around on the web and couldn't find an exploded view of the internals, so I can't suggest a specific mechanism, but it seems like something has to be broken inside the valve assembly causing fuel to be drawn from the opposite tank for this to happen.

[Jim Smith]

Ed, I did actually think "wow is this situation so unique I've stumped everybody or no one else has ever had this occur!"

I've come to the initial conclusion that the only way this can occur is through a fuel valve. However, with the balls in the valves being spring loaded and the plane stationary when the problem materialized, I just can't see how they could fail in such a manner that would cause a complete change in fuel flow. Maybe a little leakage one way or the other but not a complete change. I exercised the valves through all of their positions when I checked for proper positioning and detents, and will do my one hour test flight this week and report back to everyone if the problem persists or has 'corrected' itself. Quite frankly, I sure hope it hasn't 'corrected' itself without me taking any corrective action.

[hharney]

Jim
Just be careful as you do your flight test. Be sure to only make one engine/fuel change at a time, keeping power to one engine without changing both.

A fatal Skymaster accident earlier this year was a test flight just after having worked on the fuel system. Just be careful.....

[Jim Smith]

Going to be just running on the mains and then see if I had greater fuel burn out of one side, and none out of the other, as occurred on the last flight. Thanks for the concern. I'll be staying very close to the airport!

[Jim Smith]

Hey guys. Flew today and burned 15 gallons. Both sides took an almost equal amount upon refueling. Not sure if exercising the valves corrected the problem or this condition only occurs after burning more than 50 gallons. The problem occurred after 50 gallons used (67 total). I rarely go that high without refueling. I'll continue flying and be watching the fuel gauges for uneven burn and topping off after each flight. I'll keep everyone advised.

[Jim Smith]

After analyzing of the conditions and circumstances, discussions with several fine mechanics and Skymaster specialists including Owen Bell and Don Neiser, we've all come to the conclusion the only way this could've occurred is through one of the fuel valves. How, we're not exactly sure. Possible trash, weak spring(s), bad o-ring(s), or internal corrosion. My valves look beautiful on the outside but are original and have never been rebuilt so who knows what the insides look like. There is the possibility that one valve was slightly out of detent but it would've been only ever so slight. My future course of action and recommendation to other owners is, during your preflight make sure you hear each valve click as you put it into position, if it had been previously moved out of position from the mains. The cables from the valve selector handles to the valves do tend to get some slack and play. Adjust if necessary. Additionally, even though I'm not aware of any Cessna SID or an AD, it would be prudent to remove your valves and go through and rebuild them if they've never been rebuilt. At the very least inspect them internally. Being a 1969, mine are 44 years old and I'd have to imagine there may be some stuff going on in there that probably isn't good especially where there's any rubber concerned. You can maintain your aircraft in tip-top condition and one thing like this, that isn't required to be rebuilt unless it's broken, can take you down. There have been too many Skymaster crashes attributed to fuel exhaustion and who knows if this could be part of the reason why. Also, if you have a fuel flow/totalizer system, these add a great level of confidence with fuel usage, but continue monitoring for uneven burning from your good old fuel gauges. Thanks to everyone for your input.