++safety Alert++

Jim Smith · #1 10-15-13, 01:24 PM

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 · #2 10-24-13, 05:09 PM

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 · #3 10-25-13, 07:21 PM

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 · #4 10-26-13, 10:07 AM

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 · #5 10-26-13, 01:41 PM

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 · #6 10-26-13, 02:32 PM

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 · #7 10-27-13, 10:22 AM

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.