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 Wednesday, April 23 2014 @ 03:44 PM EDT

Fuel Tank Service Bulletin

   

Wings

There have been at least three engine failures caused by fuel pickup tubes that came loose inside fuel tanks. One aircraft was destroyed in a forced landing (the pilot survived, fortunately) and at least two other aircraft did successful forced landings. In February, Van's released a Service Bulletin (SB) on the subject. The SB calls for the tank access covers to be removed, the fuel pickup tube fittings to be drilled for safety wire, tightened, and safety wired.

It seems that two of the three incidents were on aircraft that had been built from quick build kits. The builder does not build the fuel tanks on these kits - he just has to inspect them, then install the access cover. The fuel pickup tube is in place, but not tightened. It seems that many builders miss the step in the instructions where they are told to tighten the fuel pickup tube.

I was reasonably sure that my fuel pickups were tight, but decided to open the tanks up just to be safe. It took several hours work to get the access covers off and clean the old Proseal off. I drilled the fitting the for the fuel pickup on the right tank and secured it with safety wire.

The left tank has a flop tube (fuel pickup on the end of a flexible hose) to provide fuel during inverted flight. The SB calls for the flop tube to be safety wired to its attachment elbow fitting. I wasn't crazy about this, as it means you have to pull the attachment elbow out of the tank so you can so the safety wiring. Then you have to Proseal it back in place and hope it doesn't leak. And, you have to do this all over again whenever you replace the flop tube (they gradually stiffen up as the hose ages).

Having a flop tube come loose doesn't really add a new risk, as flop tubes can get hung up inside the tank so that they aren't resting on the bottom. The result is the same in either case - a bunch of fuel in that tank becomes unusable. So, if you have a flop tube, you need to always operate the aircraft as if fuel in that tank may not all be available. In other words, you plan to use the fuel in that tank first, so that if it all doesn't feed you can switch to the other tank and land.

Given that having a flop tube come loose is no worse than having it hang up, which is a risk that I have already accepted, I decided to not put safety wire on the flop tube. I simply made sure it was very tight, and then put some Proseal between the fitting and the threads. The Proseal will be almost as good as safety wire, but it won't make it too much harder to replace the flop tube when that time comes.


For some reason, I thought I should check the resistance of the fuel senders while I was working on the tanks. This turned out to be a good idea, as the one on the right tank gave screwy results. It turned out that the fuel sender was not grounded to the tank - the Proseal acted as an insulator. I'm glad I discovered this now, as I would have needed to mix up some more Proseal whenever I found the problem. I fixed it by removing one of the fuel sender attachment screws, and put a ring terminal on it, with internal tooth lock washers above and below the ring terminal. The teeth on the lock washers made a good connection between the fuel sender and the attachment bolt, and now it seems that the attachment bolt has a good connection to the tank. Now the fuel sender resistance is correct, even without hooking up that ground wire.


I had some left over Proseal, so I used it to seal the bottom corners of the firewall. I wish I had done that a long time ago, as I spilled some oil when I pulled one of the oil hoses off to pressure test it. The spilled oil ran down the firewall, and some of it seeped inside the lower fuselage. The oil later seeped out near the right landing gear leg area. The spilled oil is inside the lower fuselage, where I can't get at it. It will probably cause untold grief when it comes time to paint the aircraft.




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