Silicone Sealants: Not For Use On Aircraft!
In this important article about fuel system integrity, I wish to relate to you a recent experience of one of the Challenger pilot/builders at our airport (who shall remain anonymous) :-).
He installed the 17-gallon aluminum fuel tank from Specialty Welding in his Challenger II. This tank picks up fuel from the bottom and this builder also used that fitting as a sump drain. It works out real well that way, except that it picks up from the very bottom.
Now this is a nice tank and it is a good addition for anyone wanting some extra range. It is made to fit behind the passenger seat and will fit - but - it is a very tight squeeze to place it into the aircraft. This builder installed a fuel probe into the top of the tank and calibrated it on the work bench.
Upon trying to install the tank he found that it will not fit through the framework into its position with that fuel probe installed. There is plenty of room to reinstall the probe after the tank is installed so he simply removed the probe.
These fuel tank probes have a standard SAE bolt pattern which ensures that the probe, body, gasket, and the bolts can only be assembled and installed in one orientation. With the tank top as high in the fuselage "as it was" made it hard to get all components lined up. Silicone was used to keep parts aligned for assembly and bolt installation. In this instance some other procedure such as using thread looped through the bolt boles of the probe components and loosely tied might have been a better plan. To make that a little easier, he used a bit of silicone to adhere the gasket in place before installing the tank. Immediately after the tank was in place, he installed the probe and put in the 5 machine screws that hold it down to the tank. The remainder of the installation was completed and the aircraft was test flown for a couple of hours over the next several days.
On a Saturday morning we started out on a short XC to a club meeting. About 6 miles from the airport I noticed he was flying a bit lower, maybe even descending. Then I saw him turn toward a ranch house and thought he was going to do a flyover for some of his many friends. Next he turned toward an open pasture and into the wind. It was then that I noticed that his Challenger's propeller was not turning. Oh Boy, here we go... an airborne engine out!!
I circled around a couple of times to make sure he was OK (he was), then I landed. He said the engine just stopped as if it ran out of fuel. We checked the fuel flow, none, we checked the fuel filter, OK. We opened the sump drain, nothing!
I blew back through the fuel line and it seemed to clear out. The fuel flow promptly returned and just as immediately... stopped again. I blew it out again and it cleared. But there was just no way we were going to try to fly this airplane out from the pasture with an unstable, unreliable fuel flow.
By this time, two more of our group had arrived in a Titan. One of them flew back to the airport and returned with fuel jugs, strapped to the struts of his Challenger I, and siphon hoseso we could empty the tank and remove it for inspection. Another couple of guys flew back to the airport and came back with a pick up and tools. We drained the tank and removed it. Upon inspection, we could see 5 little 'somethings' in the bottom of the tank.
What happened as close as we can figure it, is that each of the 5 machine screws holding down the probe took a small dab of silicone sealer down on its tip. After these were exposed to gasoline they apparently absorbed gas or otherwise 'grew'. They became heavy enough to detach from the screw tip and fell into the bottom of the tank. We managed to dump them out of the tank and each one was a gel-like glob a bit smaller than a normal pencil eraser and just the right size to go down into a fuel fitting and plug it.
The tank was reinstalled, partially filled with fuel and the aircraft flown back to the airport. Of course... none of us made the meeting that day but it was an eventful day nevertheless :-). Just about the time we were ready to leave, it started raining so we stood under our wings for a bit. As the rain ended, the farmer arrived to see what all those airplanes were doing in his horse pasture. We explained the reason for our presence there and he was very nice about it. The problem airplane (CH II LW) departed the tall grass OK as did the CH I LW. My heavy CH I Clip Wing did not get out of the field so easily. My second attempt almost worked though but as I was about to rotate, I apparently hit a small wash out hard enough to knock my GPS loose in its RAM mount. So I had to stop and investigate that.
On my next attempt, I tried a different direction. Same problem, as I would rotate, the tail would drag and the aircraft would slow down. I finally came to a small area with shorter grass and managed to rotate but couldn't get enough flying speed to lift off before I was in tall grass again. I let the nose back down to try to build up my ground speed and came to another area of shorter grass. This time I kept the nose on the ground and let the aircraft build its speed up and just before reaching the taller grass on the other side, I popped it off into ground effect... and was gone (phew).
Over the next few days, we removed the tank and cleaned it and the gasket very well. We put a stand pipe in the fuel fitting in the bottom of the tank. This is a small pipe that protrudes about two inches up into the tank. The top end of it is crimped closed and a series of small holes drilled all around its sides starting about a half inch off the bottom of the tank. This will prevent any similar fuel system flow stoppage problems and undesireable engine outs in the future.
Now this builder and many others of us (myself included) know that it is not adviseable to use silicone sealants in many of the places that we do use it. But with careful use, it is very good in many places that perhaps it should not be used. This time though, it bit. In this instance, no big problem, just a rather large inconvenience. This experienced pilot had some altitude, knew the wind direction and had chosen a flight path that offered emergency landing sites, but the outcome could have been very different.
The moral of this story is, that I caution all builders and repairman about the use of silicone sealers in non-appropriate locations. It can and will bite!!