Engine failure due to silicone

Tarheelpilot said:
I’m not licensed but I am a certified mechanic. Does that count? What do I win? I hope it’s whisky or whiskey. I like both.
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The Canadian has a license. The American has a certificate.
 
Dan Thomas said:
You're missing the whole point. Smearing RTV onto something that has a tiny hole it will push some of it through that hole and it will now be a thread or gob or drip of RTV inside the fitting. Fuel flowing past it, and vibration shaking it, will eventually pull it off and away it goes, to get into some place where it will cause trouble.
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You obviously belive this is the case but if a crack is so small that is doesn't leave a blue puddle then it to small to push/suck A significant amount (enough to clog a filter) through that same hole.

Rtv on the treads? Yes, seen it a few times. But a tiny tiny crack? Pull that fitting and show us the inside where rtv is visible.
 
pfarber said:
You obviously belive this is the case but if a crack is so small that is doesn't leave a blue puddle then it to small to push/suck A significant amount (enough to clog a filter) through that same hole.

Rtv on the treads? Yes, seen it a few times. But a tiny tiny crack? Pull that fitting and show us the inside where rtv is visible.
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The OP already said that there was tiny hole in that fitting. Argue with him, please.

It's a 310.That airplane has wing tanks. The fuel is down low. There is a boost pump between the header tank and the engine's pump. The suspect fitting was on the engine pump. The engine's pump will not let fuel run backward, due to its internal check valves, so fuel isn't going to run back out of that fitting from the pump. The boost pump also has a bypass valve that takes a tiny bit of pressure to overcome, and the pump's vanes prevent easy bypassing through them, and the fact that the tanks are BELOW the engine's fuel pump means that gravity can't push fuel through the boost pump and out of that hole in the fitting when the engine isn't running. The ONLY time fuel would leak out of it is when the boost pump is on, and in that airplane that might be only during priming. And maybe there WAS a leak and the mechanic who put that RTV on there was doing it to stop it. Stupid, but there ya go. And, as I keep saying and nobody gets, that silicone was a big patch on the outside of the fitting to stop chafing. A gob falling off the inside would not suck the silicone off the outside. It's way too tough for that.

Furthermore: I have seen the sort of damage caused by chafing against something. It often rubs through the metal far enough to leave a tiny leaf of metal across the hole, a leaf that will restrict low-pressure leakage, but be easily pushed inward with a bit of pressure. Can't say that this was the case here, but pushing silicone onto that fitting could shove it through that hole and leave that gob hanging in there. There was a silicone gob in the fuel pump, after all, and since there is the fuel strainer at the firewall that has a really fine screen in it, and the next component is the pump with it's inlet fitting, the only other way it got in there was if someone had used RTV as a thread sealant somewhere between the strainer and pump. No other way. A fitting with a tiny hole, gooped over with RTV to stop a cable's chafing on it, is a red flag.
 
Why are questions deemed to be arguments? Silly me questioning the Internet gods :rolleyes:

I guess the 'tiny hole' has to be significant if its all the way through the threads of an AN elbow. That's a couple 32's of Al to wear out.
 
pfarber said:
Why are questions deemed to be arguments? Silly me questioning the Internet gods :rolleyes:

I guess the 'tiny hole' has to be significant if its all the way through the threads of an AN elbow. That's a couple 32's of Al to wear out.
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You weren't questioning. You were indicating that you didn't think it was possible that silicone could get to the inside of that elbow.

And yes, that metal is not very thick, and in many cases it's aluminum. A steel control cable running across it will easily chew through it. In some cases it's a brass AN fitting, and brass is also easily chafed. I found a brass fitting on the fuel servo of an IO-520 chafed by the alternator belt that runs very close to it. Chafed by a rubber belt. Someone had replaced it and not made sure there was some clearance, and someone else had done some stupid stuff with the alternator, using a mash-up of parts from shock-mounted alternators and non-shock-mounted alternators that resulted in the cracking and partial failure of the alternator's lower mounting lug and dropping it enough to get the belt chewing at that fitting. If it had worn though, life would have gotten pretty uncertain. There's fuel pressure inside that fitting.

I have found the steel engine mounts deeply chafed by carb heat or mixture control cables.
 
Water WILL wear out the rock. Or cut steel. Aluminum will also cause wear on steel. Many grinding wheels are aluminum oxide.
 
Magman said:
Water WILL wear out the rock. Or cut steel. Aluminum will also cause wear on steel. Many grinding wheels are aluminum oxide.
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Nylon air brake hose will eat into a steel semi-truck frame. It gets sand and dust embedded in it, and the frame gets ground away while the plastic hose survives.
 
Wagondriver said:
I get it just fine, I understand what the story is. Apparently you must be clairvoyant to know what some mechanic did years ago on a plane you’ve never seen.
I have a real hard time believing that there was enough chafing to put a hole in a hose end but it wasn’t seen. I also have a hard time believing that fuel pump suction pulled in a big enough chunk of silicone to shut it down, but it didn’t suck in air or leak fuel.
No need to be a donkey.
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This… “I also have a hard time believing that fuel pump suction pulled in a big enough chunk of silicone to shut it down, but it didn’t suck in air or leak fuel.” …was exactly my thought when pulling that ball of rtv outta the pump inlet. All I can figure is maybe since fuel softens silicone maybe it happened a little bit at a time, over the course of ?years?. Fuel contacting the rtv on the underside of its attached surface might soften it up just enough to draw a tiny bit of it in possibly during engine start up. Then after years of that happening the glob on the inside could have grown large enough that the flow of fuel broke it off. The inside diameter of the fitting is probably much larger than the diameter of the actual hole into where the vanes of the pump live. That ball of silicone was perfectly sized to block the orifice without getting inside the pump where it could have damaged the internal workings of the pump. Dan Thomas is right in his description of how the fuel system works so the only place it could have come from was that glob on the elbow, and through that tiny little crack that was only discovered after I scraped the silicone off of the elbow. Just a theory, I really have no idea how it got in there and why it would have taken so long to do what it did. Strangest thing I ever saw.
 
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