Engine failure due to silicone

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RussellJ

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RussellJ
Been flying for 30 years and the only time I had an engine failure was due to a glob of silicone sealant. Bought myself a twin and had roughly 15 hours in it. Took off from a local airport with my CFI in the right seat, positive rate of climb, put the wheels away and then the left engine went quiet. The plane was lightly loaded so it wasn’t too big of a deal, rate of climb dropped from 1000 fpm to 400 fpm until we got things sorted out (trying to decide if one of us might have initiated a simulated engine out) and feathered the prop. Once feathered it accelerated nicely and we continued to climb to a safe altitude. After unsuccessfully trying to restart the engine we landed back at the airport to see what happened. Found a ball of silicone blocking the inlet to the engine driven fuel pump. Fuel selector valves, electric boost pumps, cross-feed valves are all upstream of that pump so there was no chance of restoring fuel to the engine in flight. It looked like a cable was chafing against an elbow on the inlet side of the pump and someone put a glob of silicone between them to stop it from rubbing. It did a good job of holding the cable but… apparently there was a tiny hole already worn into the threads of the elbow when the silicone was applied. My theory is over time as the engine driven pump was drawing fuel from the tanks it was also pulling on that glob of sealant. Once enough of the sealant was drawn into the line the flow of fuel pulled it loose blocking off the inlet to the pump. I have seen people use silicone that way plenty of times before so it didn’t bother me when I saw it there. But now I know, if there’s silicone on a fuel line, remove it and see if there’s any wear on the lines, then maybe use an Adel clamp to hold it or find a way to reroute the offending cable. Silicone and fuel don’t mix.
 
455 Bravo Uniform said:
How did you not have a fuel leak also at that moment?
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I bet there was a slow leak when the engine was off, but when on, it would be a low pressure hose, sucking air if anything. Leak was probably slow enough to evaporate before being seen.
 
Possible you lost some of the silicone blob externally (on a runway/taxiway or aerially somewherz).

Perhaps look for another source of silicone internal to the fuel system before next flight.
 
Always thought the 310 was a very nice looking plane. Of course the DHC-2 Beaver does better were I live.

2004p_310_001_16x9.jpg


upload_2022-1-27_9-17-43.jpeg

....:)
 
Just to clarify, this happened shortly after I purchased the airplane, about 12 years ago. After removing the ball of silicone from the pump inlet and replacing that worn elbow I flew the plane without issue. I sold that 310 three or four years ago, after getting out of the FBO business. I was just posting this so folks might look a little closer when they see glue holding something in the engine compartment.
 
RussellJ said:
That is weird, there was no fuel leak. A blue stain would certainly trigger further investigation. I'm open to any guesses.
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Silicone pushed onto the fitting, against the unnoticed hole, would push some of it into the fitting, where it would hang as a gob. Once it broke off, the hole would still be covered by the rest of the silicone on the outside of the fitting, preventing any leakage.

Silicone has no business being used as a sealant in any fuel or oil system. On engine gaskets it can cause serious problems; it extrudes both internally and externally when the hardware is tightened, even if minimal amounts are used. One can check, for instance, the list of approved sealants published by Continental, and not find silicone anywhere in that list.

Back in 1972 I used silicone to seal the carb body gasket in my '64 Econoline van. That silicone came off inside the carb and blocked the main jet, killing the engine. 19 years old, learned it the hard way, in a vehicle that didn't have to make a forced landing. A fairly intense lesson, that. Had to take it all apart at the side of the highway, clean it out, and put it back together.

But as a bumper between stuff, it's fine, as long as it can't be sucked or fall into anything. So watch its use around the air intake for the carb heat. No filters there.
 
My intro to silicone ( aka RTV - room temp vulcanizing) was as an Afterburner Heat Sealant on an F-100C. It was developed by General ELectric. Ironically 1 year earlier I had been employed by the GE Flight TestCenter.
 
A carbureted engine might need 4-5psi of fuel pressure AFTER the pump. The suction side would be a small suction.

Why would cured silicon be sucked theough this tiny hole when there is a a much less viscose fluid (the fuel) right there?
 
pfarber said:
A carbureted engine might need 4-5psi of fuel pressure AFTER the pump. The suction side would be a small suction.

Why would cured silicon be sucked theough this tiny hole when there is a a much less viscose fluid (the fuel) right there?
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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.
 
I think I’ve told this story before…

On the BING carburetors on ROTAX engines it can be hard to get the float bowl gaskets to stay in place as you remount the float bowls. I used a tiny bit of silicone gasket goop to hold them in place. Not long after my Sky Arrow in cruise over S GA suddenly became very rough and lost a lot of power. I declared an emergency and began looking for a forced landing spot. Fortunately it gradually cleared up and I landed at a nearby airport and could not reproduce any problem.

Back home I pulled the float bowls and found this:

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My working hypothesis is that when fuel hit the silicone it turned to jello and enough of it temporarily clogged the main jet on one side. It was later pointed out to me that silicone sealer and gas don’t mix. And to use grease instead, which just dissolves when gas hits it.

I know at least one RV fatal could be traced to that. I also ferried a damaged cropduster back to S FL from Nassau that had lost power, likely due to silicone sealant in the hopper - which was plumbed for aux fuel - making its way into and blocking the injectors.
 
There a local guy with an RV he build in which he sealed the airbox with RTV. We've told him....but cna't tellthis guy anything. We're waiting.....
 
There are very few sealants compatible with fuels and silicone isn’t one of them. For some reason it continues to get used with fuels even though there are warnings about it on the packaging.
 
It would be great if there were a handbook of "things not to do" in electrical/mechanical assemblies. Especially for the home builders. No sarcasm intended.

Things like no silicone for fuel, anti-seize or lubricant greatly affects fastener torque, epoxy doesn't necessarily adhere to cured epoxy, don't run wires through panels without grommets, don't use zinc plated fasteners around computers, stainless to stainless fasteners are likely to gall w/o anti-seize, etc. etc. Sometimes the right ways to do things can seem counter-intuitive.
 
Dan Thomas said:
One can check, for instance, the list of approved sealants published by Continental, and not find silicone anywhere in that list.
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M-0 has RTV listed for use on the "alternator assembly". Not sure what alternator part needs it but that is the only place under Lubricants, Sealants & Adhesives.

Otherwise, the manual is full of warnings about the use of RTV or silicone.
 
I think any hole big enough to let in a silicone glob is also going to let in air when the fuel pump is running and let fuel out when the pump is not running.
If someone tried to seal up a hole in a fuel line, either pressure or suction, their license should be revoked.
 
Albany Tom said:
It would be great if there were a handbook of "things not to do" in electrical/mechanical assemblies. Especially for the home builders.
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FYI: There actually are handbooks like this per se, but they work from the opposite end of the equation and show you how to do things vs what not to do. So if you apply those readily available standard practices to the work you perform you'll greatly reduce doing something you should not do. Examples of these handbooks are OEM and other standard practices manuals, FAA ACs, SAE documents, etc. All of which can be used in the E/AB world. Add in the warnings and cautions found in various OEM mx manuals one could easily look to them for guidance regardless if working on that specific item or not. There's nothing that prevents applying an acceptable practice from one OEM to another OEM product. Even in the TC world in most cases. It only depends on how much one wants to look vs just winging it.
 
Wagondriver said:
I think any hole big enough to let in a silicone glob is also going to let in air when the fuel pump is running and let fuel out when the pump is not running.
If someone tried to seal up a hole in a fuel line, either pressure or suction, their license should be revoked.
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For Pete's sake. Some of this bunch just doesn't get it. I guess they've never worked on such stuff and cannot picture what was going on.

The OP's airplane had a cable chafing on a fuel fitting. Someone smeared RTV on that fitting to stop the cable's chafing it. They did not see that there was a tiny hole in the fitting, the result of chafing just a hair too far into it. Some of the RTV was forced into the fitting and it cured there and hung there until the fuel pulled it off.

The silicone was never intended to stop a leak of any sort. The tiny hole was overlooked. The sealant was being used as an anti-chafe; the real mistake was in not replacing the chafed fitting.

The RTV remaining on the outside of the fitting prevented air being drawn in or fuel seeping out. It was put there in some thickness to stop chafing, after all.

From post #1:

"It looked like a cable was chafing against an elbow on the inlet side of the pump and someone put a glob of silicone between them to stop it from rubbing. It did a good job of holding the cable but… apparently there was a tiny hole already worn into the threads of the elbow when the silicone was applied."

Anyone who has used RTV for any amount of time knows that it can extrude into all sorts of places. It's terribly invasive stuff.

Now. Does anyone still not get it?
 
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Bell206 said:
FYI: There actually are handbooks like this per se, but they work from the opposite end of the equation and show you how to do things vs what not to do. So if you apply those readily available standard practices to the work you perform you'll greatly reduce doing something you should not do. Examples of these handbooks are OEM and other standard practices manuals, FAA ACs, SAE documents, etc. All of which can be used in the E/AB world. Add in the warnings and cautions found in various OEM mx manuals one could easily look to them for guidance regardless if working on that specific item or not. There's nothing that prevents applying an acceptable practice from one OEM to another OEM product. Even in the TC world in most cases. It only depends on how much one wants to look vs just winging it.
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To add to that: Many of those manuals are available online, free. A lot of OEM maintenance manuals and parts catalogs are there as .pdfs. They might not be up-to-date, but there's a wealth of info in them. Engine overhaul and operator's manuals. Propeller manuals. AC43.13. You can pay big bucks for them, or just do a bit of searching. I do it all the time, to use as advice here.
 
mondtster said:
There are very few sealants compatible with fuels and silicone isn’t one of them. For some reason it continues to get used with fuels even though there are warnings about it on the packaging.
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#3 (aviation) Permatex is OK with fuel but do not use a lot.
 
I had a friend who used RTV throughout his RV-10’s fuel system. An in-flight fuel blockage caused an engine out that led to a fatal crash that killed him and his granddaughter. This guy was what I regarded as a smart, conscientious builder. To this day I have no idea why he did what he did in deciding to not follow what I consider to be a fundamental best practices when it came to proper installation of flared fuel lines and associated AN fittings.
 
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BTW ... trust this is common knowledge but if not ... do not use Teflon tape on line fittings!
 
Daleandee said:
BTW ... trust this is common knowledge but if not ... do not use Teflon tape on line fittings!
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Yup.

That Continental Standard Practice Manual has a list of sealants approved, where they go on the engine or fuel system.

There are Cessna SBs addressing various fuel system issues, and they specify the sealants they want used, as well. RTV, teflon tape and others are not approved. There are Loctite sealants specified, some are teflon-goop base (not tape). Some are anaerobic sealants.

And any standard practice will tell you to NOT apply any sealant on the first couple of threads on a fitting. Sealant can extrude forward into the system as the fitting is screwed in.

Aviation Form-A-Gasket is fuel-proof but it's really old technology. It tends to harden and crack with age. Cracking means leakage. Hardening makes the fitting hard to remove. There are plenty of better sealants. But I have that old stuff and have used it when nothing else was at hand.

Good A&P textbooks talk at length about this stuff.
 
Dan Thomas said:
For Pete's sake. Some of this bunch just doesn't get it. I guess they've never worked on such stuff and cannot picture what was going on.

The OP's airplane had a cable chafing on a fuel fitting. Someone smeared RTV on that fitting to stop the cable's chafing it. They did not see that there was a tiny hole in the fitting, the result of chafing just a hair too far into it. Some of the RTV was forced into the fitting and it cured there and hung there until the fuel pulled it off.

The silicone was never intended to stop a leak of any sort. The tiny hole was overlooked. The sealant was being used as an anti-chafe; the real mistake was in not replacing the chafed fitting.

The RTV remaining on the outside of the fitting prevented air being drawn in or fuel seeping out. It was put there in some thickness to stop chafing, after all.

From post #1:

"It looked like a cable was chafing against an elbow on the inlet side of the pump and someone put a glob of silicone between them to stop it from rubbing. It did a good job of holding the cable but… apparently there was a tiny hole already worn into the threads of the elbow when the silicone was applied."

Anyone who has used RTV for any amount of time knows that it can extrude into all sorts of places. It's terribly invasive stuff.

Now. Does anyone still not get it?
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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.
 
How does a glob of silicon on the external side of a fuel anything, get inside the fuel anything? Without some sort of major failure of the system component directly adjacent to the external silicon glob? Unless, per chance, silicon was also used internally, mabe on threads, or some such.
I've actually seen such as teflon tape, or silicon, or some other kind of sealant used on flare fittings, and compression fittings. I've even seen silicon used on exhaust gaskets. :rolleyes:
 
Nobody reads and understands anymore. It's so sad. Who said ANYTHING about silicone being sucked through that hole? Who said anything about it being a hose end? Who here knows the thickness of an AN elbow's wall? Anyone besides me? Who else here is a licensed mechanic, or has any experience at all with fuel systems and their fittings? Who else here has even seen a chafed fuel fitting?
 
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bluerooster said:
I've actually seen such as teflon tape, or silicon, or some other kind of sealant used on flare fittings, and compression fittings. I've even seen silicon used on exhaust gaskets. :rolleyes:
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Sure, you'll see it. Doesn't make it legal or wise at all. Silicone on exhaust gaskets can't hurt much; it isn't going to get sucked into the engine. But it's usually there because the exhaust port face on the cylinder head has been eroded and pitted by exhaust squirting past a bad gasket. It acts like a blowtorch, and removes metal. Seen that, too. Proper fix is to have those faces milled.
 
Dan Thomas said:
Nobody reads and understands anymore. It's so sad. Who said ANYTHING about silicone being sucked through that hole? Who said anything about it being a hose end? Who here knows the thickness of an AN elbow's wall? Anyone besides me? Who else here is a licensed mechanic, or has any experience at all with fuel systems and their fittings? Who else here has even seen a chafed fuel fitting?
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Relax dude, you’re reminding me of this guy.
someone+is+wrong_zpsncnzkixr.png
 
Bell206 said:
FYI: There actually are handbooks like this per se, but they work from the opposite end of the equation and show you how to do things vs what not to do. So if you apply those readily available standard practices to the work you perform you'll greatly reduce doing something you should not do. Examples of these handbooks are OEM and other standard practices manuals, FAA ACs, SAE documents, etc. All of which can be used in the E/AB world. Add in the warnings and cautions found in various OEM mx manuals one could easily look to them for guidance regardless if working on that specific item or not. There's nothing that prevents applying an acceptable practice from one OEM to another OEM product. Even in the TC world in most cases. It only depends on how much one wants to look vs just winging it.
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Right - plenty of descriptions on how to do things, and for certified aircraft that's all you need. And from what I understand Vans is very well documented and has multiple peer resources on how to do things, too. But that doesn't include all of experimental.

In any field or trade, there's a lot of knowledge not only on how to do things, but on what things to avoid, that just aren't obvious.
 
Dan it's kinda like when soemone asks for help with his medical- not retained, no relationship. I give 'me the scoop but "Your'e all wrong doc....I have this document from the FAA and this otehr document from the FAA....and this other.

I then say, "after the denial call me back".
Unfortunatly when it's a mechanical, the probem is "call me after you survive the off airport landing".
Sigh.
 
Albany Tom said:
But that doesn't include all of experimental.
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Albany Tom said:
but on what things to avoid, that just aren't obvious.
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Sure it does. All that general guidance applies to any aircraft, boat, vehicle, whatever if you want. But the point you're missing is if you follow the guidance on how to perform the task correctly, you'll never need to know those "things to avoid." For example, written standard practice defines all torques are dry torques unless specifically stated they are wet torques with the required lubricant. Hence, why would you need a reminder that a "lubricant greatly affects fastener torque." Proactive measures always win in maintenance over reactive measures.
 
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Bell206 said:
Sure it does. All that general guidance applies to any aircraft, boat, vehicle, whatever if you want. But the point you're missing is if you follow the guidance on how to perform the task correctly, you'll never need to know those "things to avoid." For example, written standard practice defines all torques are dry torques unless specifically stated they are wet torques with the required lubricant. Hence, why would you need a reminder that a "lubricant greatly affects fastener torque." Proactive measures always win in maintenance over reactive measures.
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Yup. It would take a huge book to list all the ways one shouldn't do something. There are far more ways to do it wrong than to do it right. Nobody would get past page 20 of a book like that.
 
Dan Thomas said:
Sure, you'll see it. Doesn't make it legal or wise at all. Silicone on exhaust gaskets can't hurt much; it isn't going to get sucked into the engine. But it's usually there because the exhaust port face on the cylinder head has been eroded and pitted by exhaust squirting past a bad gasket. It acts like a blowtorch, and removes metal. Seen that, too. Proper fix is to have those faces milled.
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Thing is that the silicon burns out faster than the metal. I asked about it once, and was told by several mechanics, that they use it to hold the gasket in place while they install the exhaust. I use a piece of safety wire myself. And pull it out when the nuts are started.
 
I've seen some bizarre examples of RTV abuse. A lot of guys will use it to seal baffles and I guess because of the fact that you really can't recap a tube of RTV and expect any of it to come out the next time you open it this creates a compulsion to use the whole tube. So you see people smearing it externally along case half seams and using globs of it to create impromptu mounting clamps, grommets or electrical boots. If it's applied onto a clean fresh surface it is damn near friggin' impossible to get it all off when the time comes to clean up the mess. Yes, it's amazing stuff that will stick to dang near anything except the cured form of itself to which pretty much nothing else will adhere and it definitely has it's uses but sheesh, come on people....
 
Mineral spirits is a pretty good way to clean silicone rubber.

Do Not use mineral spirits on any silicone gasket you wish to reuse like on rocker covers...I learned this years ago working on valve cover gaskets on my Jaguar...they will soften and swell up. :)
 
Dan Thomas said:
Nobody reads and understands anymore. It's so sad. Who said ANYTHING about silicone being sucked through that hole? Who said anything about it being a hose end? Who here knows the thickness of an AN elbow's wall? Anyone besides me? Who else here is a licensed mechanic, or has any experience at all with fuel systems and their fittings? Who else here has even seen a chafed fuel fitting?
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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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