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Stay away from RTV near fuel

LettersFromFlyoverCountry

Well Known Member
The NTSB this week released the probable cause of an RV-10 crash (I'm not saying where nor who because I don't want to open that can of worms and have people miss the important point) last year in which two people died.

There were two important takeaways.

(1) Don't use RTV* in a fitting in the fuel line.

rtv.jpg


(2) If someone with lots of experience and knowledge says "don't do that", then don't do that.

* Or Teflon tape.

Carry on
 
Floscan has pipe thread. You can see RTV used for the threads and some stuck inside the Floscan. No further comment.
 
A good example of how "it's what you don't know that will kill you". Chances are the offending substance was also used upstream on the fuel valve as the blockage was in the inlet of the FF xdcr line.
 
I saw an engine oil pump that had ingested a slug of silicone RTV and it fractured the housing caused an engine failure. Oil pressure was high enough not to trip the pressure switch, but not enough to lube the engine.

RTV is only to be used in rare, very rare, situations for engine related sealing. And sparingly if used at all. Slugs of excess are just one of the failure modes, but is at the top of the probability list.
 
I can't think of a case where RTV would be ever indicated FWF. AN fittings are assembled dry and NPT fittings (like the one on the fuel flow sensor) require an aircraft-grade thread sealant like Tite-Seal. Where would RTV be the best approach anywhere FWF?:confused:
 
It is very saddening to learn that something so simple and obvious to most caused loss of life.

Regarding other comments stating to never use Teflon tape on threads. There is a type of Teflon tape made specifically for fuel and oil use, and I have used it many times on aircraft. Yes it can cause blockages is it is applied incorrectly, but so can Teflon pipe dope.
 
I can't think of a case where RTV would be ever indicated FWF. AN fittings are assembled dry and NPT fittings (like the one on the fuel flow sensor) require an aircraft-grade thread sealant like Tite-Seal. Where would RTV be the best approach anywhere FWF?:confused:

Anywhere FWF? Sealing various areas on the baffles comes to mind...
 
Anywhere FWF? Sealing various areas on the baffles comes to mind...

Sorry, I meant on the engine. I know you can use it to secure wiring, plug holes, provide chafe protection, etc. Even then you need to be careful of which product you're using and the substrates on which you're using it.
 
So what should we use?

What is commonly used in the certified aircraft world for this application? This thread mentions Loctite 567 and a special Teflon tape but are these common ways to do this?

I had an IA back when I had my Aeronca that told me to use Fuel Lube on these types of connections. He said that this prevents fuel seeps. I still use Fuel Lube on my RV and haven't had a problem.
 
What is commonly used in the certified aircraft world for this application? This thread mentions Loctite 567 and a special Teflon tape but are these common ways to do this?

I had an IA back when I had my Aeronca that told me to use Fuel Lube on these types of connections. He said that this prevents fuel seeps. I still use Fuel Lube on my RV and haven't had a problem.

I'm no expert, but this is almost a religious question (search the archives and you'll see what I mean). Fuel Lube, EZ Turn (same thing as Fuel Lube), Tite-Seal, Loctite all have been used and are acceptable I believe. Some think Fuel Lube/EZ Turn is garbage and Tite Seal is the way to go, and others are Loctite apostles. I researched this a while ago, and basically the requirement is that NPT threads require a sealant appropriate to use.
 
Doug, thanks for putting this on the front page. My wish is you would do that from now on with every Probable Cause henceforth. Fills a void for me in GA, and after all, it is all about me. :rolleyes:
 
Sorry, I meant on the engine. I know you can use it to secure wiring, plug holes, provide chafe protection, etc. Even then you need to be careful of which product you're using and the substrates on which you're using it.

The air filter for an updraft -6/6A requires modification with RTV to fit the FAB per Vans plans.
 
I'm no expert, but this is almost a religious question (search the archives and you'll see what I mean). Fuel Lube, EZ Turn (same thing as Fuel Lube), Tite-Seal, Loctite all have been used and are acceptable I believe. Some think Fuel Lube/EZ Turn is garbage and Tite Seal is the way to go, and others are Loctite apostles. I researched this a while ago, and basically the requirement is that NPT threads require a sealant appropriate to use.

True, there are a lot of opinions regarding what should/should not be used on pipe thread fittings.

The very simple answer is use something that is specifically designated as a "thread sealant".

If that is part of its name, or the specs specifically state "for sealing threads", it is probably fine to use. Stay away from anything that that does not specifically state for sealing NPT threads.
 
Here's the complete docket.

Interesting to note that the pilot had done a conditional inspection 2-3 months earlier, but on day of accideent, found a starter bolt had fallen out and wasn't engaging, and witness also said the alternator bolts had to be tightened down.

Blog posts indicated he was not very satisfied with power output of the engine and was looking for help.

Had to charge the battery after the plane wouldn't start.

Wondering if he had all electronic ignition or mags.
 
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Here's the complete docket.

Interesting to note that the pilot had done a conditional inspection 2-3 months earlier, but on day of accideent, found a starter bolt had fallen out and wasn't engaging, and witness also said the alternator bolts had to be tightened down.

Blog posts indicated he was not very satisfied with power output of the engine and was looking for help.

Had to charge the battery after the plane wouldn't start.

Wondering if he had all electronic ignition or mags.
One of the reports indicates mags. Would be interesting to see the electrical schematic. It did indicate use of the EXP Bus.
 
use of sealant

I read here to use a fuel sealant on the AN fittings and purchased some from Aircraft Spruce.

It did say on the instructions to only use sparingly and to apply one to two threads up from the opening. This appeared to make a fine seal to me and this was used on the fuel valve.

Any thoughts?
 
use of sealant

I read here to use a fuel sealant on the AN fittings and purchased some from Aircraft Spruce.

It did say on the instructions to only use sparingly and to apply one to two threads up from the opening. This appeared to make a fine seal to me and this was used on the fuel valve.

Any thoughts?
 
The nut ain't sealed

I read here to use a fuel sealant on the AN fittings and purchased some from Aircraft Spruce.

It did say on the instructions to only use sparingly and to apply one to two threads up from the opening. This appeared to make a fine seal to me and this was used on the fuel valve.

Any thoughts?

Be aware tho the sealing surface is where the inside of the flare meets the the male side of the fitting. All the threads do is pull the two together for a seal. The only thing I have ever put on these threads is a tiny bit of fuel seal so as to make removal easier. Never had a leak using this method.
 
Be aware tho the sealing surface is where the inside of the flare meets the the male side of the fitting. All the threads do is pull the two together for a seal. The only thing I have ever put on these threads is a tiny bit of fuel seal so as to make removal easier. Never had a leak using this method.

I think he is talking about pipe thread on fittings, mounting them into a fuel valve.... not flared lines attaching to AN fittings.

To the original question, it sounds like like you are in good shape with the way you installed the fittings.
 
dumb question

On the typical fuel transducer there is an npt-to-AN-flare adapter. On one side of the adapter fuel sealant is required, but the other side it's strictly verboten. I guess I don't understand the danger of having sealant on the threads of the AN fitting. I now understand it isn't necessary. And obviously putting sealant on the flare itself is bad. But what's the problem with sealant on the threads?
 
Joe--we've found in other AN/JIC applications when sealant is applied to the treads, it sometimes allows the nuts to tighten before the 2 flares meet and interlock. The seal is made by the 2 flares, not the threads, which is there for the mechanical connection. NPT is different, because of the tapered threads. Those do need sealant.
 
Ahhh. That makes sense. It's not because we're worried about sealant getting into the line. It's because the sealant can prevent the connection from sealing! ... And this is why I ask dumb questions.

Thank you,
-Joe
 
I wasn't aware that a lubricant could cause torque to be reached prematurely (premature torqulation?). I would have thought it the other way around. Thanks for explaining:D
 
I wasn't aware that a lubricant could cause torque to be reached prematurely (premature torqulation?). I would have thought it the other way around. Thanks for explaining:D

My guess(!) would be that in this case the lubricant/sealant basically fills up the space and doesn't flow out of the empty space as fast as it needs. Torquing slower might work better, but you shouldn't need anything anyway (as stated above).
 
While I can not speak for the Red Cube, the FloScan units warn that no sealant or lubrication should be used on the pipe fittings. Evidently, the sealant can migrate into the unit and gum up the impeller or otherwise inhibit it's function. Fuel lube holds up well to fuel but it will leach out of the fitting.
I like EZ-Turn Fuel Lube for pipe fittings. I use it as a lubricant not a sealant. The same is true of "plumber tape". It is not intended to seal but to lubricate the threads so they can be properly tightened to form the metal to metal contact 360 deg. or more around the thread. It is also intended to keep dissimilar metals from galling and to protect the threads from corrosion so you can get the fitting off down the line.

Fuel lube is commonly used on other components like the fuel distributor (spider) on those pipe fittings, etc....

In theory, a pipe thread should be self sealing when properly tightened right? I think so many leak because it is difficult to align 90 deg. fittings and get things correctly tight at the same time.
 
Also, a note on NPT, they are cut to mate and seal with no sealant, in my experience, they are a ONE time thread, mating and sealing to each other when tightened.
 
The NPT threads that are on all aircraft AN fittings are not intended to seal 100% without the use of thread sealant. Particularly since many of the fittings we use are 45 or 90 deg which during installation requires the insertion to be stopped at a specific orientation which may or may not have result in the fitting being fully seated 100%.
 
The NPT threads that are on all aircraft AN fittings are not intended to seal 100% without the use of thread sealant. Particularly since many of the fittings we use are 45 or 90 deg which during installation requires the insertion to be stopped at a specific orientation which may or may not have result in the fitting being fully seated 100%.

With this in mind, I wonder why they even use NPT threads? They are hard to work with, they are not so easy to seal and subject to so much troubles.

I hate NPT fittings.
 
With this in mind, I wonder why they even use NPT threads? They are hard to work with, they are not so easy to seal and subject to so much troubles.

I hate NPT fittings.

Buy Loctite 567. You'll never hate NPT fittings again.
 
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