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  #21  
Old 09-17-2021, 04:37 PM
rileyspoon rileyspoon is offline
 
Join Date: Oct 2020
Location: WINSTON SALEM
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Default Maybe someone can explain....

Anyone care to explain why sealant on A is less likely to cause an issue than B? I understand that sealant on a flared fitting is probably a waste of time and not needed, but I'm not sure I see the need to disassemble all the fittings and remove the sealant from the flared side and put it all back together. Especially now that it is holding pressure at 50psi.

Just asking for a friend.....
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  #22  
Old 09-17-2021, 07:52 PM
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rvbuilder2002 rvbuilder2002 is offline
 
Join Date: Jul 2005
Location: Hubbard Oregon
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Quote:
Originally Posted by rileyspoon View Post
Anyone care to explain why sealant on A is less likely to cause an issue than B? I understand that sealant on a flared fitting is probably a waste of time and not needed, but I'm not sure I see the need to disassemble all the fittings and remove the sealant from the flared side and put it all back together. Especially now that it is holding pressure at 50psi.

Just asking for a friend.....
If you are absolutely positively sure that no sealant made its way to the tapered mating surface that the flared tube seals against, on any of the connects completed this way, then there probably is no direct reason.

A potential indirect reason, is that any airworthiness inspector that is giving you your moneys worth, that sees any evidence of sealant at B nuts is going to question the installation. I know I would.

The reason A is acceptable is because the threads are tapered, and if sealant is applied properly (do not apply sealant to the first 1.5-2 threads) the sealant is forced towards the outside of the bore it is being threaded into.

With B, there is no way to know where sealant has gone during the tie that the line is being fiddled with to get it lined up and then get the B nut started. Sometimes when the B nut is being initially threaded on, the line pulls away from the fitting. If the was enough sealant being squished (fancy name for extruded) out of each end of the threads depending on how much was applied, it is possible that some sealant could get between the flare on the line and the conical end of the fitting. Then when the B nut is fully tightened it could extrude some of that sealant to the inside of the tube.
All hypothetical of course but still possible.
One of the top priorities in building an airplane is to do everything possible to mitigate the unknowns.
This is an unknown. You are the one that is going to have to decide what to do with it.
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Opinions, information and comments are my own unless stated otherwise. They do not necessarily represent the direction/opinions of my employer.

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Last edited by rvbuilder2002 : 09-17-2021 at 08:02 PM.
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  #23  
Old 09-17-2021, 08:44 PM
Captain Avgas Captain Avgas is offline
 
Join Date: Oct 2005
Location: Melbourne, Australia
Posts: 2,011
Default

Quote:
Originally Posted by rileyspoon View Post
Anyone care to explain why sealant on A is less likely to cause an issue than B? I understand that sealant on a flared fitting is probably a waste of time and not needed, but I'm not sure I see the need to disassemble all the fittings and remove the sealant from the flared side and put it all back together. Especially now that it is holding pressure at 50psi.

Just asking for a friend.....
Here's a few comments:

1. Firstly, you may be building in a knowledge vacuum. Are you a member of an EAA chapter. If not it might be a good idea to join one.

2. The flared fittings require no sealant and in fact it is very poor practice to put sealant on flared fittings...it's dangerous. If you have leaks at the flared fittings it is because you are not doing the flares properly (which you are not). My recommendation is that you disassemble every flared fitting with sealant on it and clean them all thoroughly. I'd also clean out all of your fuel lines in case you already have any bits of sealant in them. At the same time I'd ensure that all of your flares are within spec.

3. The sealant you are applying to the flared fittings is probably not achieving anything. If the flare leaks then fuel can escape via the collar and migrate out of the rear of the B-nut.

4. Putting sealant on flared fittings is not just an initial problem but will be a problem later on if you have to disassemble the fitting (which you will have to do on many flared fittings). At that time it will be difficult to stop bits of sealant from getting into the fuel line.

5. The torque values for flared fittings are dry torque values. Putting sealant on the threads will result in higher stresses on the aluminium flare. The 3003 aluminium tube that Vans supplies is not aviation grade and is extremely soft. The flares are therefore susceptible to deformation that can ultimately result in failure (quite common on flared fitting that are disassembled and retorqued numerous times).

6.The way you are applying sealant onto your NPT fitting is also problematic. You're applying way too much and it should never be applied to the first 2 or 3 threads to ensure it does not get into the fuel system.
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  #24  
Old 09-18-2021, 12:51 AM
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rv8ch rv8ch is offline
 
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Riley, It's really good that you are asking here, and that you are testing these lines. Small leaks can be annoying since they make your aircraft smell like fuel, and large leaks can cause a fire.

The advice you are getting here is really solid. Looking at your photos I can't agree more with Scott and Bob, although I don't have 1% of the experience these gentlemen have. What I do know is that I too have been faced with the "oh ****, I did it wrong, now I have to do it again" moment several times during my build. At first, I wanted to defend the way I did it, justify it, rationalize it. Getting over that instinct (character flaw?) was not easy. Anytime someone suggests a better way to do something, I am delighted, since I've found another way to make my aircraft safer and more reliable.

After about a dozen inspections by different people during my build, most of which had a suggestion or question, I feel like each suggestion made things better. I think the suggestions you are getting from this thread to re-do your flares and to carefully clean out the thread sealant will make your aircraft safer.

One of the best things about this community is that very experienced people genuinely want you to build a safe, reliable aircraft, and they invest time in helping "newbies" like us to do so - for free!
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  #25  
Old 09-18-2021, 12:05 PM
Ralph Inkster Ralph Inkster is offline
 
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Location: Calgary, Alberta
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This is not a solution for incorrectly flared lines, but could be for the odd stubborn properly flared fitting that wont seal.
ACS pt# 04-05006 is for 3/8"
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Last edited by Ralph Inkster : 09-18-2021 at 12:33 PM.
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  #26  
Old 09-18-2021, 05:59 PM
Birkelbach Birkelbach is offline
 
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Location: Waller, Texas
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You said yourself that out of 15 or so connections that you had two that didn't leak. You also said that it was a lot of work to tighten them enough to get them to hold at 40 psi. Both of these statements tell me that there is something fundamentally wrong in your connections. A good flared fitting seals easily. If it leaks, don't tighten it more, take it apart to see why.

These are fuel lines, don't take any chances! Take it all apart and make sure it's right. I would flush out the tubing too and make sure that they are all clean. I know it's not what you want to hear and it's frustrating but it's the right thing to do.
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  #27  
Old 09-18-2021, 06:59 PM
gasman gasman is offline
 
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Location: Sonoma County
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Quote:
Originally Posted by Birkelbach View Post
You said yourself that out of 15 or so connections that you had two that didn't leak. You also said that it was a lot of work to tighten them enough to get them to hold at 40 psi. Both of these statements tell me that there is something fundamentally wrong in your connections. A good flared fitting seals easily. If it leaks, don't tighten it more, take it apart to see why.

These are fuel lines, don't take any chances! Take it all apart and make sure it's right. I would flush out the tubing too and make sure that they are all clean. I know it's not what you want to hear and it's frustrating but it's the right thing to do.
Crushed (over torqued) flared fittings can FAIL at any time with a massive fuel leak.
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