Flare Fittings - Improving the Seal

Rob58

Rob58

Pre-takeoff checklist
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Between fuel and hydraulics I have a couple of dozen lines with flare fitting connections. I have noticed that a few of these connection seem to be leaking. Is there a method or tool to polish the flared end of a line to clean up any imperfections? I'm thinking a conventional countersink tool is a little overkill for this - I'm looking to burnish the finish. The mating AN fittings are not the problem - it's the flared end of the lines. Thanks!
 
Rob58 said:
I'm thinking a conventional countersink tool
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will eff up the end of the tube by removing metal.
I would gently re-burnish with a flare tool long before I took a countersink to the end of a line. Even then, I would be worried about overdoing it and work hardening.

But I ain't no A&P, so following my advice will likely result in flaming death raining from the sky.

https://www.aircraft-tool.com/shop/detail.aspx?id=212FB
 
Rob58 said:
Is there a method or tool to polish the flared end of a line to clean up any imperfections?
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Definitely no on the countersink. One recommended method is to remove the line and look at both flares with a 10x glass for cracks. On hard tubes pull the flare collar back and look at the radius area. If you see a ridge/mark on the exterior of tube flare at the collar interface then fitting has been over torqued and tube may need to be re-flared/refabricated--or you can try the flare seals mentioned above. I prefer the copper ones if you can find them. But if you find any cracks need to repair/replace the tube/line/fitting.

Keep in mind, there are several types of flares used in aviation: single flare, double flare, etc. Also aviation flares are 37 deg which are different than industrial 37 deg fittings and automotive flares.

Minor scratches can usually be dressed out with some high grit sandpaper but be sure to sand perpendicular to the fitting/line axis. No files. And thoroughly clean the tube afterwards. FYI, a pilot can not mess with hydraulic lines but fuel/oil/air lines are covered under 43 Appx A

The key is do not over torque the B nuts. There are various listings of recommended torques. Normally when a flare fitting is over-torqued, a fine crack develops through the tube flare and starts to leak more the tighter you turn it.
 
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I have a "double-flaring tool". It folds the end of the tubing in on itself. It leaves a lot more "squish" volume. Never needed to do anything else to prevent leakage. I've used it on racing vehicles, burning methanol. Not on an airplane, however.
 
@DaleB's recommendation is the correct method for flares that leak but are not cracked.

"DEL fitting seals are designed to eliminate leakage due to scratches on the sealing surface, improper torquing, or poor finishes on tube ends, in high or low pressure applications. For use with AN, MS, and JIC 37° flared fittings, DEL fitting seals also reduce tube flare cracking by reducing torque needed, eliminating nose damage to the fitting, and allow reuse of hardware. Approved by FAA, NASA, SAE, Air Force, Army, Navy, and major aerospace contractors."
 
The nice thing about most tubing cutters is that they have a little recess to accommodate the flare, so you can cut the tube only 1/8" to 1/4" away from the flare, and then make a new one. Typically there's enough slack/flex in the line to reconnect with the slightly shorter length.

Lots of great tutorial videos on the EAA website. Aluminum tubing is cheap...buy some and get a lot of practice making good flares.
 
I agree that cutting and creating a new flare is preferable to reconditioning an old flare. I have had a few leaks and this is a quick easy resolution.
 
Great feedback - thanks! I didn't consider that over-torquing might induce cracks, but that makes good sense. In my cavern of fuel and hydraulic lines it will be difficult to visually examine the flare end - is there some type of penetrant dye that would make a small crack easier to spot?
 
A good flashlight and a magnifying glass are easiest. The conical seals mentioned we used to use on Jetstream fuel manifolds, where the sealing surfaces were worn stainless steel and would not seal no matter how tight they were made. If we missed one, it was easy to find.

If one side is softer, that will often be the side causing issues.
 
Rob58 said:
is there some type of penetrant dye
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There is a dye penetrant but not for this application. If you can't get to it to check with a light and glass, then you need to remove the line. It is what it is.
 
I have been known to loosen the end and apply "fulelube" lightly to the cone and nut threads and torque to spec.do not do this on any fitting after the fuel filter.
 
Bell206 said:
Keep in mind, there are several types of flares used in aviation: single flare, double flare, etc. Also aviation flares are 37 deg which are different than industrial 37 deg fittings and automotive flares.
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[nitpick] automotive are 45° [/nitpick] ;)
 
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