Source: 2009 Q4 Beartracks, Eric Newton
Caution: Do Not Use automotive flaring tools for aircraft tubing. Automotive flares are done at 45 degrees, aircraft flares are done at 37 degrees. All aircraft quality flare fittings (AN) are set up to receive a 37 degree flare.
Most of the brake lines are ¼” aluminum tubing (3003-0) and the fuel lines are 3/8” aluminum tubing (3003-0). We use the 3003-0 aluminum tubing because it’s nice and flexible but durable as well. You can also use 5052-0 tubing which is a bit stiffer but more durable.
Again, aircraft flare fittings are 37 degrees. Automotive ones are 45 degrees. Make sure that you purchase the correct flaring tool (37 degree) for making up the fittings for your airplane.
Step 1. Cut the tube to the proper length using a tubing cutter. Lightly smooth the end with some fine Emory cloth and debur the inside diameter. Slip on the flare nut (AN818) and sleeve (AN819):
Step 2. Using the proper, 37 degree flare tool, flare the end of the tube:The flared tube:
Step 3. Slide the sleeve (AN819) to the flare:Now you are ready to go. You don’t need any type of sealer, Teflon tape or goop on these fittings. Just leave them dry – they are self-sealing. Don’t over-tighten them. Snug them down but don’t over-do it. It is very important that you never use Teflon tape on aircraft fuel systems.
Done incorrectly, small bits of the tape can be torn loose during tightening and clog your fuel system. Flares, like those shown above, need no sealant at all. Pipe thread fittings could use a bit of EZ Turn Lubricant or
Bakerseal to lubricate the threads so they can be tightened down without galling the threads. All fuel and brake lines should be well supported (no less than every 12” for ¼” tubing, and no less than every 16” for 3/8”tubing) against vibration, chafing and movement.
Also, all tubes should have gentle sweeping bends of a proper radius (minimum 9/16” radius for ¼” tubing and minimum 15/16” radius for 3/8” tubing). Kinking of the tube, even a mild one, is cause for rejection and
replacement. Lever-type tubing benders are inexpensive and available from discount tool dealers like Harbor Freight.