BY JOE NORRIS
REGARDLESS OF THE AIRCRAFT you’re working on, sooner or
later you’ll need to fabricate a fuel, oil, or hydraulic line. The
most common way to connect these lines is with AN fittings.
These fittings require the tubing to be terminated with a
37-degree flare. Note that this is different than the flare used on
automotive lines, which are typically terminated with a
45-degree flare. This means that you’ll need proper tools to
install the fittings on your aviation lines. Don’t use your
automotive brake-line flaring tool as this will not give you
the proper flare and will end up causing you problems.
Let’s take a look at the process for fabricating these
STEP 1/ Here’s a completed flared tubing connection, as well as the individual parts you’ll
need to make one. The tubing is typically 5052-0
aluminum; you’ll need to know the outside
diameter of the tubing to get the correct fittings.
You’ll need an AN819 coupling sleeve and an
AN818 coupling nut for each connection.
STEP 5/ Next you’ll need an aviation flaring
tool. This tool will actually form the 37-degree
flare on the end of the tubing. There are several
styles, but each will do the same job. The flaring
tools are typically set up to flare a variety of tubing diameters, so you’ll need to pick the proper
die on the tool for the tubing diameter you’re
working with. The tool will also include a depth
gauge that will allow you to seat your tubing in
the tool to the proper depth so that the flare will
be correctly formed.
STEP 2/ You’ll need several tools. First you’ll need a way to cut the tubing. You can cut it with
a hacksaw or cutoff wheel, but the best way is to
use a tubing cutter, as shown here. A few turns
around the tube with the tubing cutter, tightening the cutter a little at a time and not trying to do
too much at once, will result in a clean, straight,
true cut and make the rest of the installation go
STEP 6/ Lubricate the tool with some light oil
and turn the handle until the tool bottoms in the
die. Don’t force it! You don’t need a lot of pressure to accomplish your goal. Just turn it until it
feels like it has bottomed out and stop there.
STEP 3/ Deburr the end of your tube. Use a
hand file or Scotch-Brite wheel for the outside.
For the inside you can use a deburring tool as
shown here, or you can do it by hand with the
tool that comes with your tubing cutter. You want
a nice, clean end with no burrs so that your flare
will turn out straight and true.
STEP 4/ Don’t forget to put your AN818 coupling
nut and AN819 coupling sleeve on your tube before
you form the flare. This is especially important if
you’ve already flared the other end of the tube,
otherwise there’s no way to get the coupling sleeve
or nut on the tube after you’ve flared the end.
STEP 7/ You’ll end up with a nice, clean, true
flare. You’ll want to carefully inspect the flare to
make sure you didn’t crack the edge (which will
happen if you put too much pressure on the tool
when forming the flare). You’ll also want to blow
out the tube with some compressed air after you’ve
formed the flares on both ends to make sure there’s
no debris or contamination in the tube before
installing it on your aircraft.
STEP 8/ Here’s your finished product! The coupling nut and sleeve will tighten the flare against
the appropriate fitting on your aircraft to complete
a simple, leak-free connection.