Just as time is nature’s way of keeping everything from happening at once, the tube is nature’s way of getting gases and liquids from one place to another without a mess. Airplanes abound in tubes, hoses and pipes containing fuel, air and hydraulic fluid.
In some cases, to resist vibration and allow relative movement, tubes take the form of flexible hoses. But where flexibility isn’t required, there’s no need for heavy and expensive hoses. For that reason, most kit aircraft use light, cheap and easy-to-form 3003 semi-soft aluminum tubing where rigid lines make sense.
Keeping fluids inside an aluminum tube is relatively easy. Designing joints that allow tubes to connect to vibrating pieces of equipment, or one another, without leaking—that’s a lot harder. The solution is to install fittings that will allow fluids to pass, run the tubing between them and fasten the tube to the fitting with a threaded fastener.
The most common connection for fuel, vent and brake lines is a single-flare fitting. (High pressure hydraulic lines require different materials and methods, but they don’t appear on a lot of small homebuilt airplanes, so we won’t deal with them here.) A flared connection requires four pieces of hardware, at least three tools and several operations for fabrication and assembly. What could be more fun?
Making the Connection
First, make sure fittings and tubes are matched for size. Most aluminum tube in small airplanes will be either ¼ inch (brake lines, fuel vent lines, and perhaps fuel lines for small engines) and ⅜ inch (usually for fuel lines for engines of 100 horsepower or more). The fittings are machined from aluminum (anodized a beautiful deep blue) or steel (gray).
Steel fittings are often preferred on engine cases and sumps, but most of the time fittings within the airframe will be aluminum. Flare fittings (often called AN fittings) are available in a wide variety of elbows, tees and union, each with its own designator number. On at least one end is a male cone, with an included angle of 37°. (Somewhere, long ago, aircraft engineers concluded that 37° was the magic, non-leaking angle. Automotive engineers, however, settled on 45°. The two systems are not compatible, however, so be careful not to use automotive tools or fittings in your airplane.)
Behind the cone are threads that will accept a nut (part number AN818 – (n)(x), where n is the diameter tubing in sixteenths of an inch and x designates the material. An AN818-4D nut fits ¼-inch tubing, and the D means aluminum. Within that nut is a collar that slips around the tube [AN819-(n)(x)] and rests against the back of the flare. So the four necessary pieces are the fitting, the nut, the collar and the tube.
Connecting the tubing to the fitting requires slipping the nut and collar onto the tube, and then forming (flaring) the end of the aluminum tube to a female 37° cone. To make the connection, the male and female flares are mated up, and the nut is threaded onto the fitting. As it tightens, it pulls the collar, squeezing the flared tube onto the male cone of the fitting and forming a leakless seal. This is not a difficult operation, but it must be done in the correct order with the right tools.
Preparing the Tube
Soft aluminum tube should be cut with a tubing cutter, not a hacksaw or a snip. Tightening the cutter slightly every turn leaves a nice, square end. After making the cut, polish the end of the tube with fine crocus cloth, emery paper or a Scotch-Brite wheel.
The best tool for flaring the tube is the rotary flaring tool. This hefty beast isn’t cheap, but it has rotating dies that will fit all common tubing sizes, stops that ensure the tubing is in the right place for forming and clamps that will hold it there. It will last for generations.
Most flaring tools have a triangular, foldable blade. This is used for cleaning the burr left by the cutter on the inside of the tubing. It works well enough on larger sizes, but at sizes of less than about ⅜ inch it’s usually best to use a deburring bit.
Once the dies (top and bottom) are rotated to the proper size, the nut and collar are slipped over the tube, and the end of the tube is inserted into the dies. A little finger on the back of the dies serves as a stop. When the tube hits the stop, close the top of the tool and tighten the clamp. This sounds simple, but holding the tool, moving the tubing and tightening the clamp requires three hands. The operation gets much easier if you clamp the tool in a vise and bring the tubing to it.
Put a drop of light oil or a dab of Boelube on the cone of the flaring tool. Spin the cone down into the tube and watch it force its way into the tube, spreading the end and creating the flare. (Oops! Did you forget to put the collar on first? Cut the flared end off the tube and start over.)
Take a good look at the stretched aluminum around the circumference of the flare. You will probably see some tiny stretch marks, but there should be no cracks or splits. Polish the edges with a scrap of 320 wet/dry sandpaper and blow the finished tube out with compressed air to remove deburring/sanding chaff.
Connect the Dots
Mate the flared end of the tube with the conical end of the fitting. Slide the collar down the tube until it rests on the top of the flare. It must be square to the fitting. Any slight angle will make if difficult to start the nut, and if you do get it started, you run the risk of splitting the aluminum flare. Slide the nut over the far end of the tube and engage the threads on the AN fitting. Tighten to the specified torque—assuming aluminum fittings, it’s 40 to 65 inch-pounds for ¼-inch tubing, 75 to 125 inch-pounds for ⅜-inch tubing. Getting a torque wrench onto fittings is usually difficult and often impossible, so an alternative method is to tighten the nut as tightly as possible by hand, and then use a wrench to tighten it once more flat. This is not a place for bulging biceps. Too much torque will simply crush and split the aluminum flare. Use a dab of TiteSeal or Fuel Lube on the nut threads, but don’t get any on the mating flare surfaces.
What Might Go Wrong?
The typical mistake is to push too much tubing into the flaring tool (bypassing the little finger). A bigger flare is not a better flare. In fact, the aluminum will often refuse to stretch far enough and crack as the flare is formed. Other problems could arise from ragged or crooked cuts on the end of the tubing or failing to clean the tube properly.
With the right tools, some practice and a big stamp on your forehead reminding you not to forget the AN818 collar, flare fittings are easy. And properly made, they can last the life of the airplane.