Best Practices – Plumbing

Many airplane builders use rigid aluminum tubing for fuel and vent lines, and occasionally hydraulic lines, in their projects. This material is relatively inexpensive and serves builders well for these applications if pressures are not too high, but there are better and worse ways to use aluminum tubing. Some builders may need to work with stainless steel for high-pressure hydraulic lines, but most builders will not, so we will mainly look at aluminum tubing.

Types of Aluminum Tubing

There are two types of aluminum tubing available for our use: the more flexible 3003-0 (what Aircraft Spruce calls Versatube) and the stronger 5052-0 alloy. Versatube (3003) is less expensive and will work for fuel and vent lines in most applications. It can also be used for vacuum lines, but this is obviously not popular these days since almost no one is building a new plane with a vacuum system. It is not recommended for hydraulic systems due to its fairly low strength and working pressure. Another disadvantage of 3003 tubing is that it comes in rolls and must be straightened. Depending on how picky you are, this is a minor task or a real nuisance. Tubing straighteners are available to purchase or you can make your own, but that just becomes one more task or expense to absorb into your project budget and schedule. On the other hand, 3003 tubing is easier to bend.

Stronger 5052 tubing comes in straight sticks, so straightening it is not an issue, but it is more difficult to bend. It is not so hard to bend as to be a problem in most cases, but it will always require the use of a tubing bender. It is best to also use a bender for 3003, but adjustments can often be bent by hand if you don’t need to bend it too much. The main advantage of 5052 tubing, besides its straightness, is its greater strength. It is good for about twice the working pressure of 3003 tubing. This means that it is adequate for un-boosted hydraulic lines for brakes. However, its 1500 psi maximum working pressure may not be sufficient for hydraulic landing gear applications. Needless to say, 3003 should never be used for hydraulic applications with any mechanically boosted pressure. Check with your kit maker to be sure if this is a concern. Both 3003 and 5052 are well suited for fuel and vent lines. Other applications should be carefully considered.

Bending Aluminum Tubing

Just about every application will require some bending, so getting a tool to do the job well is important. Trying to make bends without such a tool is a bad idea because without the right tool, it is impossible to keep the tubing from collapsing as you bend it. This collapsing will reduce the flow through the tube. Tubing benders start at about $30 and go up from there. If you have more than one size of tubing to bend be sure to get a bender that will handle all sizes, if possible. No need to get two tools when one will do the job.

Making a single bend when you have plenty of tubing doesn’t require much thought. Just place the tube in the tool, bend the desired amount, and remove. However, to make a series of bends successfully, some planning is required. For example, if your bender produces a one-inch radius in 3/8-inch tubing (about the minimum for that diameter), you will need to move the starting point one inch to allow for the bend. Consider a 12-inch section with a 90-degree bend at each end. The straight portion will be 12 inches long minus one inch off each end to allow for the bends, giving you a straight section of 10 inches.

With more complex bend patterns it is easier to start off with a template made of bent wire such as welding rod. Then just match the template. Working from one end and making frequent checks along the way will save you scrapped tubing. When making your wire template, be sure to factor in the tubing bend radius and not make sharp bends in your template that cannot be duplicated in the tubing.

The 3003 alloy will allow easier tweaking of complex shapes that don’t quite fit, but the downside may have a less attractive appearance. It is also easy to collapse 3003 tubing as you are making those adjustments. On the other hand, the neater appearance of 5052 tubing will often be paid for by being somewhat more difficult to handle.

Cutting

It is best to cut tubing with a tubing cutter, but it can also be cut with a hacksaw or metal-cutting band saw. If you use a saw, be sure to use a fine-tooth blade (32 teeth per inch). In any case, the cut needs to be square to be suitable for flaring later.

After cutting be sure to clean up the end and remove any burs. A flat file or belt sander can work for this if used with care. Be sure to remove any rolled over material left on the inside of the cut. This is especially common when using tubing cutters. A countersink tool works well for this, but be careful not to remove too much material. This is much like deburring a rivet hole. Remove enough material to make a clean hole but not enough to actually cut a countersink into it. I like to make a quick pass over the cut end with a Scotch-Brite wheel when I am done, just to clean everything up. The final step is to clean out any debris left in the tube. Usually a blast of compressed air will do the job.

Joining Tubing to Other Parts

Tubing can be joined to other parts with a flare fitting, compression fitting, or by slipping a flexible hose over the end. Flare fittings using standard AN parts are the most common, so we will look at them first. The tube is prepared for joining by slipping on an AN818 nut and an AN819 sleeve and then flaring the end using an aviation flaring tool. Please note that automotive flaring tools are set for a different angle and are not acceptable for aviation use. The nut and sleeve then secure the flared end to another AN fitting that is designed to work with these parts.

Photos show the two most common aviation flaring tools. They can both do a good job. The one-piece tool is easier to use but is not as good for situations where a bend is close to a flare fitting. The type with the separate yoke is slightly more cumbersome to use but will work better when the flare is close to a bend. The cost is about the same, so there is no advantage there one way or another. Note that the one-piece tool will not work with stainless steel tubing such as you will need for high-pressure hydraulic lines. The tools with separate yokes may work with stainless steel, but double check the literature before buying to be sure.

The one-piece tools have a built-in stop to correctly position the tubing for the flare, but with the others you will need to allow a small length of tubing to project above the tool to allow for sufficient material for the flare. About a penny thickness will do the job. A dime or a quarter will also work if you find yourself penniless. Be sure to check your flares for cracks. Any cracked flare needs to be replaced before use.

For those seeking the best possible flare, the double lap flaring tool is the answer. On the plus side it produces a superior flare, but on the downside the tool costs about $600. This is not a necessary expense for the typical builder, so it is only mentioned as a point of information. Do not be tempted to buy a less expensive automotive double flaring tool. It produces a 45-degree flare and not the recommended 37-degree flare used for aviation.

Compression fittings are also available under the Swagelok brand. These fittings are handy to use, but more expensive and heavier than AN fittings. They have their place, but most builders will not use them because of these shortcomings.

Sometimes it will be convenient to attach a flexible hose onto the end of a rigid tube. If this is to be done, it is a good practice to roll a bead into the end of the rigid tube to help secure the hose and clamp. Inexpensive bead rollers are available from Aircraft Spruce for 3/8-inch .035-wall tubing, the most common size for fuel lines. Bead rollers are available from other sources for other sizes. Be sure to push the hose on far enough to get past the bead and allow for the width of the hose clamp.

Support

Long runs of rigid tubing should be supported about every 12 inches for 1/4-inch lines and every 16 inches for 3/8-inch lines, preferably with a cushioned Adel clamp. In some situations, it may be desirable to move supports even closer. The point is to prevent vibration or other potentially destructive movement. This guidance comes from FAA AC 43.13-1B, a copy of which should be at hand for anyone building an airplane.

Damaged Tubes

Tubing may get damaged or flattened in fabrication or in use. AC 43.13-1B again gives us guidance as to how much flattening or scratching is acceptable. A tube may be flattened to no less than 75% of its original diameter and still be useable. Ultimately the serviceability of a fuel line depends on its ability to flow fuel at the required rate, which may or may not allow for flattening.

Scratches to tubing may not exceed 10% of the tubing wall thickness. In other words, 3/8-inch tubing with a wall thickness of .035 inch may not have any scratches that are more than .0035 inch deep. This is not much of a scratch, and is admittedly hard to measure. The basic rule is when in doubt, throw it out. Deep scratches can cause stress risers that will lead to cracks in the future. Nobody wants a cracked fuel line in their airplane.

Routing

Rigid lines should be routed in a way that keeps them clear of excessive heat, such as exhaust systems. They also need to be protected from chafing. The need for support has already been mentioned. Any transition from a rigid line to an Aeroquip flexible line should be supported as close to the joint as possible. As a general rule, the most direct routing is preferred, and the fewest possible number of bends will produce better flow than more convoluted layouts. This must be weighed against other considerations, but it should always remain a goal.

Vent lines need to have runs that do not produce any low points except at their exit and origin. No dips in between these two points should be permitted. Similarly, fuel lines should always be run downhill from the fuel tank to the system drain. If this is not possible, a drain must be installed at any low point. With tailwheel airplanes, this should be checked in the level and parked positions, because low points may be created in one attitude that are not present in the other. Metal lines should never be placed in a position where they are likely to conduct electricity. Lines routed near batteries need to be protected. Rigid fuel lines run in the engine compartment should be protected with firesleeve or a similar material, just as flexible lines would.

A neat and secure installation should give you many years of reliable service requiring virtually no maintenance. It is worth the trouble to do it correctly.