“Make Sure the Pipe Fits”
Pipe Fittings in Aviation.
Pipe threads are used in virtually every aircraft built today, from ultralights to Boeing’s finest. In sport aviation we use the pipe thread fittings throughout the aircraft, including fuel systems, brakes, oil, hydraulic, coolant and even pitot static systems. The pipe fitting has been around for quite some time, however, after looking through the FAA’s extensive selection of maintenance publications, this seems to be an area that has only been touched on lightly. We continue to see confusion, problems, and even accidents as a result of misunderstanding the subject of pipe fittings.
Pipe Thread Types
The most common types of tapered pipe thread used in the United States are the NPT (National Pipe Tapered Thread), The ANPT (Aeronautical National Pipe Tapered Thread), And the NPTF (National Pipe Tapered Fine Thread)
The NPT (National Pipe Tapered Thread) are constructed using a “Seller’s” thread (truncated peaks and valleys) with threads having an included angle of 60° and a 1.7899° taper. (Figure 1)
The ANPT (Aeronautical National Pipe Tapered Thread) is essentially an NPT thread that undergoes additional quality assurance requirements.
The NPTF (National Pipe Tapered Fine Thread) is also referred to as Dryseal American National Standard Tapered Pipe Thread. (ANSI B1.20.3) This thread was designed to provide a leak free seal without the use of Teflon tape or sealing compound. It is essentially the same thread as the NPT with the root and crest of the threads modified to provide an interference fit during installation. Time has shown that this often works okay during the initial installation, but the use of fittings with the NPTF thread on subsequent removal and re-installation will almost certainly leak without the use of a thread sealant.
Pipe Thread Size
An experienced aircraft builder or mechanic can very easily identify the size of a pipe fitting visually from across the workbench. With the limited sizes which we use in small aircraft, and a little bit of practice, you will soon be able to identify them by sight as well. The most common sizes used in light aircraft are 1/8”, 1/4”, and 3/8”. Until you become familiar with the different sizes, you can use the size chart (Figure 2) to help with identification. Either place the physical fitting directly onto the diagram or take a measurement of the end of the pipe fitting with a pair of calipers.
For easy identification aircraft aluminum fittings are anodized blue and steel fittings are plated black. Commercial grade Brass fitting are, of course, brass colored. (Figure 3)
The problem of leakage arises as a result of the design of the threads. Particularly on an NPT thread where the root and the crest are truncated. (Figure 5) This design allows for a helical passage from the inside of the fitting through the root and the crest of each thread to allow fluid to pass around the perimeter of the thread until it can escape presenting itself as a leak. (Figure 6)
The purpose of thread sealant is to fill in this passage way between the crest of the male and female thread. It is not possible to stop a leak on a pipe thread by tightening the fitting. This is because tightening the fitting does not eliminate this passage way. If you find that you have a pipe fitting that is leaking, the only method by which you can eliminate the leak is to remove the fitting, reapply thread sealing compound, reinstall and tightened properly.
Tips for proper installation of fittings with the NPT (National Pipe Tapered Thread)
Tip 1. Before installation inspect both the fitting and the boss, the boss is the raised area on a component that is drilled and threaded for the fitting. (Figure 7) Inspect both the fitting and the boss for damage and cleanliness. Clean, if needed.
Tip 2. Apply the proper thread sealant to the threads of the fitting to be installed. The type of thread sealant that will be used will primarily depend on the type of fluid used in that system. Each manufacturer of a system component may also decide the proper thread sealant to be used considering factors other than the type of fluid such as temperature, environment, vibration, and whether the part is removed for service routinely. The amount of thread sealant needed is limited to the amount that will fill the void where the threads are truncated on the fitting. Ensure that no thread sealant extends to the end of the threads where it could be ingested into the fluid system. Many an engine has quit because of thread sealant or Teflon tape being ingested into a carburetor or fuel injection system plugging a fuel passage.
Note regarding the use of Teflon Tape: Although there are some instances where the manufacturer recommends the use of Teflon tape on a pipe fitting, this is an area to tread carefully. The potential of Teflon tape being ingested into a fluid system is a high enough risk that most manufactures recommend the use of a pipe sealant instead. In addition, the use of Teflon tape substantially reduces the amount of friction during installation. This can lead to over torque of the fitting and cracking of the boss in which it is installed. And because of the low friction of the Teflon tape, this also leads to loosening of the fitting in high vibration environments.
Tip 3. Install the fitting ensuring that it remains concentric to the hole in the boss during the installation process. Pipe threads are notoriously easy to cross thread. The fitting should rotate 2 – 3 turns smoothly by hand until finger tight. If it rotates less than 2 turns or more than 3 1/2 turns, this is an indication that there is a possible problem.
Tip 4. Once installed finger tight, continue to tighten using a wrench. For the smaller size fittings 3/4” and smaller tighten between 2 – 3 turns beyond finger tight. Because of the nature of the tapered pipe thread, the tightening procedure is somewhat discretionary. Although torque guidelines can be found in some publications, the proper installation requires that you develop a “feel” for the proper torque. A good rule of thumb is to turn until the fitting is tight enough to prevent movement or loosening and then slightly more, if needed, for proper fitting alignment. Not tightening enough could result in the fitting becoming loose and leaking. In contrast over torquing could result in cracking the boss. (Figure 8) Cracking is particularly common when over torquing where there is a thin cross-section at the boss and the material is manufactured from aluminum or magnesium. The quality of the threads both internal and external very greatly and will have some effect on the amount of tightening necessary. After final installation, you should have between 3 1/2 and 6 threads fully engaged.
Tip 5. Ensure that there are no side loads or bending loads applied to pipe fittings. There are literally hundreds of examples of system failures and airplane crashes as a result of improperly side-loading a pipe fitting.
The photo to the right is a classic example of improper use of threaded pipe fittings. In this case, a Rotax 912S powered Tecnam light sport aircraft was modified to accommodate the installation of a Hobbs meter pressure switch (Figure 9). The original oil pressure sender was removed from the oil pressure port on the side of the engine. A commercial grade brass tee fitting was installed with a 1/8 inch brass pipe nipple into the oil pressure port. The oil pressure sender and the Hobbs meter pressure switch were installed into the tee fitting. The added weight and the extended arm of these two components resulted in overloading and subsequent failure of the brass pipe nipple. This caused a loss of oil pressure and an off field landing, resulting in substantial damage to the aircraft (Figure 10).
Now that we understand a little bit more about the use of pipe fittings in aircraft, we should be able to approach the pipe fitting from a little different perspective. We have provided some generic information and rules which should help you to make better choices regarding the use of pipe fittings. However, the tips and information here should never take precedence over the manufacturer’s recommendations for your particular aircraft.
For additional information about the use of pipe fittings in aviation applications we recommend the following references:
Mechanics Tool Box (John Schwarner)
FAA-H-8083-30 Aviation Maintenance Technician Handbook General