Aircraft and engine manufacturers are responsible for the original installation of identification markers, but the aviation mechanic is responsible for the replacement when it becomes necessary. Tapes and decals are generally placed on both ends of a line and at least once in each compartment through which lines run. In addition, identification markers are placed immediately adjacent to each valve, regulator, filter, or other accessories within a line. Where paint or tags are used, location requirements are the same as for tapes and decals.

Depending on the type and use, fittings have either pipe threads or machine threads. Pipe threads are similar to those used in ordinary plumbing and are tapered, both internal and external. External threads are referred to as male threads and internal threads are female threads.

When two fittings are joined, a male into a female, the thread taper forms a seal. Some form of pipe thread lubricant approved for particular fluid application should be used when joining pipe threads to prevent seizing and high-pressure leakage. Use care when applying thread lubricant so that the lubricant does not enter and contaminate the system. Do not use lubricants on oxygen lines. Oxygen reacts with petroleum products and can ignite (special lubricants are available or oxygen systems).

Machine threads have no sealing capability and are similar to those used on common nuts and bolts. This type of fitting is used only to draw connections together or for attachment through bulkheads. A flared tube connection, a crush washer, or a synthetic seal is used to make the connection fluid tight. Machine threads have no taper and do not form a fluid-tight seal. The size of these fittings is given in dash numbers, which equal the nominal outside diameter in sixteenths of an inch.

When a fluid line passes through a bulkhead, and it is desired to secure the line to the bulkhead, a bulkhead fitting should be used. The end of the fitting that passes through the bulkhead is longer than the other end(s), which allows a locknut to be installed, securing the fitting to the bulkhead.

Fittings attach one piece of tubing to another or to system units. There are four types: (1) bead and clamp, (2) flared fittings, (3) flareless fittings, and (4) permanent fittings (Permaswage™, Permalite™, and Cyrofit™). The amount of pressure that the system carries and the material used are usually the deciding factors in selecting a connector.

The beaded type of fitting, which requires a bead and a section of hose and hose clamps, is used only in low- or medium-pressure systems, such as vacuum and coolant systems. The flared, flareless, or permanent-type fittings may be used as connectors in all systems, regardless of the pressure.

A flared tube fitting consists of a sleeve and a nut. [Figure 9-18] The nut fits over the sleeve and, when tightened, draws the sleeve and tubing flare tightly against a male fitting to form a seal. Tubing used with this type of fitting must be flared before installation. The male fitting has a cone-shaped surface with the same angle as the inside of the flare. The sleeve supports the tube so that vibration does not concentrate at the edge of the flare and distributes the shearing action over a wider area for added strength. Fitting combinations composed of different alloys should be avoided to prevent dissimilar metal corrosion. As with all fitting combinations, ease of assembly, alignment, and proper lubrication should be assured when tightening fittings during installation.

Standard AN fittings are identified by their black or blue color. All AN steel fittings are colored black, all AN aluminum fittings are colored blue, and aluminum bronze fittings are cadmium plated and natural in appearance. A sampling of AN fittings is shown in Figure 9-19. Figure 9-20 contains they are not interchangeable. AN flared fittings are easily recognized, because they have a cone at the end of the fitting while the MS flareless fitting has a straight end. [Figure 9-21]

MS flareless fittings are designed primarily for high-pressure (3,000 psi) hydraulic systems that may be subjected to severe vibration or fluctuating pressure. [Figure 9-22] Using this type of fitting eliminates all tube flaring, yet provides a safe and strong, dependable tube connection. The fitting consists of three parts: a body, a sleeve, and a nut. [Figure 9-23] The internal design of the body causes the sleeve to cut into the outside of the tube when the body and nut are joined. The counterbore shoulder within the body is designed with a reverse angle of 15° for steel connectors and 45° for aluminum fittings. This reverse angle prevents inward collapse of the tubing when tightened and provides a partial sealing force to be exerted against the periphery of the body counterbore.

A popular repair system for connecting and repairing hydraulic lines on transport category aircraft is the use of Permaswage™ fittings. Swaged fittings create a permanent deburr (2), oil (3), and revolve tool around tube while tightening

connection that is virtually maintenance free. Swaged fittings are used to join hydraulic lines in areas where routine disconnections are not required and are often used with titanium and corrosion-resistant steel tubing. The fittings are installed with portable hydraulically-powered tooling, which is compact enough to be used in tight spaces. [Figure 9-24] If the fittings need to be disconnected, cut the tubing with a tube cutter. Special installation tooling is available in portable kits. Always use the manufacturer’s instructions to install swaged fittings. Typical Permaswage™ fittings are shown in Figure 9-25.