Chapter Objective: Upon completion of this chapter, you will have a working knowledge of the fabrication and maintenance of aircraft hydraulic and pneumatic hoses and their associated hardware. 

You are responsible for maintaining a portion of the hundreds of feet of fluid and air lines and various hardware and seals found in modern-day aircraft. The maintenance of these lines frequently involves fabrication and replacement of hose and hose assemblies. To be able to select the proper type of hose and hose assemblies and their hardware, you will need a basic knowledge of the type, size, and material from which items are to be made.

Learning Objective: Identify the various types of hose, hose assemblies, hardware, tools, and equipment used on naval aircraft.

Hose assemblies are used to connect moving parts with stationary parts and in locations subject to severe vibration. Hose assemblies are heavier than aluminum-alloy tubing and deteriorate more rapidly. They are used only when absolutely necessary. Hose assemblies are made up of hose and hose fittings. A hose consists of multiple layers of various materials. An example of the hose most often used in medium-pressure applications is shown in figure 5-1.

There are two basic types of hose used in military aircraft and related equipment. They are synthetic rubber and polytetrafluoroethylene, commonly known as Teflon@ or PTFE. 

Bulk hose identification will vary with the materials from which the hose is constructed. It is important that you are able to clearly identify the proper hose to be used by recognizing the various hose markings.

Figure 5-1.Medium pressure synthetic rubber hose, MIL-H-8794.

Synthetic Rubber Hose 

Synthetic rubber hose has a seamless synthetic rubber inner tube covered with layers of cotton and wire braid, and an outer layer of rubber impregnated cotton braid. The hose is provided in low-, medium-, and high-pressure types.

Synthetic rubber hose (if rubber-covered) is identified by the indicator stripe and markings that are stencilled along the length of the hose. The indicator stripe (also called the lay line because of its use in determining the straightness or lie of a hose) is a series of dots or dashes. The markings (letters and numerals) contain the military specification, the hose size, the cure date, and the manufacturers federal supply code number. This information is repeated at intervals of 9 inches. Refer to figure 5-2.

Size is indicated by a dash followed by a number (referred to as a dash number). The dash number does not denote the inside or outside diameter of the hose. It refers to the equivalent outside diameter of rigid tube size in sixteenths (1/16) of an inch. A dash 8 (-8) mates to a number 8 rigid tube, which has an outside diameter of one-half inch (8/16). The inside of the hose will not be one-half inch, but slightly smaller to allow for tube thickness.

The cure date is provided for age control. It is indicated by the quarter of the year and year. The year is divided into four quarters.

The cure date is also marked on bulk hose containers in accordance with Military Standard 129 (MIL-STD-129). 

Synthetic rubber hose (if wire-braid covered) is identified by bands wrapped around the hose at the ends and at intervals along the length of the hose. Each band is marked with the same information (fig.5-2).

Teflon Hose The Teflon hose is made up of a tetrafluoro- ethylene resin, which is processed and extruded into tube shape to a desired size. It is covered with stainless steel wire, which is braided over the tube for strength and protection. The advantages of this hose are its operating temperature range, its chemical inertness to all fluids normally used in hydraulic and engine lubrication systems, and its long life. At this time, only medium-pressure and high-pressure types are available. These are complete assemblies with factory-installed end fittings. The fittings may be either the detachable type or the swaged type. When failures occur, replacement must be made on a complete assembly basis.

Teflon hose is identified by metal bands or pliable plastic bands at the ends and at 3-foot intervals. These bands contain the hose military specification number, size indicated by a dash (-) and a number, operating pressure, and the manufacturers federal supply code number. Refer to figure 5-2.