Fire detector sensing elements are located in many high-activity areas around aircraft engines. Their location, together with their small size, increases the chance of damage to the sensing elements during maintenance. An inspection and maintenance program for all types of continuous-loop systems should include the following visual checks. These procedures are examples and should not be used to replace the applicable manufacturer’s instructions.
Sensing elements of a continuous-loop system should be inspected for the following:
1. Cracked or broken sections caused by crushing or squeezing between inspection plates, cowl panels, or engine components.
2. Abrasion caused by rubbing of the element on cowling, accessories, or structural members.
3. Pieces of safety wire or other metal particles that may short the spot-detector terminals.
4. Condition of rubber grommets in mounting clamps, which may be softened from exposure to oils or hardened from excessive heat.
5. Dents and kinks in sensing element sections. Limits on the element diameter, acceptable dents and kinks, and degree of smoothness of tubing contour are specified by the manufacturer. No attempt should be made to straighten any acceptable dent or kink, since stresses may be set up that could cause tubing failure. (See illustration of kinked tubing in Figure 9-14.)
6. Nuts at the end of the sensing elements [Figure 9-15] should be inspected for tightness and the presence of a safety wire. Loose nuts should be retorqued to the value specified by the manufacturer’s instructions. Some types of sensing element connection joints require the use of copper crush gaskets, which should be replaced any time a connection is separated.
7. If shielded flexible leads are used, they should be inspected for fraying of the outer braid. The braided sheath is composed of many fine metal strands woven into a protective covering surrounding the inner insulated wire. Continuous bending of the cable or rough treatment can break these fine wires, especially those near the connectors.
8. Sensing element routing and clamping should be inspected carefully. [Figure 9-14] Long, unsupported sections may permit excessive vibration that can cause breakage. The distance between clamps on straight runs, usually about 8–10 inches, is specified by the manufacturer. At end connectors, the first support clamp is usually located about four to six inches from the end connector fittings. In most cases, a straight run of one inch is maintained from all connectors before a bend is started, and an optimum bend radius of three inches is normally applied.
9. Interference between a cowl brace and a sensing element can cause rubbing. [Figure 9-16] This interference may cause wear and short the sensing element.
10. Grommets should be installed on the sensing element so that both ends are centered on its clamp. The split end of the grommet should face the outside of the nearest bend. Clamps and grommets should fit the element snugly. [Figure 9-17]