FIRE DETECTION AND EXTINGUISHER SYSTEM - MAINTENANCE PRAC¬TICES

FIRE DETECTION AND EXTINGUISHER SYSTEM - MAINTENANCE PRAC­TICES\

INTRODUCTION

Fire detector sensing elements are located in many high-activity areas around aircraft engines. Their location, together with their small size, in­creases the chances of damage to the sensing ele­ments during maintenance. The installation of the sensing elements inside the aircraft cowl panels pro­vides some measure of protection not afforded ele­ments attached directly to the engine. On the other hand, the removal and re-installation of cowl panels can easily cause abrasion or structural defects to the elements. A well-rounded inspection and mainte­nance program for all types of continuous-loop sys­tems should include the following visual checks. These procedures are provided as examples and should not be used to replace approved local main­tenance directives or the applicable manufacturer's instructions.

FIRE DETECTION MAINTENANCE PRACTICES

Sensing elements should be inspected for:

(1)       Cracked or broken sections caused by crushing or squeezing between inspection plates, cowl panels, or engine components.

(2)       Abrasion caused by rubbing of element on cowling, accessories, or structural mem­bers.

(3)       Pieces of safety wire or other metal parti­cles which 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 or kinks, and degree of smoothness of tubing contour are specified by manufacturers. 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 5.1).

(6)      Loose nuts or broken safety wire at the end of the sensing elements (figure 5.2). Loose nuts should be re-torqued to the value specified in the manufactur­er's instructions. Some types of sensing element connections require the use of copper crush gaskets. These gaskets should be replaced any time a connection is separated.

(7)      Broken or frayed flexible leads, if used. The flexible lead is made up 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. Broken strands can also protrude into the insulated gasket and short the center electrode.

(8)      Proper sensing element routing and clamping (figure 5.3). Long unsup­ported sections may permit excessive vibration which can cause breakage. The distance between clamps on straight runs is usually about 8 to 10 in., and is speci­fied by each manufacturer. At end connec­tors, the first support clamp is usually lo­cated about 4 to 6 in. from the end connector fittings. In most cases, a straight run of 1 in. is maintained from all connectors before a bend is started, and an optimum bend radius of 3 in. is normally adhered to.

 (9)     Rubbing between a cowl brace and a sensing element (figure 5.3). This inter­ference, in combination with loose rivets holding the clamps to skin, may cause wear and short the sensing element.

(10) Correct grommet installation. The grom­mets are installed on the sensing element to prevent the element from chafing on the clamp. The slit end of the grommet should face the outside of the nearest bend. Clamps and grommets (figure 5.4) should fit the element snugly.

(11)    Thermocouple detector mounting brack­ets should be repaired or replaced when cracked, corroded, or damaged. When replacing a thermocouple detector, note which wire is connected to the identified plus terminal of the defective unit and connect the replacement in the same way.

(12)    Test the fire detection system for proper operation by turning on the power supply and placing the fire detection test switch in the "TEST" position. The red warn­ing light should flash on within the time period established for the system. On some aircraft an audible alarm will also sound.

In addition, the fire detection circuits are checked for specified resistance and for an open or grounded condition. Tests required after repair of replacement of units in a fire detection system or when the system is inoperative include:

(i) Checking the polarity, ground, resistance and continuity of systems that use ther­mocouple detector units, and

(ii) Resist­ance and continuity tests performed on systems with sensing elements or cable detector units.

In all situations follow the recommended practices and proce­dures of the manufacturer of the type system with which you are working.