AIRCRAFT FIRE DETECTION SYSTEM - TROUBLESHOOTING

FIRE DETECTION SYSTEM - TROUBLESHOOTING

The following troubleshooting procedures repre­sent the most common difficulties encountered in engine fire detection systems.

(1)       Intermittent alarms are most often caused by an intermittent short in the detector system wiring. Such shorts may be caused by a loose wire which occasionally touches a nearby terminal, a frayed wire brushing against a structure, or a sensing element rubbing long enough against a structural member to wear through the insulation. Intermittent faults can often best be located by moving wires to re­create the short.

(2)       Fire alarms and warning lights can occur when no engine fire or overheat condition exists. Such false alarms can most easily be located by disconnecting the engine sensing loop from the aircraft wiring. If the false alarm continues, a short must exist between the loop connections and the control unit. If, however, the false alarm ceases when the engine sensing loop is disconnected, the fault is in the discon­nected sensing loop, which should be ex­amined for areas which have been bent into contact with hot parts of the engine. If no bent element can be found, the shorted section can be located by isolating and disconnecting elements consecutively around the entire loop.

(3)       Kinks and sharp bends in the sensing ele­ment can cause an internal wire to short intermittently to the outer tubing. The fault can be located by checking the sen­sing element with a megger while tapping the element in the suspected areas to pro­duce the short.

(4)       Moisture in the detection system seldom causes a false fire alarm. If, however, moisture does cause an alarm, the warning will persist until the contamination is re­moved or boils away and the resistance of the loop returns to its normal value.

(5)       Failure to obtain an alarm signal when the test switch is actuated may be caused by a defective test switch or control unit, the lack of electrical power, inoperative indicator light, or an opening in the sen­sing element or connecting wiring. When the test switch fails to provide an alarm, the continuity of a two-wire sensing loop can be determined by opening the loop and measuring the resistance. In a single wire, continuous-loop system, the centre conductor should be grounded.