Alarm Bell
A. The alarm bell warns of fire or overheat conditions in the APU or engine with a steady ringing. The bell is controlled by the aural warning and master fire warning lights control section of the engine and APU fire detection accessory unit (Fig. 2). The alarm bell is located in the aural warning devices box, refer to Chapter 31, Aural Warning and Call Devices.
6. Remote Horn
A. The remote horn (loudspeaker) warns of fire or overheat conditions in the APU. The remote horn is located in the right main wheel well adjacent to the APU control panel. Audio tones for the horn are generated by a multi-vibrator circuit (in miscellaneous switching module) which is turned on and off by the aural warning and master fire warning lights control section of the engine and APU fire detection accessory unit. The horn will operate only when the airplane is on the ground.
7. Master Fire Warning Lights
A. The master fire warning lights are located on lightshield module P7. The lightshield is located in a direct line of sight for both the captain and first officer. When a fire or overheat condition occurs the fire warning lights will alert the crew to the type of problem that exists.
B. The master fire warning light will alert the crew of a fire or overheat condition in the APU. The master fire warning lights are controlled by aural warning and master fire warning lights control section of the engine and APU fire detection accessory unit (Fig. 2). A fire warning light is located on either side of the lightshield for the captain and first officer. Either light may be depressed for bell or remote horn cutout and extinguishing the lights. The lights are also used by the engine fire detection system.
8. Sensing Elements
A. A sensing element provides fire detection inside the APU shroud. The element is mounted on the APU. The sensing element is shrouded and attached by quick-release clamps. The sensing element consists of one sensor mounted in a loop within a perforated metal tube.
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B. The sensor is a thermistor device, which has a resistance inversely proportional to temperature; that is, as the sensor temperature is raised by heating, its resistance lowers. The sensor is composed of two wires embedded in a thermistor material, which is incased in a heavy wall inconel tube for high strength at elevated temperatures. The electrical connector at each end of the sensor is completely insulated by ceramic insulators. The sensor is completely shrouded in a one-half inch diameter perforated stainless steel tube or armor between the electrical connectors. The shroud (armor) protects the sensor from common hazards such as breakage due to vibration, abrasion of sensor against airplane structure and damage from maintenance activity in the area. The sensors are supported away from the sides of the armor by teflon-impregnated asbestos bushings at approximately two and one-half inch intervals. The tubing provides sufficient clearance for the sensor, to protect it from objects striking and denting the armor, while also providing a structurally rigid support for the sensors.
C. The resistance of sensor is a logarithmic average function and varies inversely with its length, the increments of length being essentially resistances in parallel. For a given installation, the heating of less than a full length on a sensor will require it to be heated to a higher temperature for the resistance to decrease to the alarm point. This characteristic permits the system to integrate the varying temperatures throughout the installation rather than sensing only the highest local temperature. One wire of the sensor is grounded to the outside case of the sensor. The other wire receives power from the detector control unit. The system is thus continuously monitoring the resistance between the "hot" wire and ground through the sensor case or through the other wire which is grounded to case (Fig. 1).
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