(d)
At Engine and APU Fire Control Panel P8-1, hold the TEST switch to the FAULT position and verify the DET FAULT 1 light on panel P8-1 is illuminated.
(e)
Hold TEST switch to OVHT/FIRE position and verify the following indications are present: 1) Both FIRE WARN lights on P7 panel are illuminated. 2) Both MASTER CAUTION lights on panel P7 are illuminated. 3) OVHT/DET annunciator light on panel P7 is illuminated. 4) Eng-1 and Eng-2 overheat lights on panel P8-1 are illuminated. 5) Eng-1 and Eng-2 fire switches are illuminated. 6) The alarm bell sounds.
D. Restore Airplane to Normal
(1) Remove electrical power if no longer required (Ref 24-22-0 MP).
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APU FIRE DETECTION SYSTEM - DESCRIPTION AND OPERATION
EFFECTIVITY
CA B2501 thru B2505
1. General
A. The APU fire detection system provides fire detection in the auxiliary power unit (APU). Fire detection is provided by shrouded sensing elements mounted on the APU (Fig. 1). The detection system is an electrical, heat sensing, system that responds to a general overheat condition or a localized fire condition by activating warning lights, an alarm bell and a remote horn, and shutting down the APU.
B. Fire detection elements are installed in two areas of the APU; the engine area and the exhaust area. One sensing element is installed on the engine. Two sensing elements are installed on the heat shield and muffler assembly.
C. The APU fire detection system consists of the fire protection system module, engine and APU fire detection accessory unit, APU remote control panel, alarm bell, remote horn, master fire warning lights and sensing elements. The APU fire detection system sensor (thermistor type) is enclosed in a shrouded sensing element which is connected to the transistorized engine and APU fire detection accessory unit. The control module contains the APU detector control card and associated wiring to the APU. For engine coverage, refer to 26-11-0, Engine Fire Detection System. The APU detector control card monitors the electrical resistance of the APU sensor. The control card circuit discriminates between an overheat condition or a short circuit. The fire warning lights, alarm bell, (and remote horn if the airplane is on the ground) will actuate when a temperature rise causes the sensor element resistance to drop to the design value corresponding to the alarm temperature. The APU DET inoperative light, on the fire protection module, will illuminate when a short is sensed in the detection circuit.
D. The APU fire detection system can be tested in flight or on the ground by operation of the fire test switch on the fire protection module (Fig. 2). The fire test switch circuit checks continuity through the system by electrically simulating a fire condition on the APU. This energizes APU fire warning horn and light in the right wheel well, the alarm bell in the aural warning devices box in the control cabin, the fire warning lights on P7, and APU fire handle lights on the fire protection module; or causes the APU detector inoperative light to illuminate.
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APU Fire Detection System Component Location 551
26-15-0 Figure 1 (Sheet 1) Feb 20/87
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553 APU Fire Detection System Component Location
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26-15-0 Figure 2 May 1/75
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2. Fire Protection System Module Components
A. The fire protection system module components described in the following paragraphs will be only those associated with the APU fire detection system. The fire protection system module is located on control stand aft electronic panel P8. For further information on the fire protection system module, refer to 26-00, Fire Protection.
B. APU Detector Inoperative Light
(1) An amber detector inoperative light, mounted on the front panel, provides an indication of a short circuit in the APU fire detection system. A short circuit on the sensors will cause the steady illumination of the APU detector inoperative light. The light is also connected to the master dim/test system (Ref Chapter 33).
C. Warning Light
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