S 422-034

 (13)
 Install the smoke sensor in the detector assembly. S 712-021

 (14)
 Do the Lavatory Smoke Detector Operational Test (AMM 26-14-00).

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757 MAINTENANCE MANUAL
 APU FIRE DETECTION - DESCRIPTION AND OPERATION
______________________________________________
 1.  General_______
 A.  A dual-loop fire detection system is installed in the APU compartment. This system monitors temperatures in the compartment, and provides alerts for a fire condition.
 B.  The APU fire detection system uses power from the 28-volt DC battery bus, with alternate power from the right main DC bus. The system circuit breakers are on overhead circuit breaker panel P11. The system contains no on/off switches.
 C.  The system is composed of two dual-loop detectors. Thus, each loop of the system has two detector elements, one from each detector.
 D.  Each detection loop has a control card. The system has one fire/overheat logic/test card, and a fire/overheat test panel.
 E.  When the airplane is on the ground, an APU fire indication is given at the nose landing gear - on the LTS/APU/INTERPHONE panel, as well as in the cockpit.
 F.  The detection system operates in AND logic on the ground and in the air. Thus, both loops of the system must sense a fire or fault condition in order for an alarm to be given.
 2.  Component Details (Fig. 1)_________________
 A.  APU Fire Detectors (Graviner)
_____________________________
 (1)  
A fire detector consists of two sensing elements which are attached to a support tube by quick-release mounting clamps. Each sensing element is a resistor-capacitor network, with resistance varying as a function of temperature.

 (2)  
At low temperatures, the impedance of the sensing element is mainly resistive. As temperature increases, the resistance drops, thus the impedance becomes more reactive. The detector card senses the change as a fire signal. A pure resistance will not be sensed by the detector card as a fire, but as a fault.

 (3)  
The fire detectors are installed above and below the APU. One detector forms a 180 degree arc around the air inlet above the APU and the other detector is below the APU, on an access door.

 B.  APU Fire Detectors (Systron Donner)
___________________________________
 (1)  A detector consists of two sensing elements which are attached to a support tube by quick-release mounting clamps. Each sensing element contains an inert gas, a gas-emitting core material, and has a responder on one end. The responder contains two pressure switches and provides the electrical interface with the airplane wiring.
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757
MAINTENANCE MANUAL

P11 CIRCUIT BREAKER PANEL

APU Fire Detection Component Location
Figure 1 (Sheet 1)

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757
MAINTENANCE MANUAL

SYSTRON DONNER FIRE DETECTOR
RESPONDER

APU Fire Detection - Component Location
Figure 1 (Sheet 2)

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