C.  Test Switch
(1)  The pushbutton test switch tests the short discriminator circuitry of the control cards. Actuation of the test switch will cause the fault lights on the control module and accessory unit to illuminate.
D.  Fault Lights
(1)  
The fault lights illuminate due to fault signal from the control cards or during fault testing. The lights extinguish when the fault is removed or the test switch is released.

(2)  
The cockpit fault light will not illuminate when an alarm condition is detected by one loop while the other loop is in a fault condition.

4.  Alarm Bell
A.  The alarm bell warns of fire with a steady ringing. The bell is controlled by the control card in the engine and APU fire detection accessory unit. The alarm bell is in the aural warning devices box.
5.  Master Fire Warning and Master Caution Lights (Fig. 1)
A.  The master fire warning and master caution lights are located on lightshield module P7.
B.  The master fire warning lights are controlled by control card in engine and APU fire detection accessory unit. Either light may be pressed for bell cutout and extinguishing of the master fire warning lights.
C.  The master caution and annunciator lights are controlled by the master caution light control section of the fire control module. The overheat detector annunciator light is at the left side and a master caution light is located at each side of the lightshield. Either master caution light may be pressed to reset the master caution system. The annunciator light may be pressed to recall the warning on the lightshield if the fault still exists.
6.  Detectors (Fig. 1)
A.  Detectors provide overheat and fire detection at each engine. Detectors are mounted above the engine on the horizontal firewall and on the bottom of the engine and are attached by quick-release clamps. Each detector consists of two sensing elements mounted in a perforated metal tube. Each sensing element detects overheat and the fire temperature. Each detector is composed of two wires imbedded in thermister material which is encased in a heavy wall inconel tube for high strength at elevated temperatures. The electrical connectors at each end are ceramic insulated. The inconel tubes are shrouded in a perforated stainless steel tube and supported by insulated bushings at intervals. The shroud protects the sensor from breakage due to vibration, abrasion against airplane structure, and damage from maintenance activity.
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H79671
5C9  Engine Fire Detection System Truth Table 
Jun 20/84  Figure 3  26-11-02 
Page 7 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

B.  Each dual-element detector is connected in series such that two identical, electrically independent, continuous sensing loops are formed. Each loop senses the temperature of its surrounding area. If the temperature reaches or exceeds the alarm value, the resistance of the sensing loop decreases to cause the detection accessory unit to trigger an alarm. The two sensing elements of each detector sense both fire and overheat conditions and the detection accessory unit interpret the corresponding resistance change.
7.  Operation (Fig. 2 and 3)
A.  Functional Description
(1)  
Temperature rise in the engine area causes resistance drop in the detector sensing elements. When the alarm value is reached, the fire detection accessory unit signals an alarm. The accessory unit interprets the resistance change to determine whether it is fire or overheat and signals applicable lights and alarm to actuate.

(2)  
When the loop select switch is set to NORMAL, the system operation is AND-logic. When the loop select switch is set to A or B, system operation is single-loop. The truth table lists alarm and light indications corresponding to each type of operation.

(3)  
The FIRE WARN lights may be extinguished and the alarm bell may be silenced by pressing BELL CUTOUT switch or either FIRE WARN light. The OVERHEAT, DET FAULT and fire switch lights extinguish only when the fire, overheat or fault is removed.

B.  Control
(1)  Control of the engine fire detection system is automatic when power is provided, the battery switch is set to ON, and the FIRE PROTECTION DETECTION and MASTER WARN AND CONTROL circuit breakers are closed.
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ENGINE FIRE DETECTION SYSTEM - TROUBLE SHOOTING
EFFECTIVITY
WA ALL EXCEPT N4502W thru N4530W, N4569N, N4571M, N235WA CA B-2509
　
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