I. Both Loops Operative (AND-Logic)
COCKPIT  ELECTRONIC  
INDICATION *[2]  EQUIP. RACK *[2] 
LOOP A  LOOP B  INOP 
DETECTORS *[1]  DETECTORS *[1]  FIRE LIGHT  CONTROL MODULE LIGHTS *[2] 
&  FAULT 
FIRE  FAULT  FIRE  FAULT  ALARM BELL  LIGHT  LOOP A FAULT  LOOP B FAULT 
1.  0  0  0  0  0  0  0  0 
2.  0  0  0  1  0  0  0  1 
3.  0  0  1  0  0  0  1  1 
4.  0  1  0  0  0  0  1  0 
5.  0  1  0  1  0  1  1  1 
6.  0  1  1  0  1  0  1  0 
7.  1  0  0  0  0  0  1  1 
8.  1  0  0  1  1  0  0  1 
9.  1  0  1  0  1  0  0  0 
II.  Loop A Operative, Loop B De-Selected (single-loop) 
COCKPIT  ELECTRONIC  
INDICATION *[2]  EQUIP. RACK *[2] 
 LOOP A  LOOP B  INOP 
DETECTORS *[1]  DETECTORS *[1]  FIRE LIGHT  CONTROL MODULE LIGHTS *[2] 
&  FAULT 
FIRE  FAULT  FIRE  FAULT  ALARM BELL  LIGHT  LOOP A FAULT  LOOP B FAULT 
10.  0  0  - - 0  0  0  -
11.  0  1  - - 0  1  1  -
12.  1  0  - - 1  0  0  -
III.  Loop B Operative, Loop A De-selected (single-loop) 
13.  - - 0  0  0  0  - 0 
14.  - - 0  1  0  1  - 1 
15.  - - 1  0  1  0  - 0 
*[1] 0 indicates no signal, 1 indicates signal 
*[2] 0 indicates device off, 1 indicates device on 

5C9  APU Fire Detection System Truth Table 
Jun 20/84  Figure 3  26-15-02 
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 - Unpublished Work - See title page for details.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.
9.  Operation (Fig. 2 and 3)
A.  Functional Description 
(1)  
Temperature rise in the APU 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 and signals applicable lights and alarms 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 and remote horn may be silenced by pressing BELL CUTOUT switch or HORN CUTOUT switch or either FIRE WARN light. The DET FAULT and fire switch lights extinguish only when the fire or fault is removed.

B.  Control
(1)  Control of the APU 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.
505 
26-15-02 Page 8  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  Feb 20/85 

APU FIRE DETECTION SYSTEM - TROUBLE SHOOTING
EFFECTIVITY
WA ALL EXCEPT N4502W thru N4530W, N4569N, N4571M, N235WA CA ALL EXCEPT B2501 thru B2505
1.  General
A.  Trouble with the fire detection system will usually be indicated by false alarm or absence of fire warning lights or alarm bell when the test switch is actuated. If a short circuit has occurred the APU DET FAULT light will probably be illuminated. If an open circuit has occurred, the APU warning lights and master fire warning lights will not be illuminated when the system is tested. Lights on the fire control module and master fire warning lights should be tested by operating the master lights test circuits (Ref 33-15-0) before assuming the detection system is faulty.
　
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