(b)  
Captains fire warning light on P7.

(c)  
First officers fire warning light on P7.

(d)  
Horn cutout on APU panel in right wheel well.

(9)  
Release test switch to off position.

(10)
Hold the overheat fire test switch to the fire position. Press the air-sensing switch on the front panel of the landing gear warning module in the electronic equipment rack. Observe that the fire warning light on the APU control panel in the wheel well flashes and the APU horn does not sound.

(11)
Release switches.

(12)
Attempt to pull out APU fire switch. Switch should remain locked.

C.  Restore Airplane to Normal Configuration
(1)  
Close engine-1, engine-2, wheel well wing body overheat and cargo smoke (Passenger/Cargo Convertible Airplanes only) circuit breakers on load control center P6.

(2)  
Determine whether there is any further need for electrical power to the airplane; if not, remove external power.

500 
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APU FIRE DETECTION SYSTEM - DESCRIPTION AND OPERATION
EFFECTIVITY
WA ALL EXCEPT N4502W thru N4530W, N4569N, N4571M, N235WA CA ALL EXCEPT B2501 thru B2505
1.  General
A.  The APU fire detection system provides fire detection in the auxiliary power unit (APU) area. Fire detection is provided by shrouded detectors on the APU. The detection system is an electrical heat sensing system that responds to fire by activating warning lights, an alarm bell, and a remote horn.
B.  The APU fire detection system consists of the engine and APU fire control module, engine and APU fire detection accessory unit, alarm bell, remote horn, master warning and caution lights, and detectors. Each sensor is connected to the engine and APU fire detection accessory unit. The accessory unit contains control cards, test circuits and warning component control circuits. The detector control card monitors the electrical resistance of their associated sensors. The control card circuit discriminates between a fire or a short circuit and actuates fire warning lights, remote horn, and alarm bell. The detector inoperative light will illuminate when a short is sensed in the detection circuit.
C.  The APU fire detection system can be tested by operation of the TEST switch. The TEST switch circuit checks continuity through the system by simulating fire on the APU. This energizes the alarm bell, remote horn, and fire warning lights. The detector test switch on engine and APU fire detection accessory unit tests the detector inoperative circuit and causes the detector inoperative light to illuminate.
2.  Engine and APU Fire Control Module Components (Fig. 1)
A.  APU fire detection system components of the fire control module are the loop select switch, warning lights, test switch and bell cutout switch. The fire control module is located on aft control stand electronic panel P8.
B.  Loop Select Switch
(1)  The loop select switch permits selection of dual-loop AND-logic when set to NORMAL, or single-loop when set to A or B. When the switch is set to NORMAL, signals from both elements in a detector are processed by detection accessory unit, which actuates the appropriate fire or fault signal. When the switch is set to A or B, the detection accessory unit permits the selected loop to monitor fire, adjusts control logic to recognize absence of the other loop, and suppresses fault signals from the other loop.
C.  Warning Lights
(1)  
Red fire switch light in the fire switch handle provides APU area fire warning. A fire signal from the detection accessory unit will cause steady illumination of the fire switch lights. The warning light consists of several bulbs covered by a red translucent plastic lens in the fire switch handle.

(2)  
Detector fault light provides detector fault warning. A fault signal from the detection accessory unit will cause steady illumination of the APU DET FAULT light.

Feb 20/85 26-15-02 Page 1
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.

H79594 L20685 T79897
APU Fire Detection System Component Location  5C9 
26-15-02  Figure 1 (Sheet 1)  Oct 20/83 
Page 2 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

505  APU Fire Detection System Component Location 
Feb 20/85  Figure 1 (Sheet 2)  26-15-02 
Page 3 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

APU Fire Detection System Simplified Schematic  5C9 
26-15-02  Figure 2  Jun 20/84 
　
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