A. Functional Description
______________________
 (1)  
ON GUN 001-003
 smoke detector blowers

 (a)  
When electrical power is applied to the equipment, blower 1 starts to draw air through the system. The blower 1 delay relay prevents blower 2 from receiving electrical power for 10 seconds. Before the 10 seconds (60 seconds on GUN 206) has elapsed, blower 1 creates a vacuum in the plenum, causing the plenum pressure switch to activate. The activated pressure switch deactivates the blower 1 delay relay, making blower 2 a back-up blower.

 (b)  
If blower 1 now fails, the plenum pressure switch deactivates. The blower 1 delay relay energizes after 10 seconds. The relay activates a latching circuit to hold the relay, and switches electrical power to blower 2. It also sends a signal to EICAS (Engine Indicating and Crew Alerting System) to display the appropriate maintenance message - FWD or AFT DET FAN, after 10 seconds (EICAS delay).

 (c)  
Blower 2 now draws air through the system causing the plenum pressure switch to activate. This causes the EICAS message -CARGO DET AIR, to go off. If blower 2 now fails, the EICAS message - CARGO DET AIR will come on after 20 seconds (EICAS delay).

 (2)  
ON GUN 004-999;
 smoke detector blowers

 (a)  
When electrical power is applied to the equipment, blower 1 starts to draw air through the system. The blower 1 delay relay prevents blower 2 from receiving electrical power for 60 seconds. Before the 60 seconds has elapsed, blower 1 creates a vacuum in the plenum, causing the plenum pressure switch to activate. The activated pressure switch deactivates the blower 1 delay relay, making blower 2 a back-up blower.

 (b)  
If blower 1 now fails, the plenum pressure switch deactivates. The blower 1 delay relay energizes after 60 seconds. The relay activates a latching circuit to hold the relay, and switches electrical power to blower 2. It also sends a signal to EICAS (Engine Indicating and Crew Alerting System) to display the appropriate maintenance message - FWD or AFT DET FAN, after 60 seconds (EICAS delay).

 (c)  
Blower 2 now draws air through the system causing the plenum pressure switch to activate. This causes the EICAS message -CARGO DET AIR, to go off. If blower 2 now fails, the EICAS message - CARGO DET AIR will come on after 20 seconds (EICAS delay).

 (3)  
Smoke Detection

 (a)  
With all system power on, the blowers force air through the detectors. When either detector senses smoke, a signal is sent to the AFOLTS card.

 (b)  
The AFOLTS card senses the signals from each loop and determines the appropriate output to the flight crew. See Table 1.

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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
757 MAINTENANCE MANUAL
 INPUTS TO AFOLTS | AFOLTS
 DETECTOR 1 DETECTOR 2 | OUTPUTS

 .............................................................
 ALARM ALARM | FIRE
 ALARM -----| EICAS DET 1
 -----ALARM | EICAS DET 2

 TABLE 1
 Logic System Operation
 Cargo Compartment Smoke Detection
 AND Configuration

 (c)  A cargo compartment fire indication is given by the following:
 1)  The appropriate red FWD or AFT CARGO FIRE switchlight, on the APU/cargo fire control panel (on the P8 control stand), comes on.
 2)  The red FIRE light, on captain's instrument panel P1-3, comes on. 3) The appropriate fire warning message - FWD or AFT CARGO FIRE, is displayed on EICAS (P2). 4) The red master warning lights, on glareshield panels P7, come on. 5) The fire bell sounds, on the flight deck aural warning speakers.
 (d)  
A dual-detector fail indication is given by the following: 1) The amber FAIL P-RESET switchlight, on the P8 control stand, comes on. 2) The advisory message - FIRE/OVHT SYS, is displayed on EICAS.

 3)  The appropriate system maintenance messages are displayed on EICAS: a) FWD CARGO DET 1 and 2. b) AFT CARGO DET 1 and 2.

 (e)  
The FAIL P-RESET switchlight is pressed to reset the fail light circuitry. This also turns off the FAIL P-RESET switchlight, and clears the EICAS advisory message - FIRE/OVHT SYS.
　
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