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ALL ú ú 08 Page 5 ú Dec 20/93
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
757
MAINTENANCE MANUAL
3. Operation (Fig. 2)
_________
A. Functional Description
(1)
When a fire is detected in the APU compartment, the APU fire switch handle red warning light comes on and the fire warning bell sounds through the aural warning speakers. The red APU FIRE light on the P62 APU shutdown remote control panel also comes on.
(2)
After a fire is detected in the APU compartment, a solenoid energizes releasing a mechanical interlock on the fire switch handle shaft. The handle is then operable. The mechanical interlock is manually released by pressing the button located behind the fire handle.
(3)
When the airplane is on the ground and a fire is detected, the APU will shut down and the fire extinguisher bottle will discharge automatically. When the airplane is in flight and a fire is detected, the APU will shut down automatically but the extinguisher bottle must be discharged manually.
(4)
When the APU fire switch handle is pulled out, the APU automatically shuts down and the extinguishing bottle is armed. The following also happens: the APU generator field relay and generator circuit breaker are tripped; the APU fuel shut-off valve closes; the APU air supply valve closes; the fire bell shuts off; the external fire warning horn and APU fire light on P62 shut off.
(5)
Extinguishing occurs when the APU fire handle is pulled out and rotated in either direction. After rotation of the handle, an electrical signal is applied to the squib cartridge of the extinguisher bottle. The electrical signal detonates the bottle squib producing a rupture in the retaining disc allowing the extinguishing agent to be released. The extinguishing agent runs to an outlet nozzle at the bottom of the APU fire wall and is discharged into the APU compartment.
(6)
Decreasing bottle pressure, by discharge or leakage, activates the bottle pressure switch. The following occurs when a decrease in pressure is detected: the APU BTL DISCH light on APU/CARGO fire control panel P8 comes on, and a APU BTL DISCH message appears on the EICAS display.
B. APU Extinguishing System Test
(1)
Squib Test
(a) The squib test control panel checks the integrity of extinguisher bottle squibs. Pressing the TEST switch sends a signal to the squib discharge cartridge. If the squib and circuit continuity are good, the green APU light on the panel will come on.
(2)
Pressure Switch Test
(a) ON GUN 051; Manually activating the bottle pressure switch tests discharge light circuit continuity. When the manual override test hex key is turned 1/4 turn counterclockwise, the amber APU BTL DISCH light comes on and a APU BTL DISCH message appears on the EICAS display indicating a successful test.
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ALL ú ú 11 Page 6 ú Dec 20/93
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
757
MAINTENANCE MANUAL
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
AIR/GND RELAY
SPEED CARD
APU Fire Extinguishing Schematic
Figure 2
A
757 MAINTENANCE MANUAL
(b) ON ALL EXCEPT GUN 051; Manually activating the bottle pressure switch tests discharge light circuit continuity. When the ground test pushbutton is pressed and held, the amber APU BTL DISCH light comes on and a APU BTL DISCH message appears on the EICAS display indicating a successful test.
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