The fire detection card supplies fire alarm 1 and fire alarm 2 outputs, the same as in the attended mode.
The fire detection card also supplies an unattended fire signal. This signal causes the APU fire extinguisher to discharge automatically.
BITE
BITE monitors the fire detection card and loops for faults. BITE also does a full system test for these conditions:
*
When the system first gets power
*
After a power interrupt
*
Every five minutes of operation.
BIT data goes to the AIMS through the systems ARINC 629 buses. The primary display system shows advisory or status messages. The MAT shows maintenance messages.
System faults include detector open circuits, detector short circuits, and defective cards. If a fault involves only one loop, a status message shows and the fire detection system still operates.
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APU FIRE DETECTION -FUNCTIONAL DESCRIPTION -FLIGHT DECK INDICATION CIRCUIT
If the power-up BIT finds an open loop, the system changes to single-loop operation. Both ends of the loop are possibly disconnected. If the loop opens after the power-up BIT, the system stays in the dual-loop mode until the next electrical power-up. A loop which is open in only one place still operates. The related maintenance message specifies that loop 1 or loop 2 is open.
If a loop has a short circuit or if a card is defective, the system changes to single-loop operation. A status message shows.
If the fault involves both loops, the fire detection system does not operate. Advisory and status messages show.
Training Information Point
After you do maintenance, use the fire/overheat test switch to make sure the system operates correctly. Be careful not to pull the APU fire switch. The switch is unlocked during the test.
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APU FIRE DETECTION -FUNCTIONAL DESCRIPTION -FLIGHT DECK INDICATION CIRCUIT
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APU FIRE DETECTION -FUNCTIONAL DESCRIPTION -P40 PANEL INDICATION CIRCUIT
General
Fire indications at the P40 service and APU shutdown panel are different on the ground and in the air. For both conditions, the battery bus supplies power to the circuit through the APU fire switch in the flight deck.
Ground Operation
If there is an APU fire, the APU fire detection card makes a ground for the APU remote warning relay. The relay energizes and supplies power to the APU fire warning light and the horn interrupter. If the airplane is on the ground and the APU external shutdown relay is not energized, the APU horn interrupter energizes. This supplies power to the APU fire warning horn. The horn interrupter causes the warning horn to sound intermittently.
If you pull the APU fire switch in the flight deck, the warnings at the P40 service and APU shutdown panel go away. If you push the APU fire shutdown switch on the P40 panel, the APU external shutdown relay energizes and stops the horn.
Air Operation
In flight, the air/ground relay is in the AIR position. The warning horn does not operate.
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APU FIRE DETECTION -FUNCTIONAL DESCRIPTION -P40 PANEL INDICATION CIRCUIT
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APU FIRE DETECTION -INDICATIONS
Flight Deck Indications
If there is an APU fire, these indications occur in the flight deck:
*
The master warning lights come on
*
The fire warning aural operates
*
An APU fire EICAS warning message shows
*
The APU fire warning light comes on.
Also, because the APU stops automatically, the APU fault light comes on and an EICAS advisory message shows APU shutdown.
Service and APU Shutdown Panel Indications
If the airplane is on the ground, these indications occur at the P40 service and APU shutdown panel:
*
The APU fire warning horn operates intermittently
*
The APU fire warning light comes on.
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777 AIRCRAFT MAINTENANCE MANUAL
APU FIRE DETECTION -INDICATIONS
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