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1. General
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A. The control cabin aural warning system provides characteristic audio signals to alert the pilots to: an abnormal takeoff condition, landing condition, pressurization condition, mach-airspeed condition, an engine, APU, or wheel well fire, and calls from the crew call system or selcal systems. The aural warning and call devices are installed in an aural warning devices box. The box is on the right side of the forward section of the control stand in the control cabin (Fig. 1).
B. Inputs to the aural warning devices box are from: the flight and ground crew call system and selcal systems (Chapter 23), the mach-airspeed warning system (Chapter 34), the engine, APU and wheel well fire detection systems (Chapter 26), cabin altitude warning system (Chapter 21), the takeoff warning system, and the landing gear warning system.
C. On some airplanes, a parking brake ON aural warning is provided. The intermittant takeoff warn horn will sound when the airplane is in a takeoff configuration, thrust levers are advanced into takeoff range, and the parking brakes are engaged (not released).
2. Aural Warning Devices Box
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A. The aural warning devices box contains solid state horn, chime, logic and oscillator circuits, mounted on a printed circuit board, which provide the horn audio and chime audio to a loudspeaker whenever the appropriate circuits are keyed (Fig. 2). A second printed circuit board contains the oscillators and solid-state switching circuits used with a mechanical bell and clacker. The printed circuit boards may be easily removed from their connectors and replaced as needed. The power amplifier transistors are on heat sinks installed directly on the box. A single multipin connector on the backside of the box completes all circuits to the box. The components use 28 volts dc obtained through circuit breakers on load control center P6.
B. Circuit board A1 is used with the bell. The board contains a multivibrator and a solid-state switching (gate) circuit. When the multivibrator is keyed by a ground from the fire warning system, the multivibrator turns a gate on and off, thus causing power through the bell to be completed to ground and the bell to sound at the multivibrator rate.
C. Circuit board A2 contains a multivibrator, horn sound (tone) generation circuits, chime sound (tone) generation circuits, amplifiers, oscillator, and logic circuits required to produce appropriate sounds from the loudspeaker. The circuit board also contains an autopilot circuit disconnect wailer, and a buzzer circuit which is not used at present. Whenever the steady horn circuit is keyed by a ground from the landing gear warning circuits, an oscillator circuit provides a horn sound which is coupled to a driver (amplifier), power amplifier and loudspeaker. When the intermittent circuit is keyed by a ground from the takeoff warning or cabin altitude warning circuts, the oscillator is turned off and on by the multivibrator circuit, thus producing an intermittent horn sound.
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31-26-00
ALL ú ú 01 Page 1 ú Mar 15/94
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A 737-300/400/500MAINTENANCE MANUAL
FLAP TAKEOFF WARNING SWITCH,
TAKEOFF AND LANDING GEAR WARNING SWITCHES (CONTROL STAND)
Aural Warning System Component Location
Figure 1
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31-26-00
ALL ú ú 01 Page 2 ú Mar 15/94
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A 737-300/400/500MAINTENANCE MANUAL
Aural Warning System Simplified Schematic
Figure 2 (Sheet 1)
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31-26-00
ALL ú ú 01 Page 3 ú Mar 15/94
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A 737-300/400/500MAINTENANCE MANUAL
Aural Warning System Simplified Schematic
Figure 2 (Sheet 2)
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