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Cabin Altitude Warning System
_____________________________
 A.  The cabin altitude warning system provides an audible signal when cabin pressure approaches an altitude deterimental to passenger comfort.
 B.  The cabin altitude warning system consists of a pressure switch, a cutout switch and a cutout relay. The system provides an intermittent sound signal when cabin altitude exceeds approximately 10,000 feet.
 C.  The pressure switch is an aneroid type switch. When cabin pressure as sensed in the cabin port reaches approximately 10,000 feet, the difference in pressure inside and outside the bellows closes the switch contacts and provides a ground for the aural warning devices box.
Fire Detection System
_____________________
 A.  The fire detection system provides visual and aural warnings from sensors in each engine, auxiliary power unit (APU), and wheel well areas. The fire protection system module in the control cabin provides controls and visual indicators. Control circuits are contained in the engine and APU fire detection accessory unit located in the electronic equipment rack. When a fire is detected, the control circuit provides a ground to activate the alarm bell in the aural warning devices box.
Mach Airspeed Warning System
____________________________
 A.  The mach airspeed warning system provides a warning to the flight crew when the airplane exceeds mach airspeed limits. Two mach airspeed warning systems, No.1 and 2, are installed on the airplane.
 B.  Each system consists of a mach airspeed indicator located on pilot's panel, MACH AIRSPEED WARNING TEST switch located on the aft overhead panel, and an aural warning clacker located in the aural warning and call device box. The mach airspeed indicator displays maximum operating airspeed (Vmo) as a function of airspeed and altitude from the digital air data computer. The indicator provides an output signal to actuate the aural warning clacker when the maximum operating speed is exceeded.
 C.  A self test may be performed on each system by pressing the MACH AIRSPEED
 WARNING TEST switch. The clacker sound when the test switch is pressed.Operation
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 A.  Landing Gear Warning
 (1)  A continuous steady horn will sound if any landing gear is not down and locked when certain conditions exist on the airplane. The warning control circuits and monitor circuits are in the landing gear module. Only those portions of the circuit which control keying of the aural warning box are discussed (Fig. 2). The conditions which must exist to keep the continuous horn from sounding in the flight mode are as follows: all landing gear must be down and locked when the flaps are in the landing range, or all landing gear must be down and locked when the flaps are not fully retracted and a thrust lever is retarded to idle.
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
737-300/400/500MAINTENANCE MANUAL
 (2)  
The conditions which cause the landing warning horn to sound or not sound are shown schematically in Fig. 2. The continous sound landing gear warning horn will sound with landing gear not down and locked when:

 (a)  
Flaps at positions 1 thru 10 and either or both thrust levers (S139 and S140) is at a position of less than 11 degrees above idle. Horn can be silenced with horn cutout switch.

 (b)  
Flaps at position 15 and both thrust lever (S814 and S815) is in the retarded position. Horn cannot be silenced with horn cutout switch when both throttles are retarded.

 (c)  
Flaps at position greater than 15 and thrust levers in any position. Horn cannot be silenced with horn cutout switch.

 

 (3)  
For a discussion of landing gear position indications, refer to Chapter 32.

 B. Takeoff Warning
 (1)  
The intermittent warning horn sounds when either thrust lever is advanced beyond approximately 30 degree from the idle stop position and one or more of the conditions that follow is present:

 -speedbrake handle is not in down detent
 -flaps are outside of takeoff range
 -stabilizer is out of green band range
 -leading edge flaps are not extended. The horn can be silenced only by correcting the configuration or by retarding the thrust levers.

 (2)  
Logic and relay circuits in the landing gear module complete the control circuit to the aural warning devices box. The relay is energized when an engine thrust lever is advanced beyond approximately 30 degrees thrust lever angle (TLA) and the landing gear ground sensing proximity switch indicates that the airplane is on the ground. A ground is then completed to arm the logic circuit for control of the horn. A ground from a speed brake switch, flap position switch, stabilizer up limit switch, or stabilizer down limit switch causes the horn to sound. As the airplane leaves the ground, the relay is de-energized and the warning horn control circuits disconnected.
　
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