6.  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.
7.  Mach /Airspeed Warning System
A.  The mach/airspeed warning system provides a warning to the flight crew when the airplane approaches a critical speed.
B.  A switch assembly is built within the captain's and the first officer's mach airspeed indicators. Under conditions of high mach airspeed, the switch will trigger a clacker located in the aural warning devices unit. The clacker will sound any time the indicated airspeed exceeds the maximum operating airspeed or Mach number. The clacker will remain energized until the airplane airspeed or Mach number is reduced. 
C.  Self-test of the system can be performed by pressing the MACH AIRSPEED WARN TEST switch on overhead panel P5.
8.  Operation
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 for the horn not to operate 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 downand locked when the flaps are less than full up and the thrust levers are retarded to idle.

(2)  
If any landing gear is not down when either one of the above two conditions exist, the steady horn circuits will be keyed. If the flaps are less than full up but not in the landing range and the thrust lever is retarded and all landing gear are not down, the horn can be silenced by pressing the thrust lever horn reset switch just to the right of the thrust levers on the control stand. For a discussion of landing gear position indicating, refer to Chapter

32.

(3)  
The conditions which cause the landing warning horn to sound or not sound are shown schematically in Fig. 2. When the flaps are at the up limit, the horn is inhibited and cannot sound. When the flaps are not fully up and either thrust lever is retarded, ground potential is applied to the horn causing it to sound unless the landing gear are all down and locked. The horn can be silenced by pressing the reset button. The horn also sounds when the flaps are placed in the landing range and the gear is not down and locked.

(4)  
Because the takeoff and landing flap ranges overlap on some airplanes, engine EPR signals are used to differentiate between the takeoff and landing conditions to prevent spurious landing warnings during climbout after gear retraction. If the flaps are within the minimum landing range but not beyond the takeoff range and the EPR of both engines is less than 1.55 and the gear is not down and locked, the horn will sound. If the EPR of either engine is at least 1.7 1 and the flaps are in the takeoff range, the horn will not sound if the gear is not down and locked. When the flaps are in the landing range, the horn will sound if the gear is not down and locked.

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May 15/78  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  31-26-0 Page 9 

B  Takeoff Warning
(1)  
The intermittent warning horn sounds when the thrust levers are advanced to a position which actuates the takeoff warning switches and provides an EPR of 1.4 or greater if the speed brakes are not down, or the flaps are not in the takeoff range, or the stabilizer trim is not in the takeoff range. The horn can only be stationed 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 whenever the EPR exceeds 1.4 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, TE 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 in the landing gear module is de-energized and the warning horn control circuits disconnected. Test switches on the front of the engine accessory unit module allow testing of the EPR takeoff warning system.
　
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