C. Pressurization Warning
(1) A two per second pulse intermittent horn sounds if at any time the cabin pressure exceeds 10,000 feet during flight. An aneroid switch provides a horn control ground through the de-energized warning horn cutout relay. The horn may be silenced by pressing the altitude horn cutout switch on overhead panel P5. This energizes and latches the relay, thus disconnecting the control ground. The relay is de-energized only after the condition has been corrected, thus resetting the circuit.
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D. High Mach/Airspeed Warning
(1)
The aural warning clacker will sound any time the airplane exceeds mach airspeed limits. The clacker is controlled by an electronic overspeed limit switch in the mach airspeed indicator. The switch provides a ground to the clacker when closed by an overspeed condition. The clacker will sound at approximately 8 Hz until airplane airspeed is reduced. The aural warning clacker cannot be silenced except by decreasing the speed of the airplane.
(2)
The clacker can be tested by pressing the associated MACH AIRSPEED WARNING TEST switch on the aft overhead panel P5. When the switch is pressed, 28 volts dc is applied to the overspeed light switch which provides a ground to the clacker. The clacker will sound as long as the test switch is held (Ref. Chapter 34).
E. Fire Warning
(1)
A bell will sound if at any time a fire is occurring in either engine, the APU, or the main wheel well. The bell will also sound when the fire system is tested by pressing the fire test button on the fire detection control panel on the control stand. The bell may be silenced by pressing either the master fire warning light switch above the center instrument panel, the bell cutout switch on the fire dectction control panel, or the APU remote cutout switch on APU panel P28 in the main wheel well (Ref Chapter 26).
(2)
Fire sensing circuits in the fire detection module sense whether a fire is in the main wheel well, engines, or APU by changes in the associated fire detection element resistance. When a sensing circuit is operated by an element, a bell control gate circuit is operated and a ground is provided for the bell circuits in the aural warning devices box. Each engine gate circuit is connected to the fire sensing circuit. The APU is connected to the APU gate circuit by the APU fire sensing circuit. The gates may be turned off and the bell silenced by pressing one the fire detection reset button. Voltage is then completed to SCR circuits which bias off each gate. The gates are reset whenever the fire condition is corrected.
F. Crew Call
(1) A high chime will sound any time a captain call button is pressed at the external power panel, the forward attendants' panel, or the aft attendants' panel. Each pressed call button completes a 28-volt dc pulse which operates the high chime circuits in the aural warning devices box (Ref Chapter 23).
G. Selcal
(1) A two tone high-low chime or a single tone chime will sound whenever a call has been received by a HF/VHF communication system and decoded by the selcal decoder. When the decoder accepts a call signal, a circuit in the decoder completes 28 volts dc to the chime control circuits in the aural warning devices box. The chime then sounds each time 28 volts dc is applied to the control circuit. Whether the chime is repeated or not depends on the selcal system requirements. If the chime is repeated, the chime may be stopped by pressing a reset button on the selcal system control panel (Ref Chapter 23).
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AURAL WARNING SYSTEM - TROUBLE SHOOTING
_______________________________________
1. Prepare for Trouble Shooting____________________________
A. Supply electrical power (MM 24-22-00/201).
B. Supply hydraulic power (MM 29-15-00/201).
C. Make sure these circuit breakers on the P6 panel are closed:
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