C. Pressurization Warning
(1) An 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 clacker will sound if at any time the indicated airspeed (IAS) or mach number exceeds the limits. The clacker cannot be silenced except by decreasing the speed of the airplane.
(2)
Control of the clacker is from the mach/airspeed switch in each pilot's mach/airspeed indicator. The two switches are connected electrically in parallel. The relay in each switch is energized by 28 volts dc from the mach/airspeed system. The aneroid in each switch will open a relay circuit if the airspeed or mach number are exceeded for the given conditions. The de-energized relay then completes a ground for control of the clacker. The system can be tested by operating a mach test switch on overhead panel P5. When the test switch is pressed, 28 volts dc is connected to the ground side of each mach/airspeed switch relay, thus biasing each relay off and causing the clacker to sound for 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 detection control panel or the APU remote cutout switch on APU panel P28 in the main wheel well. Since the engines have both 400 and 750°F elements, the bell may also be silenced, by selecting the other element (provided the other element does not indicate a fire) with the detector selector switches on the fire detection control panel. A similar switch for the APU on the fire detection control panel connects either 400°F element A or 400°F element B to the warning system (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 through a detector selector switch to either the 400 or 750°F sensing circuit. The APU has two 400°F elements (A and B); either one may be connected to the APU gate circuit by the APU detector selector switch. The gates may be turned off and the bell silenced by pressing one of the reset buttons. Voltage is then completed to SCR circuits which bias off each gate. The gates are reset whenever the fire condition is corrected.
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F. Crew Call (If Activated)
(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 (If Activated)
(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. Provide electrical power.
B. Provide trailing edge flap system A hydraulic power.
C. Check that following circuit breakers on panel P6 are closed:
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