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airlines in the same area would be required to monitor the heavy
communications traffic on a single channel to determine if any messages
were for their flight. For SELCAL ( selective calling system ) operation,
each airplane is assigned a unique four-letter code. Each letter in the code
correspond to a different audio tone. A SELCAL (tone) decoder is
programmed to respond only to that assigned code and is connected to
the output of a receiver. The ground stations modulate their transmission
with the appropriate code tones to call an individual airplane. When the
proper tones are received, the decoder operates alert devices in the flight
deck to advise the crew to listen to the network frequency.)
[2] The aural warning system consists of monitor circuits and an aural
warning devices unit. The monitor circuits interface with the airplane
systems to detect certain potentially dangerous flight control
configurations, critical system warning, and flight crew alerts. The aural
warning devices unit contains a speaker for horn and chime signals, 2
clackers and bell which provide audio alert signals to the flight crew. The
aural warning system interfaces with the following systems:
 Landing gear warning system
 Takeoff warning system
 Cabin altitude warning system
 Digital flight control system
 Mach airspeed warning system
 Fire detection system
 Crew call system
 Selcal system
Fig. 45-1 aural warning system
[3] The aural warning devices unit produces several different sounds: any
one of which alerts the crew to conditions that require immediate
attention. The unit is 10 inches long, 5 inches wide, 2 inches deep and is
mounted vertically on the aft right side of the P9 panel. Two electrical
connectors at the rear of the unit provide interface with the airplane
wiring. The unit contains chime circuits, wailer and horn circuits, speaker,
fire warning bell and overspeed clackers.
Fig. 45-2 aural warning devices unit
[4] Figure 45-3 illustrates the aural warning system schematic. There are
3 separate aural warning devices in the aural warning devices unit. They
are the clackers, module A1, and module A2. Module A1 contains driver
circuitry for the bell. Module A2 contains the chime, wailer, intermittent
horn, and steady horn circuitry. The highest priority warning is amplified
and sent to the speaker. Module A2 receives 28 v dc from the 28 v dc
BAT BUS through a AURAL WARNING circuit breaker on the P6
panel.

Fig. 45-3 aural warning system schematic
Module A1 receives 28v dc from the 28v dc BAT BUS through a
MASTER WARNING AND CONTROL circuit breaker on the P6 panel.
The clackers receive 28v dc from the 28v dc ELEX-1 through a circuit
breaker on the P18 panel for system 1 and from the 28v dc ELEX-2
through a circuit breaker on the P6 panel for system 2.
[5] The conditions which cause an input to the aural warning devices unit
are displayed on the graphic. The HI chime circuit receives an input from
the crew call system . The HI chime circuitry generates an output which
is amplified and sent to the speaker. The HI-LO chime circuitry receive
an input from the SELCAL system. The input is applied to the HI chime
generator and through a 0.3 second time delay to the LO chime generator.
The HI chime circuitry generates a HI chime output which is amplified
and sent to the speaker. The LO chime circuits generate a LO chime
output 0.3 seconds after the HI chime which is amplified and sent to the
speaker following the HI chime.
[6] The steady horn circuit receives an input from the landing gear
warning system which starts the horn oscillator. The output of the horn
oscillator is sent to a driver and amplifier before being applied to the
speaker. The intermittent horn circuit receives inputs from the takeoff
warning or the cabin altitude warning systems. Either of these inputs will
start the horn oscillator. The input is applied to an interrupter circuit. The
interrupter circuit causes the horn oscillator to be interrupted twice each
second. The interrupted output of the horn oscillator is sent to a driver
and amplifier before being applied to the speaker.
[7] The wailer circuit receives an input from the digital flight control
system (DFCS). The input starts the horn oscillator. The input is applied
to a modulator circuit at the same time. The modulator circuit modulates
the horn oscillator at a rate of 1.4 times per second. The modulated
output of the horn oscillator is sent to a driver and amplifier before being
applied to the speaker.
[8] Clacker 1 receives an input from the F/O mach airspeed warning
　
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