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 G.  AF 121, 122, 157, 161, 183, 184; The altitude alert computer automatically resets to the approach mode whenever the difference between the selected altitude and the actual altitude exceeds 900 feet because of airplane altitude change or new altitude selection on ALT SEL control.
 2.  Altitude Alert Computer_______________________(Fig. 1)
 A.  The altitude alert computer evaluates data from the airplane central air data computer; autopilot mode select panel (altitude select function) and first officer's barometric altimeter, and computes the deviation from a preselected altitude.
 B.  The altitude alert computer is located behind the flight engineer's instrument panel.
 3.  Altitude Select Panel_____________________(Fig. 1)
 A.  The altitude select panel enables the pilots to preselect an altitude in 100-foot increments from 0 to 50,000 feet. The selector panel is located on the autopilot-flight director mode select panel on the pilots' lightshield.
 4.  Annunciators____________(Fig. 1)
 A.  The altitude alert annunciators are located on the captain's and first officer's instrument panels. Steady alert annunciators indicate that the airplane is approaching the selected altitude and flashing alert lights indicate that the airplane is deviating from the selected altitude.
 5.  Aural Warning Module
____________________
 A.  The aural warning module provides a 2 (| 1/2) second warning signal when the airplane is 900 feet from preset altitude during approach mode, and 350 feet from preset altitude during deviation mode.
 EFFECTIVITY
 

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 B. The aural warning module is located at the flight engineer's instrument panel.
 EFFECTIVITY

 

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 6.  Altitude Alert Test Switch (Fig. 1)
__________________________
 A.  The ALTITUDE ALERT TEST switch on the center instrument panel allows a confidence test of the altitude alert system. Pressing the TEST switch disables the inhibit circuit so that the system will respond to the simulated altitude variations.
 7.  Landing Gear Module
___________________
 A.  The landing gear module is located in the lower equipment bay. The deviation mode of operation is disabled when the landing gear is extended
 8.  Operation
_________
 A.  Functional Description (Fig. 2 and 3). On airplanes with Sundstrand Data Control analog altitude alert computer 965-0184-001:
 (1)  
The preset altitude is selected on the autopilot-flight director mode select panel. The altitude select function enables the pilots to preselect altitude in 100-foot increments from 0 to 50,000 feet.

 (2)  
Central air data computer No. 2 provides fine and coarse barometric altitude signals. The two signals are applied to two differential synchros in the autopilot mode select panel, where they are compared to the selected altitude. The first officer's altitude indicator provides fine altitude correction to compensate for barometric pressure variations.

 (3)  
The fine altitude output signals from the autopilot mode select panel and the corrected fine altitude signal from the first officer's altitude indicator are applied to the fine synchro converter circuits in the altitude alert computer to be converted to constant amplitude variable phase signals.

 (4)  
Since the airplane altitude may vary above or below the selected altitude, the output of the phase-to-dc converter may be a positive or negative voltage. The absolute value detector converts the output so it is always a positive signal. The output of the absolute value detector is then applied to two level detectors. The high level detector enables the logic circuits when the altitude difference is 900 feet. The low level detector disables the logic circuits when the altitude difference is less than 300 feet.

 (5)  
When an altitude is selected on the ALT SEL control, the approach-deviation latch in the altitude alert computer is latched to approach mode. As the airplane reaches 900 feet from the selected altitude, the level detectors enable the logic circuits, which in turn activate the aural warning module and simultaneously turn on the annunciators. As the airplane reaches 300 feet from the selected altitude, the low level detector is disabled to turn off the annunciators. At the same time, the approach-deviation latch is activated for deviation mode.

 (6)  
The aural warning circuit operates on a continuous ground signal. The aural warning system cycles the tone associated with the altitude alert system for only 2 (| 1/2) seconds, even though the ground is left connected.
　
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