A.  The altitude alerting system provides aural and visual alerting signals to the pilots when the airplane approaches or deviates from a selected altitude. The selected altitude is set manually by the pilots.
B.  The altitude alerting system includes an altitude alert controller on the forward electronic control panel, an aural alerting unit on the overhead panel and altitude alert annunciators on the captain's and first officer's instrument panel (Fig. 1).
C.  The altitude alert controller receives 26-volt ac power from the No. 1 electronic power bus or from the standby power bus. The aural alerting unit receives 28-volt dc power from No. 2 electronic power bus.
D.  Control input to the altitude alerting controller originates in the No. 1 air data computer; fine and coarse altitude synchro signals are fed to the captain's altimeter where barometric correction is applied. The corrected signal is then fed to the altitude alerting controller.
E.  The aural alerting unit sounds 1 to 2 seconds and the altitude alert annunciators illuminate when the airplane approaches or deviates from a selected altitude, either in ascent or descent. The altitude alert controller is automatically reset to approach mode when a new altitude is selected, or the airplane altitude exceeds 1000 feet from the selected altitude.
2.  Altitude Alert Controller
A.  The altitude alert controller is located on the forward electronic control panel. The front panel of the controller has an altitude alert select control and altitude counter.
3.  Altitude Alert Annunciators
A.  Two dimmable, push-to-test altitude alert annunciators are located on the captain's and the first officer's instrument panel. Dimming is controlled by the master dim control.
5C8
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K79301
Altitude Alerting System Component Location  577 
34-17-02  Figure 1  Sep 20/81 
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4.  Audio Alerting Unit
A.  The audio alerting unit is located at the overhead panel. It provides an audio alerting signal, which is a musical tone, of 1 to 2 seconds duration when activated by the altitude alerting controller.
5.  Operation
A.  The altitude alerting system operates over a range from sea level to 50,000 feet to provide pilots with audio and visual alerting signals when a preset altitude is being approached from above or below. Synchros in the altitude alerting controller maintain a continuous comparison with the altitude synchros in captain's altimeter.
B.  When the synchros in the altitude alerting controller receive signals indicating the airplane altitude to be 1000 feet from the altitude manually preset on the altitude alerting controller, the output logic circuit is activated, providing a ground path to the audio alerting unit and alerting annunciators. The audio alerting unit sounds the sequential musical tone 1 to 2 seconds and the altitude alerting annunciators illuminate. When the airplane reaches 375 feet from the selected altitude, the annunciators go off.
C.  No alert signal occurs when the airplane is less than 375 feet from the selected altitude. As the airplane deviates more than 375 feet from the selected altitude, the audio alerting unit sounds for 1 to 2 seconds and the annunciators illuminate. At 1000 feet from selected altitude, the annunciators go off.
D.  If preset altitude is changed or the airplane altitude exceeds 1000 feet from preset altitude, the latching circuit is automatically reset to the acquisition (approach) mode.
E.  The failure monitor in the controller continuously monitors the validity signal from captain's altimeter, synchro signals and electrical power to the controller. Loss of any of these signals will bring the OFF flag into view. 
577 
Sep 20/81  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  34-17-02 Page 3 

K79303 K79306
Altitude Alerting System - Operating Diagram  504 
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577  Simplified Altitude Alerting System Schematic 
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ALTITUDE ALERTING SYSTEM - TROUBLE SHOOTING
EFFECTIVITY
PI FF GJ AP  N789N ALL EXCEPT N37AF thru N58AF ALL EXCEPT B2509 thru B2510 N458AC and N725AL thru N726AL 
　
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