O.  Disengage and Warning (Fig. 13)
(1)  
The disengage and warning logic controls the visual and/or aural disengage and BITE warnings. The warning lights flash and audio sounds whenever the system is disengaged. A second push of the disengage switch or press-to-reset operation of either light is required to terminate the warning. The warning lights illuminate steady and cannot be extinguished when AFCS BITE is being used. Warning light power is provided by the airplane batter bus.

(2)  
When the AP system is engaged, engage voltage from either switch is applied to gate (1) or inverted to a low at gate (7). This results in a not warn condition. System disengagement via either disengage switch opens the engage interlock to disengage the AP system and supply 28 vdc power to gates (2), and (4). Gate (3) output remains low forcing a low output from gate (4) which inverts to a high at gate 5 to provide a high output to gate (7). Gate (7) engage inputs are inverted to enable a high output and initiate a warning.

(3)  
When the AP disengage switch is released, the state of gates (2) and (3) changes to high, however the output of gage (4) remains low (warning condition). The second push of the disengage switch generates a high output from gate (4) which inverts to a low at gate (5) when the lag time elapses, the (6) and (7) go low and the warning ceases.

(4)  
AP disengagements resulting from engage interlock fault (disengage switches not used) result is similar warning operation. Loss of engage voltage (1) and no warn voltage (2) results in a high output from gate (3). Gate (4) remains low to generate a warning. A single push of either disengage switch or light will change the state of gate (4) to high and the warning will cease.

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16. Altitude Alert
A.  The altitude alert function provides visual and/or aural indications to both pilots of approach toward or departure from the selected MCP altitude. The altitude alert function is inhibited during approach by the glide slope mode capturing or by the flaps being lowered to 25 or more degrees (flaps greater than 25 position). Selected baro-corrected altitude is used to determine the distance to the selected MCP altitude. Loss of altitude will prevent that channel’s altitude alert from operating.
B.  Loss of valid selected baro-corrected altitude in both channels will cause: The aural altitude alert to sound once, the altitude alert lights to flash (reset by rotating the altitude select knob), the Selected Altitude window display 50,000 feet. Single channel altitude becoming valid will reset the altitude alert lights and enable the selected altitude display window to follow altitude select knob rotation.
C.  Channel A provides the altitude alert function unless; the B selected baro-corrected altitude is valid and B autopilot engaged, or no autopilot engaged and only the B flight director is ON, or none of above and A selected baro-corrected altitude is invalid.
D.  The altitude alert function used altitude error developed from the MCP selected altitude and the selected DADC baro-corrected altitude. The altitude alert function sounds the aural warning and illuminates the altitude alert lights steady when the airplane approaches within 900 feet of the MCP selected altitude. As the airplane approaches within 300 feet, the lights go off. If the airplane departs more than 300 feet from the selected altitude, the function sounds the aural warning and flashes the altitude alert lights until the airplane either departs more than 900 feet from the selected altitude or returns to within 300 feet of the selected altitude. In either case, the alert lights then extinguish.
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Automatic Stabilizer Trim Schematic May 20/82 Figure 13 22-11-01 Page 59/60 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.

16. BITE Operation
A.  The Built-In-Test Equipment (BITE) IS designed to determine system faults, provide a complete check of the system internal operation and verify proper operation of the interfacing systems and flight controls. The BITE tests are controlled by operator switching on the PDC system CDU (Fig. 14). The PDC CDU mode selector and pushbutton switches are used to enter and exit BITE and to communicate with the AP/FD system. The PDC CDU CRT display is used to provide instructions and ask questions to guide the operator through the desired type of test and provide a visual indication (readout) of normal operation or specific LRU faults. The system is designed for a single operator to perform a complete system check or trouble shoot a fault to the LRU level from the airplane flight deck without the use of test equipment.
　
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