(c)  
AIRPLANES WITHOUT CONTROL WHEEL STEERING; when the airplane is in the air, the F/D SELECT signal is also used to generate a MODE ON output. This is used to prepare the FCC mode logic to activate the CMD or FD modes. The MODE ON output will also result from the following conditions. 1) Autopilot engaged with F/D OFF. 2) Autopilot engaged with F/D ON since the A/P mode engage

 option is used. The FCC holds the airplane bank angle at the time of F/D or CMD engagement.
 (5)  Autopilot Engagement Timing (Fig. 7)
 (a)  CMD Engage Sequence
 1)  GUN 001-050, 052-999; when a CMD switch/light is pressed, MCP software debounces the switch contacts and encodes the engage request for transmission to the appropriate FCC. The FCC varifies the request and then applies power to the elevator and aileron ARM solenoids. Once armed these servos synchronize with the actual position of the flight controls.
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MAINTENANCE MANUAL

ELEV ENGAGE SOL ENERGIZED AIL ENGAGE SOL ENERGIZED
CMD
OFF
GUN 051
GUN 001-050,052-999

Autopilot Engagement Timing Diagram Figure 7

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MAINTENANCE MANUAL

 2)  GUN 051; when a MCP engage switch is placed in the CMD position, MCP software debounces the switch contacts and encodes the engage request for transmission to the appropriate FCC. The FCC verifies the request and then applies power to the elevator and aileron ARM solenoids. Once armed, these servos synchronize with the actual position of the flight controls.
 3)  A pre-engage test monitors the synchronization and verifies the servo's response to command. Upon completion of the test, the FCC powers the elevator and aileron engage solenoids.
 4)  GUN 001-050, 052-999; the dot-bar matrix in the CMD switch/light is lighted by an engage request accepted signal within 41 milliseconds after the ARM solenoids are energized.
 5)  GUN 051; the engage switch hold solenoid is energized by a CMD HOLD signal within 41 milliseconds after the arm solenoids are energized.
 (6)  Autopilot/Flight Director System ARM and ENGAGE logic (Fig. 8)
 (a)  
GUN 051; a CMD engage request is initiated when a single MCP engage switch is placed in the CMD position. The MCP software verifies that the switch activity is valid and only then encodes the activity into an engage request digital transmission. The engage request is sent to the appropriate FCC. FCC software and hardware monitors determine if the engage request can be implemented.

 (b)  
AIRPLANES WITH CONTROL WHEEL STEERING; a CMD or CWS engage request is initiated when the appropriate MCP switch/light is pressed. The MCP software verifies that the switch activity is valid and only then encodes the activity into an engage request digital transmission. The engage request is sent to the appropriate FCC. FCC software and hardware monitors determine if the engage request can be implemented.

 (c)  
Once the engage request is received, the FCC energizes the appropriate arm solenoids if the FCC is not inhibited. The FCC is inhibited if: 1) aileron or elevator arm solenoids are already energized 2) electrical power monitor detects a failure 3) MCP parameters or interface is invalid

 (d)  
Once armed, the A/P channel transitions from ARM to OFF if: 1) aileron or elevator engage solenoids are energized 2) autopilot disengage switch is pressed

 (e)  
GUN 051; after the arm solenoids are energized, a hold enable signal is sent to CMD HOLD circuitry. This circuitry applies 28 VDC to the MCP engage switch hold solenoid. The energized hold solenoid keeps the engage switch in the CMD position.

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