*
Autopilot disengage switches
*
AFDC cross channel data
*
PFCS
*
ADIRU
*
SAARU
*
ILS
*
RA
*
Backdrive actuators.
PFCs
The PFCS controls the elevator, aileron, and rudder. When the PFCs receive valid engage requests from the AFDCs, the PFCs change control of these surfaces from the flight crew to the autopilot. The PFCs also command the AFDCs to engage the backdrive actuators.
Backdrive Actuators
The left and right AFDCs have rudder pedal, control wheel, and control column backdrive actuators. The center AFDC does not control backdrive. For this reason, the autopilot does not engage with only the center AFDC.
The engage relays for the backdrive actuators are in the AFDCs.
Logic in the AFDCs monitor the electrical components in the backdrive actuators.
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AFDS -FUNCTIONAL DESCRIPTION -ENGAGE INTERLOCKS INTRODUCTION
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General
Cruise engage refers to engagement of the pitch and roll axis. Autoland engage refers to the added engagement of the yaw axis.
Cruise Engage
Pitch and roll engagement starts when the flight crew pushes either autopilot engage switch on the MCP. The LEDs in both switches come on if there is a valid AFDC.
The autopilot engage logic in the AFDCs receives the engage request from the MCP. Each valid AFDC sends a pitch and roll engage request signal to the three PFCs when these conditions are true:
*
The left or right AFDC is valid with a valid column and wheel backdrive actuator (backdrive channel)
*
There are no engage inhibit faults.
All PFCs receive the pitch and roll engage request from each AFDC. Signal selection and fault detection logic in each PFC checks for these conditions:
*
There is a valid pitch and roll command from one of the AFDCs
*
The PFCS is in the normal mode.
When both of these conditions are true, the PFCs:
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*
Accept and acknowledge the pitch and roll engage request
*
Change from pilot control to autopilot control of the pitch and roll axis.
Each valid AFDC engages in pitch and roll. The control wheel and control column backdrive actuators connected to the master AFDC also engage.
AFDC Master Selection
The AFDC master selection changes each time the autopilot engages. This also changes which MCP processor controls the MCP displays.
Autoland Engage
The autopilot arms the yaw axis when the pilot pushes the APP switch on the MCP.
The AFDCs send a yaw engage request to the PFCs when these conditions are true:
*
The autopilot is engaged in pitch and roll
*
The airplane is below 1500 feet of radio altitude
*
Glideslope (G/S) mode is active
*
Localizer (LOC) mode is active and the airplane is on course
*
There are no faults that prevent yaw engagement.
The PFCs check for these conditions:
*
The autopilot is engaged in pitch and roll
*
At least two AFDCs request yaw engage
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* The PFCs receive at least two valid pitch, roll and yaw commands.
When the conditions are true, the PFCs do this:
*
Accept and acknowledge the yaw engage request
*
Change from pilot control to autopilot control of the yaw axis.
Each valid AFDC engages in yaw. Also, the pedal actuators and the other wheel and column actuators engage.
Backdrive Actuator Force Fight
The AFDCs do a test on the six backdrive actuators after the backdrive actuators engage. The test is a force fight test.
The AFDCs supply a small bias command of opposite polarity to each backdrive actuator set. The AFDCs monitor the current supplied to each actuator. A weak actuator disengages.
All six actuators must engage to get a LAND 3 status. For example, a LAND 2 status occurs if an actuator does not engage.
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