General
In normal mode operation of the PFCS, the PFCs calculate stabilizer pitch commands based on:
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Trim switches signals
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Column cutout function
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Elevator control schedule
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On ground stabilizer control
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Elevator offload function
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Control path logic
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Rate control
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Stabilizer position limit
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Failure monitor.
The PFCS determines the priority of the stabilizer control. The alternate pitch trim levers always have the highest priority.
In the air, the pitch trim switches, the control columns, and the autopilot send inputs to the PFCs for use in the elevator control schedule. The elevator control schedule sends commands for the elevator control and for the elevator offload function. The elevator offload function controls the operation of the horizontal stabilizer.
On the ground, elevator offload does not operate. The pitch trim switches directly control the stabilizer and the control columns directly control the elevators.
Trim Switches Signals
The trim switches signals function analyzes the input from:
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The alternate pitch trim lever switches
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The pitch trim switches.
Alternate Pitch Trim Lever Switches
The alternate pitch trim levers have priority over any other input command. They mechanically command both STCMs and cause the two hydraulic motors to move the stabilizer at the rate of speed available at the time.
The alternate pitch trim lever switches send discrete signals through the AIMS to the PFCs. These signals cause the PFCs to stop electrical control of the stabilizer, prevent failure indication, and prevent auto shutdown. They also change the trim reference speed in the elevator control schedule.
If the autopilot is engaged when a pilot moves the alternate pitch trim levers, the lever command causes the movement of the stabilizer but the autopilot stays engaged.
Pitch Trim Switches
The pitch trim switch signals operate in pairs. The signals from both the control and arm switches are necessary from the captain or the first officer for the PFC logic to calculate a command signal.
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HORIZONTAL STABILIZER CONTROL -FUNCTIONAL DESCRIPTION -NORMAL MODE
On the ground, the pitch trim switches control the stabilizer. The stabilizer moves at high speed with two hydraulic motors.
In the air, the pitch trim switches send an input signal to the elevator control schedule to change the trim reference speed.
If the autopilot is engaged when a pilot moves the pitch trim switches, the PFCs ignore the pitch trim switch signals and the autopilot stays engaged.
If the captain and first officer pitch trim switches send opposite signals, the PFCs do not use the signals and ignore the input.
If the signal from an arm or control pitch trim switch is not valid due to an ACE or an ARINC 629 bus failure, the PFCs uses a single pitch trim switch to calculate a command signal.
When there is no input from the alternate pitch trim lever switches, the pitch trim switch signals go to the column cutout function.
Column Cutout
The column cutout function uses the signals from the column position transducers. It stops the pitch trim switch commands when the pilot moves the column in the opposite direction of the pitch trim switch command.
EFFECTIVITY
BEJ ALL
Elevator Control Schedule
In the air, commands from the pitch trim switches, the control columns, and the autopilot cause the elevator control schedule to calculate an elevator output command. The pitch trim switches change the trim reference speed in the elevator control schedule. The PFCs then command the ACEs to move the elevator. The elevator command also goes to the elevator offload function. The alternate pitch trim lever switches also change the trim reference speed in the elevator control schedule.
On Ground Stabilizer Control
On the ground, the pitch trim switch signals go directly to the control path logic function. The PFCs then calculate commands to operate the stabilizer with two hydraulic motors at the high rate of speed.
Elevator Offload
The elevator offload function operates in the air. It increases the aerodynamic efficiency of the airplane and keeps the elevator available for full deflection.
The PFC uses amount, direction, and duration of the elevator command to calculate the amount and direction of the elevator offload command to the stabilizer.
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HORIZONTAL STABILIZER CONTROL -FUNCTIONAL DESCRIPTION -NORMAL MODE
There are two elevator actuation levels in the elevator offload function: the lower and the upper level. Small elevator commands may be more than the lower level and larger elevator commands may be more than the upper level. The PFCs calculate these levels as a function of airspeed and roll attitude of the airplane. The levels decrease with increased airspeed and increase with decreased airspeed.
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