The warning message STABILIZER shows when there is an unscheduled stabilizer motion and the auto shutdown has failed. The warning message also shows when all of these conditions occur for four seconds:
*
Both the right and center stabilizer channels have failures, or the STABILIZER C and the STABILIZER R advisory messages show while the STABILIZER CUTOUT advisory message does not show

*
Both elevator surfaces have a minimum of one PCU in operation, or the right and center hydraulic systems have failures.

The caution message FLIGHT CONTROLS shows for one of these conditions:
*
Both stabilizer paths are not operative

*
Two elevator PCUs and one stabilizer path are not operative

*
Two elevator PCUs are defective on one elevator

*
Captain and first officer wheel, column, or pedal position transducers are different by more than 15% with the airplane on the ground

* The elevator force fight equalization function senses a large force fight between the PCUs.
The advisory message STABILIZER C or STABILIZER R shows for one of these conditions:
*
There is an auto shutdown of the center or right stabilizer channel

*
Center or right cutout switch is in the CUTOUT position

*
Captain or the first officer pitch trim switches signals are stopped by the column cutout function for more than 15 seconds.

The advisory message STABILIZER CUTOUT shows when both the right and center stabilizer cutout switches are in the CUTOUT position.
The status message FLIGHT CONTROL SYS shows when one or more stabilizer control channel is defective. The message also shows when there is a double fault in one of these:
*
Stabilizer position modules

*
Rate solenoids on the STCMs

*
Pitch trim switches

*
Alternate pitch trim lever switches.

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HORIZONTAL STABILIZER CONTROL -FUNCTIONAL DESCRIPTION -NORMAL MODE

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HORIZONTAL STABILIZER CONTROL -FUNCTIONAL DESCRIPTION -SECONDARY M0DE

General

In secondary mode operation of the PFCS, the PFCs calculate stabilizer pitch commands based on:
*
Column cutout function

*
Secondary stabilizer control

*
Stabilizer position limit

*
Failure monitor.

Operational Priority

The alternate pitch trim levers always have the highest priority of stabilizer control. The pitch trim switches have secondary priority.
Alternate Pitch Trim Levers

Except for rate control, the alternate pitch trim lever operation in secondary mode is the same as in normal mode. They supply first priority of stabilizer control.
Because there is no rate control in secondary mode, the hydraulic motors operate at high rate of speed.
Pitch Trim Switches

The only available electrical control in secondary mode is from the captain’s and the first officer’s pitch trim switches.
Anytime the pilot moves the pitch trim switches, the PFC directly commands one STCM at high rate of speed. The stabilizer then moves with one hydraulic motor at a speed of 0.25 degree per second.
If the captain’s and first officer’s pitch trim switches send opposite command signals, the PFCs stops the command process.
Column Cutout
The column cutout function stops the pitch trim switch commands when the pilot moves the column in the opposite direction of the switch commands.
Stabilizer Position Limits
PFCs supply the same stabilizer position limits as in the normal mode.
Failure Monitor
With the stabilizer cutout switches in the NORM position, the PFC monitors the commanded STCM for correct stabilizer operation.
The failure monitor finds the same malfunctions as in the normal mode. When it detects a malfunction, the failure monitor commands the ACE to energize the auto shutdown relay of the commanded STCM.

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HORIZONTAL STABILIZER CONTROL -FUNCTIONAL DESCRIPTION -SECONDARY M0DE

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HORIZONTAL STABILIZER CONTROL -FUNCTIONAL DESCRIPTION -DIRECT MODE

General

In direct mode operation of the PFCS, the ACEs directly control the stabilizer pitch commands.
The ACE does not monitor the stabilizer operation for failure. If the PFCS is in direct mode and there is a stabilizer failure, the pilots must manually shut down the STCM with the cutout switches.
　
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