The alternate pitch trim levers always have the highest priority of stabilizer control. The pitch trim switches have secondary priority.
Alternate Pitch Trim Levers
The first priority of stabilizer control in the direct mode is the alternate pitch trim levers. Their operation in direct mode is the same as in secondary mode.
Because there is no rate control in direct mode, the hydraulic motors operate at high rate of speed.
Pitch Trim Switches
The second priority of stabilizer control in the direct mode is the captain’s and the first officer’s pitch trim switches. They supply the only available electrical control of the STCMs in direct mode.
Anytime the pilot moves the pitch trim switches, the ACE 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.
Stabilizer Position Limits
The limit switches in the stabilizer position modules limit stabilizer travel in the direct mode. These limits are:
* +3.75 degrees of airplane nose down trim
* -10.75 degrees of airplane nose up trim.
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HORIZONTAL STABILIZER CONTROL -FUNCTIONAL DESCRIPTION -STABILIZER ELECTRICAL CONTROL
General
The ACEs send the electrical commands to control the stabilizer. In normal and secondary modes, the PFC calculate the commands and send them to the ACEs. In the direct mode, the ACEs uses the commands from the pitch trim switches.
The STCM must have both an arm and a control signal from its two ACEs to operate the stabilizer ball screw assembly.
Normal Mode
In normal mode, the pitch trim switch signals operate in pairs. The signals from both the control and arm pitch trim switches are necessary from the captain (left) or the first officer (right) for the PFC logic to calculate a command for the ACEs.
In the air, a command from the pitch trim switches causes a command to the elevator. Only the elevator offload function of the PFCs can control the stabilizer while the airplane is in the air. The command can be for single or dual path, low or high rate.
On the ground, an input from the pitch trim switches causes a PFC stabilizer command to the ACEs. This command is for dual path at high rate.
When the flaps are retracted, the PFC logic stops the movement of the stabilizer at:
EFFECTIVITY
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* +2.25 degrees of airplane nose down trim
* -10.25 degrees of airplane nose up trim.
When the flaps are extended, the SPM limit switches stop the stabilizer at +3.75 degrees of nose down trim. The PFC logic still stops the movement of the stabilizer at -10.25 degrees of airplane nose up trim.
If there is an ACE or an ARINC 629 flight control bus failure, the PFC permits a single pitch trim switch, arm or control, to command pitch trim as long as the signal path from the switch to the PFC is valid.
Secondary Mode
In secondary mode, the pitch trim switch signals also operate in pairs. Both arm and control signals are necessary for the PFC logic to calculate a command.
An input by the pitch trim switches always causes a stabilizer command for single path at high rate. The PFCs alternate which stabilizer path operates.
The stabilizer limits are the same as in the normal mode.
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HORIZONTAL STABILIZER CONTROL -FUNCTIONAL DESCRIPTION -STABILIZER ELECTRICAL CONTROL
Direct Mode
In direct mode, the pitch trim switch signals operate individually through the ACE to which they connect. Pitch trim switch arm and control signals are necessary to the two ACEs that connect to the STCM. The captain’s input causes a command to the C STCM while the first officer’s input causes a command to the R STCM. The command is always at high rate.
The limit switches in the SPM that connect the ACE and the STCM stop the movement of the stabilizer at:
* +3.75 degrees of airplane nose down trim
* -10.75 degrees of airplane nose up trim.
If both pilots make a pitch trim switch command at the same time and in the same direction, the ACEs send commands in dual path. The two STCMs operate both motors and brakes. The stabilizer then moves at full speed.
If both pilots make a pitch trim switch command at the same time but in opposite direction, the ACEs send commands in dual path in opposite rotation. The two STCMs operate both motors and brakes. The stabilizer then does not move, or may move slowly if one motor turns faster than the other.
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