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General
In normal mode operation of the PFCS, the PFCs calculate pitch control commands based on this data:
*
Column-based command logic
*
Elevator column control
*
Speed stability function
*
Pitch rate
*
Flare compensation
*
Stall protection function
*
Overspeed protection function
*
Elevator rig position function
*
Elevator command limiting function
*
Force fight equalization
*
Elevator offload function
*
Elevator feel logic
*
Autopilot command.
Column-Based Command Logic
The column based command logic receives signals from the column position transducers and the column force transducers. When the pilot applies a force on the control column, the logic function selects the column position transducer signal by mid-value selection and averaging.
When the force on the column is less than 2.5 lb, the column force transducer sends a near zero signal and the PFC logic considers the columns at neutral. The column based command logic then supplies a zero position signal to the elevator column control.
The column based command logic does not function during autopilot operation.
Elevator Column Control
The elevator column control calculates the elevator command as a function of the column position transducer input.
When the force transducers detect that there is no force at the column, the elevator column control function uses the zero position signal. This zero position signal resets the calculation of the control function.
When the autopilot is engaged, the elevator column control uses the autopilot pitch command to calculate the elevator command.
Speed Stability Function
The speed stability function operates only in the air. It does not function during autopilot operation.
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The speed stability function receives computed airspeed (CAS) signals from the ADIRU and SAARU. It also receives the pitch trim switch signals from the ACEs and the alternate pitch trim lever switch signals from the AIMS.
The speed stability function controls the elevator to maintain the trim reference speed. The trim reference speed is the speed last selected by the pitch trim switches or the alternate pitch trim lever switches.
When the pilot moves the pitch trim switches, the trim reference speed changes. The speed stability function sends an elevator command so the airplane can reach the new trim reference speed.
When the pilot moves the alternate pitch trim levers, the levers cause a stabilizer movement and the lever switches change the trim reference speed. The new trim reference speed matches the speed reached by the new position of the stabilizer.
The speed stability function supplies automatic elevator commands during these conditions:
*
Thrust changes
*
Turbulence
*
Flap and slat extension and retraction
*
Speedbrake operation
*
Landing gear extension and retraction
*
Ground effect
* Banked turns less than 30 degrees.
Pitch Rate
The ADIRU pitch rate data supplies a feedback signal to the pitch command for pitch damping.
Flare Compensation
The flare compensation function operates when the airplane descends through 50 feet of radio altitude. The flare compensation function supplies a pitch-down command that simulates the natural attitude of the airplane in ground effect.
Stall Protection Function
The stall protection function uses the angle of attack from the ADIRU. It also uses the computed airspeed (CAS) and the mach number. The FSEU supplies the position of the flaps and slats and the WEU supplies stall data.
The stall protection function operates only in the air.
If the airplane approaches a stall condition, the stall protection function sends a signal to the elevator for pitch-down command. The function also increases the column feel force in the column aft direction.
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Overspeed Protection Function
The overspeed protection function monitors the CAS and Mach number. If there is an overspeed condition, the function sends a signal to the elevator for a pitch-up command.
Elevator Rig Position Function
The elevator rig position function is active only in the air when the flaps are at the 25 or 30-unit positions. The function monitors the position of the horizontal stabilizer to move the neutral position of the elevator to a maximum of 4 degrees trailing-edge-up. This elevator position increases the down lift of the horizontal stabilizer during landing. 中国航空网 www.aero.cn 航空翻译 www.aviation.cn 本文链接地址：777 AMM 飞机维护手册 飞行操纵 Flight Controls 2(11)
