2A-27-20: Elevator Pitch Control
1. General Description:
The aircraft has a dual elevator installation to control aircraft pitch attitude. The
elevators are composed of a baked graphite-epoxy material. Each of the cockpit
yokes is connected to one of the aircraft elevators. The pilot yoke is connected to
the left elevator, the copilot yoke to the right elevator. Each yoke is also connected
to the other by a mechanical torque tube beneath the cockpit floor. Since both
yokes are interconnected, moving one yoke moves both elevators.
Braided steel cables run from each yoke to hydraulic assist actuators in the tail of
the aircraft. The cables are routed beneath the aircraft floor using pulley
connections to clear other installed equipment. The cables mate with the hydraulic
assist actuators via bellcranks that translate pulley rotational motion into forward
and aft motion. The actuators each have a single shaft powered by two piston
chambers, one chamber for each (left and right) hydraulic system. Both hydraulic
systems normally power the actuators, but one system is sufficient for full elevator
movement. The actuators are connected to the respective elevator by linkages
and bellcranks, moving the elevator up or down about the pivot points on the aft of
the horizontal stabilizer. The deflection range of the elevators is twenty-four
degrees (24°) up and thirteen degrees (13°) down.
Each connection of yoke to elevator is a continuous loop. Incorporated into the
loops adjacent to the actuators is a bungee cylinder filled with viscous fluid to
resist yoke / elevator movement in order to provide artificial feel to each yoke.
Each elevator also has a stability spring incorporated into the cable linkage to
provide a forward pull to the control yoke and to contribute additional feel input.
Both sides of the hydraulic actuators are monitored to assure correct operation.
The cockpit cable input motion must result in a corresponding actuator output
motion, and similarly the output side of the actuator should not move without
cockpit input. If input and output do not correspond, actuator hydraulic pressure is
bypassed to prevent movement of the elevator.
Anytime hydraulic pressure to the actuators is bypassed or lost (in the instance of
dual hydraulic system failure) the elevators remain operable with manual yoke
movement that positions the actuator shaft and connecting linkages to the
elevator. Control forces will be higher, since normal hydraulic assist provides a six
(6) to one (1) boost advantage to move the elevator surfaces.
Each elevator is equipped with a trim tab that uses aerodynamic pressure to aid in
positioning the control surface. The trim tabs are controlled manually by rotating a
wheel on the cockpit pedestal or electrically using switches on the control yokes.
Manual trim uses a dedicated braided wire connection from the cockpit to a
mechanical linkage in the tail. Electrical switch trim movement commands an
electric servo to move the same linkage.
Both the elevators and elevator trim incorporate Rotary Variable Differential
Transducers (RVDTs) to feed back position information to the autopilot for
elevator control and trim and to the Modular Avionics Units (MAUs) for formulation
of control surface position display on the Flight Controls 2/3 synoptic page. RVDTs
measure the angle of the elevators and trim tabs and transmit an electrical signal
proportional to displacement from a neutral position.
When the autopilot is engaged, the elevators are positioned by electric servos that
move parallel cable connections to the hydraulic actuators. The autopilot also
uses the electric trim servo to move the trim tabs, minimizing hydraulic actuator
force.
OPERATING MANUAL
PRODUCTION AIRCRAFT SYSTEMS2A-27-00
Page 8
August 14/03
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