the tab forces the hinged elevator into the opposite direction. As the
elevator is moved from the neutral position (faired with the horizontal
stabilizer), it also encounters pressure forces from the airstream, thus the
amount of elevator movement from neutral is determined by a balance of
airstream forces acting on both the trim tab and the elevator. Since the
surface area of the trim tab exposed to the airstream is less than the
surface area of the elevator, the elevator is deflected in much smaller
increments than trim tab displacement (excluding other factors, trim tab
effectiveness is a ratio of tab surface area to elevator surface area).
The flight crew manually controls the amount of trim tab deflection by
moving control wheels on either side of the center console. The wheels are
hubs connected to a common axial shaft, so that moving one wheel moves
the other. The shaft is connected to a continuous loop of wire cables that
connect through a series of pulleys and bellcranks to the elevator trim tabs.
Rotating a trim wheel forward positions the trim tab up forcing the elevator
down resulting in an aircraft nose down moment. Trim wheel rotation aft
results in an aircraft nose up moment.
The flight crew has the option of electrically moving the elevator trim tabs.
A pushbutton, labelled PITCH TRIM ENG / DISENG, located to the left of
the standby flight instruments on the lower instrument panel enables
electrical operation of trim switches mounted on the outboard side of the
control yokes. Electric pitch trim is normally engaged. The amber DISENG
legend in the pushbutton will illuminate if the button is not pushed in to
engage electric trim. The yoke trim switches are composed of split halves.
Both halves must be moved in the same direction to move the elevator
trim. The split switch design helps to prevent accidental trim input. The
switches are wired to an electric servomotor that is located in the tail of the
aircraft and incorporated into the cable linkage to the trim tabs. An electric
signal from a cockpit trim switch results in the servomotor rotating an
attached pulley, moving the elevator trim cable loop in the desired
direction. The manual trim wheels on the pedestal will rotate with electric
trim inputs, since the control cabling is a continuous loop.
When the autopilot is engaged, autopilot servomotors move the elevators
and also control elevator trim with the same servomotor employed by the
yoke electric trim switches. The PITCH TRIM ENG / DISENG switch is
automatically engaged whenever the autopilot is engaged (to enable
autopilot trim). However the reverse is not true - electric pitch trim will not
disengage when the autopilot is disengaged, but must be selected off with
the switch.
Elevator trim must be within a defined range for takeoff, with the specific
setting within the range determined by aircraft Center of Gravity (COG) and
takeoff gross weight. The limits of the acceptable pitch trim range are eight
degrees nose up to nineteen degrees nose up (8° - 19° up). The takeoff
elevator trim range is marked in green on the trim setting indices of the
manual trim wheels at each side of the center pedestal. The same limits
are shown in a green band on the trim scale of the Flight Controls 2/3
synoptic page. Failure to set pitch trim within the defined range will result in
a warning annunciation and CAS message as the power levers are
advanced for takeoff.
OPERATING MANUAL
PRODUCTION AIRCRAFT SYSTEMS2A-27-00
Page 12
August 14/03
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