Longitudinal Trim
SECONDARY CONTROL SYSTEM
MAINTENANCE
Longitudinal or pitch trim can be accomplished in
several ways. On aircraft with a nonmoveable
Organizational maintenance of the secondary
horizontal stabilizer, trim could be provided by a trim
flight control system includes checking system
tab arrangement or deflection of the elevators in much
operation, rigging, periodic inspection, lubrication,
the same manner as described for the lateral trim
isolation of malfunctions, and replacement of faulty
systems.
components.
Aircraft with a movable horizontal stabilizer and
Proper operation of the gearboxes, interconnecting
elevators are longitudinally trimmed by changing the
splined shafts, and screw jack actuators are dependent
angle of incidence of the stabilizer. Moving the
on proper lubrication. Lack of proper lubrication will
four-way trim control switch on the stick grip fore or
generally result in binding and excessive loading of
aft will raise or lower the leading edge of the stabilizer
torque tube assemblies. Lack of proper lubrication
to provide the angle of incidence necessary for
promotes corrosion. Space and time limitations during
balanced flight. An electric trim motor and actuator
shipboard operations often detract from the timely
arrangement provides movement of the stabilizer.
access to some of the slat and flap actuators. In many
cases a wing spread and extension of the surfaces are
Aircraft that use a movable horizontal stabilizer
necessary. Attention to these corrosion-prone areas
for longitudinal control trim do so by varying the
will materially contribute to trouble-free operation of
neutral position of the control linkage, which, in turn,
the screw jack mechanisms.
moves the surface. For example, longitudinal trim is
provided by varying the position of the artificial-feel
Repair of most of the gearboxes and screw jack
actuators at the intermediate level of maintenance is
bungee, repositioning the linkage, and setting up a new
limited to replacement of nuts, bolts, washers, gaskets,
neutral position for the stabilizer linkage. Anytime a
bearings, and shims. At the intermediate level of
new neutral is introduced by the trim actuator, the
maintenance, components of a secondary flight control
power valve shuttle is displaced. The stabilizer
system may be disassembled for routine maintenance,
assumes a new neutral location, changing the attitude
such as cure date seal and miscellaneous parts
of the aircraft. The trim inputs may be provided by the
replacement.
pilot or the automatic flight control system. The
actuator has two operating speeds-high speed for
NOTE: Before disassembly of any component,
manual trim and low speed for AFCS trim.
reference should be made to the "Intermediate
Maintenance" section of the applicable MIM or
Directional Trim
accessories manual to determine repair procedures and
test equipment requirements. If the component is
Directional trim is necessary to compensate for
beyond the repair capability of a given activity, it
yaw of the aircraft. Rudder trim is basically similar to
should be forwarded through channels to an authorized
the aileron trim. When the momentary throw rudder
higher level repair activity.
trim switch moves left or right, the trim actuator
The repair process for many of the flap hydraulic
energizes to move the load-feel bungee, repositioning
components will generally include the following
the rudder power mechanism input crank. The rudder
considerations:
linkage and the rudder are repositioned accordingly to
a new neutral position.
1. Clean the disassembled part, using a suitable
solvent followed by air drying with low-pressure air.
Most aircraft with power-controlled actuators
work in a similar manner, using an electric trim
2. Inspect all parts, using a strong light and some
actuator to change the neutral position of linkage,
means of magnification, or one of the nondestructive
deflecting the rudder to maintain the desired
methods of metal inspection. Threaded parts are
directional stability. Like the lateral and longitudinal
inspected for crossed, stripped, worn, or otherwise
trim systems, rudder trim action can be accomplished
damaged threads. Springs are checked for distortion,
manually or automatically. Trim position indicators
permanent set, and alignment. Spring alignment may be
provide a cockpit indication of the amount of trim or
verified by rolling them on a smooth, flat surface. The
surface deflection required by each trim system.
free length, compressed length, and reflected load of the
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