Pivot point
Stabilator
Antiservo tab
Figure 6-21. An antiservo tab attempts to streamline the control
surface and is used to make the stabilator less sensitive by opposing
the force exerted by the pilot.
Figure 5-16. The movement of the elevator is opposite to the
direction of movement of the elevator trim tab.
Nose-down trim
Nose-up trim
Trim tab
Elevator
Trim tab
Elevator
Tab up—elevator down
Tab up—elevator up
Figure 6-20. The movement of the elevator is opposite to the
direction of movement of the elevator trim tab.
control pressures that may exist for that flight condition. As
power, pitch attitude, or configuration changes, retrimming
is necessary to relieve the control pressures for the new
flight condition.
Balance Tabs
The control forces may be excessively high in some aircraft,
and, in order to decrease them, the manufacturer may use
balance tabs. They look like trim tabs and are hinged in
approximately the same places as trim tabs. The essential
difference between the two is that the balancing tab is coupled
to the control surface rod so that when the primary control
surface is moved in any direction, the tab automatically
moves in the opposite direction. The airflow striking the tab
counterbalances some of the air pressure against the primary
control surface and enables the pilot to move the control more
easily and hold the control surface in position.
If the linkage between the balance tab and the fixed
surface is adjustable from the flight deck, the tab acts as a
combination trim and balance tab that can be adjusted to a
desired deflection.
Servo Tabs
Servo tabs are very similar in operation and appearance to the
trim tabs previously discussed. A servo tab is a small portion
of a flight control surface that deploys in such a way that it
helps to move the entire flight control surface in the direction
that the pilot wishes it to go. A servo tab is a dynamic device
that deploys to decrease the pilots work load and de-stabilize
the aircraft. Servo tabs are sometimes referred to as flight tabs
and are used primarily on large aircraft. They aid the pilot in
moving the control surface and in holding it in the desired
position. Only the servo tab moves in response to movement
of the pilot’s flight control, and the force of the airflow on
the servo tab then moves the primary control surface.
Antiservo Tabs
Antiservo tabs work in the same manner as balance tabs
except, instead of moving in the opposite direction, they move
in the same direction as the trailing edge of the stabilator.
In addition to decreasing the sensitivity of the stabilator, an
antiservo tab also functions as a trim device to relieve control
pressure and maintain the stabilator in the desired position.
The fixed end of the linkage is on the opposite side of the
surface from the horn on the tab; when the trailing edge of the
stabilator moves up, the linkage forces the trailing edge of the
tab up. When the stabilator moves down, the tab also moves
down. Conversely, trim tabs on elevators move opposite of
the control surface. [Figure 6-21]
Ground Adjustable Tabs
Many small aircraft have a nonmovable metal trim tab on the
rudder. This tab is bent in one direction or the other while on
the ground to apply a trim force to the rudder. The correct
displacement is determined by trial and error. Usually, small
Figure 6-24. Basic autopilot system integrated into the flight
control system.
Figure 6-23. Some aircraft, including most jet transports, use an
adjustable stabilizer to provide the required pitch trim forces.
Adjustable stabilizer
Nose down
Nose up
Trim motor or trim cable
Jackscrew
Pivot
Figure 6-22. A ground adjustable tab is used on the rudder of many
small airplanes to correct for a tendency to fly with the fuselage
slightly misaligned with the relative wind.
adjustments are necessary until the aircraft no longer skids
left or right during normal cruising flight. [Figure 6-22]
Adjustable Stabilizer
Rather than using a movable tab on the trailing edge of the
elevator, some aircraft have an adjustable stabilizer. With this
arrangement, linkages pivot the horizontal stabilizer about
its rear spar. This is accomplished by the use of a jackscrew
mounted on the leading edge of the stabilator. [Figure 6-23]
On small aircraft, the jackscrew is cable operated with a trim
wheel or crank. On larger aircraft, it is motor driven. The
trimming effect and flight deck indications for an adjustable
stabilizer are similar to those of a trim tab.
Autopilot
Autopilot is an automatic flight control system that keeps an
aircraft in level flight or on a set course. It can be directed by
the pilot, or it may be coupled to a radio navigation signal.
Autopilot reduces the physical and mental demands on a pilot
and increases safety. The common features available on an
autopilot are altitude and heading hold.
The simplest systems use gyroscopic attitude indicators and
magnetic compasses to control servos connected to the flight
control system. [Figure 6-24] The number and location of
these servos depends on the complexity of the system. For
example, a single-axis autopilot controls the aircraft about the
longitudinal axis and a servo actuates the ailerons. A three-axis
autopilot controls the aircraft about the longitudinal, lateral, and
vertical axes. Three different servos actuate ailerons, elevator,
and rudder. More advanced systems often include a vertical
speed and/or indicated airspeed hold mode. Advanced autopilot
systems are coupled to navigational aids through a flight director.
The autopilot system also incorporates a disconnect safety
feature to disengage the system automatically or manually.
These autopilots work with inertial navigation systems,
global positioning systems (GPS), and flight computers to
control the aircraft. In fly-by-wire systems, the autopilot is
an integrated component.
Additionally, autopilots can be manually overridden. Because
autopilot systems differ widely in their operation, refer to
the autopilot operating instructions in the Airplane Flight
Manual (AFM) or the Pilot’s Operating Handbook (POH).
Chapter Summary
Because flight control systems and aerodynamic
characteristics vary greatly between aircraft, it is essential
that a pilot become familiar with the primary and secondary
flight control systems of the aircraft being flown. The
primary source of this information is the AFM or the POH.
Various manufacturer and owner group websites can also be
a valuable source of additional information.