CHAPTER 9
FIXED-WING FLIGHT CONTROL SYSTEMS
Chapter Objective: Upon completion of this chapter, you will have a working
knowledge of the functions of fixed-wing flight controls (primary and secondary)
and the associated maintenance requirements to include major assembly
removal/installation and alignment procedures.
A flight control system is either a primary or
secondary system.
Primary flight controls provide
longitudinal (pitch), directional (yaw), and lateral
(roll) control of the aircraft. Secondary flight controls
provide additional lift during takeoff and landing, and
decrease aircraft speed during flight, as well as
assisting primary flight controls in the movement of
the aircraft about its axis. Some manufacturers call
secondary flight controls auxiliary flight controls. All
systems consist of the flight control surfaces, the
respective cockpit controls, connecting linkage, and
necessary operating mechanisms.
The systems discussed in this chapter are
representative systems. Values such as tolerances,
pressures, and temperatures provide better under-
standing of the text material. You should bear in mind
that these values are for representative units and are
not accurate for all systems. When actually
performing the maintenance procedures discussed,
you should consult the current maintenance
instruction manual (MIM).
TYPES OF FLIGHT CONTROL
SYSTEMS
Learning Objective: Identify the two basic
types of flight control systems.
A flight control system includes all the com-
ponents required to control the aircraft about each of
the three flight axes. A simple flight control system
may be all mechanical; that is, operated entirely
through mechanical linkage and cable from the
control stick to the control surface.
Other more
sophisticated flight control systems may use electrical
or hydraulic power to provide some or all of the
“muscle” in the system. Still others combine all three
methods.
MECHANICAL (UNBOOSTED) FLIGHT
CONTROL SYSTEM
A typical, simple, mechanical (unboosted) flight
control system is the one used in flight training
aircraft.
The flight control surfaces (ailerons,
elevators, and rudder) are moved manually through a
series of push-pull rods, cables, bell cranks, sectors,
and idlers. Figure 9-1 schematically illustrates the
elevator portion of a mechanical (unboosted) flight
control system. The control stick is mounted in such
a way that it can pivot backwards and forwards on its
mounting pin. The control stick is connected to a
push-pull rod attached to its lower end. As the stick is
moved fore and aft, it causes the elevators to be
deflected proportionately.
The push-pull tube (rod) that connects to the
lowest point of the control stick extends aft to the
pulley. Notice that the function of the pulley is to
change the direction of the push-pull action from fore
and aft to up and down. The second push-pull tube
(rod) connects the forward cable sector and the pulley,
and causes the sector to rotate according to the stick
movements.
Figure 9-1.—Mechanical (unboosted) flight control system.
9-1
