CHAPTER 9 FIXED-WING FLIGHT CONTROL SYSTEMS

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