The stabilizing surfaces of an aircraft consist of vertical and horizontal airfoils. These are known as the vertical stabilizer (or fin) and the horizontal stabilizer. These two airfoils, together with the rudder and elevators, form the tail section, For inspection and maintenance purposes, the entire tail section is considered a single unit of the airframe, and is referred to as the "empennage."

The primary purpose of the stabilizers is to stabilize the aircraft in flight; that is, to keep the aircraft in straight and level flight. The vertical stabilizer maintains the stability of the aircraft about its vertical axis. This is known as "directional stability." The vertical stabilizer usually serves as the base to which the rudder is attached. The horizontal stabilizer provides stability of the aircraft about the lateral axis. This is "longitudinal stability." It usually serves as the base to which the elevators are attached.

At high speeds, forces acting upon the flight controls increase, and control of the aircraft becomes difficult. his problem can be solved through the use of power-operated or power-boosted flight control systems. These power systems make it possible for the pilot to apply more pressure to the control surface against the air loads. By changing the angle of attack of the stabilizer, the pilot maintains adequate longitudinal control by rotating the entire horizontal stabilizer surface.

Construction features of the stabilizers are in many respects identical to those of the wings. They are usually of an all-metal construction and of the cantilever design. Monospar and two-spar construction are both commonly used. Ribs develop the cross-sectional shape. A "fairing" is used to round out the angles formed between these surfaces and the fuselage.

The construction of control surfaces is similar to that of the wing and stabilizers. They are usually built around a single spar or torque tube. Ribs are fitted to the spar near the leading edge. At the trailing edge, they are joined together with a suitable metal strip or extrusion. For greater strength, especially in thinner airfoil sections typical of trailing edges, a composite construction material is used.

The flight control surfaces are hinged or movable airfoils designed to change the attitude of the aircraft during flight. Flight control surfaces arc grouped as systems and are classified as being either primary or secondary. Primary controls are those that provide control over the yaw, pitch, and roll of the aircraft. Secondary controls include the speed brake and flap systems. All systems consist of the control surfaces, cockpit controls, connecting linkage, and other necessary operating mechanisms.

The systems discussed in this chapter are representative of those with which you will be working. However, you should bear in mind that changes in these systems are sometimes necessitated as a result of later experience and data gathered from fleet use. Therefore, prior to performing the maintenance procedures discussed in this chapter, you should consult the current applicable technical publications for the latest information and procedures to be used.