patrol aircraft. The rudder is operated by foot pedals on all types of aircraft. Secondary Group The secondary group includes the trim tabs and spring tabs. Trim tabs are small airfoils recessed into the trailing edges of the primary control surface. Each trim tab hinges to its parent primary control surface, but operates by an independent control. Trim tabs let the pilot trim out an unbalanced condition without exerting pressure on the primary controls. Spring tabs are similar in appearance to trim tabs but serve an entirely different purpose. Spring tabs are used for the same purpose as hydraulic actuators. They aid the pilot in moving a larger control surface, such as the ailerons and elevators. Auxiliary Group The auxiliary group includes the wing flaps, spoilers, speed brakes, and slats. WING FLAPS.—Wing flaps give the aircraft extra lift. Their purpose is to reduce the landing speed. Reducing the landing speed shortens the length of the landing rollout. Flaps help the pilot land in small or obstructed areas by increasing the glide angle without greatly increasing the approach speed. The use of flaps during takeoff serves to reduce the length of the takeoff run. Some flaps hinge to the lower trailing edges of the wings inboard of the ailerons. Leading edge flaps are used on the F-14   Tomcat   and F/A-18   Hornet. Four types of flaps are shown in figure 4-10. The  plain flap forms the trailing edge of the airfoil when the flap is in the up position. In the split flap, the trailing edge of the airfoil is split, and the lower half is hinged and lowers to form the flap. The  fowler flap  operates on rollers and tracks, causing the lower surface of the wing to roll out and then extend downward. The   leading edge flap operates like the plain flap. It is hinged on the bottom side. When actuated, the leading edge of the wing actually extends in a downward direction to increase the camber of the wing. Landing flaps are used in conjunction with other types of flaps. SPOILERS.—Spoilers are used to decrease wing lift. The specific design, function, and use vary with different aircraft. On some aircraft, the spoilers are long narrow surfaces, hinged at their leading edge to the upper surfaces of the wings. In the retracted position, they are flush with the wing skin. In the raised position, they greatly reduce wing lift by destroying the smooth flow of air over the wing surface. SPEED BRAKES.—Speed brakes are movable control surfaces used for reducing the speed of the aircraft. Some manufacturers refer to them as   dive brakes; others refer to them as   dive flaps. On some aircraft, they're hinged to the sides or bottom of the fuselage. Regardless of their location, speed brakes serve the same purpose—to keep the airspeed from building too high when the aircraft dives. Speed brakes slow the aircraft's speed before it lands. SLATS.—Slats are movable control surfaces that attach to the leading edge of the wing. When the slat is retracted, it forms the leading edge of the wing. When the slat is open (extended forward), a slot is created between the slat and the wing leading edge. High-energy air is introduced into the boundary layer over the top of the wing. At low airspeeds, this action improves the lateral control handling characteristics. This allows the aircraft to be controlled at airspeeds below normal landing speed. The high-energy air that flows over the top of the wing is known as  boundary layer control air. Boundary layer control is intended primarily for use during operations from carriers. Boundary layer control air aids in catapult takeoffs and arrested landings. Boundary control air can also be accomplished by directing high-pressure engine bleed air across the top of the wing or flap surface. 4-10 SPLIT FLAP LEADING EDGE FLAP PLAIN FLAP FOWLER FLAP ANf0410 Figure 4-10.—Types of flaps.