on the underside of each wing. The jacking points may also be used as tiedown fittings for securing the aircraft. Various  points  on  the  wing  are  located  by  station number.  Wing  station  0  (zero)  is  located  at  the  center line of the fuselage. All wing stations are measured in inches outboard from that point, as shown in figure 1-2. STABILIZERS 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 con- sidered 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 usuall y of an all-metal construction and of the cantilever design. Monospar and two-spar construction are both com- monly  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. FLIGHT CONTROL SURFACES 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. Primary Flight Control Systems The  primary  flight  controls  are  the  ailerons, elevators,  and  rudder.  The  ailerons  and  elevators  are operated from the cockpit by a control stick on fighter aircraft. A wheel and yoke assembly is used on large aircraft such as transports and patrol planes. The rudder is operated by rudder pedals on all types of aircraft. The  ailerons  are  operated  by  a  lateral  (side-to-side) movement of the control stick or a turning motion of the wheel  on  the  yoke.  The  ailerons  are  interconnected  in the control system and work simultaneously, but in opposite directions to one another. As one aileron moves downward to increase lift on its side of the fuselage, the aileron  on  the  opposite  side  of  the  fuselage  moves upward to decrease lift. This opposing action allows more lift to be produced by the wing on one side of the fuselage  than  on  the  other  side.  This  results  in  a controlled  movement  or  roll  because  of  unequal  forces on the wings. The aileron system can be improved with the use of either powered controls or alternate control systems. The  elevators  are  operated  by  a  fore-and-aft movement of the control stick or yoke. Raising the elevators causes the aircraft to climb. Lowering the elevators causes it to dive or descend. The pilot raises 1-5