Pre-stall  Stalled Stalled  Figure 5-69.  Wingtip pre-stall.  Figure 5-70.  T-tail stall.  to cause a low-speed Mach buffet. This very high AOA has  the effect of increasing airflow velocity over the upper surface  of the wing until the same effects of the shock waves and  buffet occur as in the high-speed buffet situation. The AOA  of the wing has the greatest effect on inducing the Mach  buffet at either the high-speed or low-speed boundaries for  the aircraft. The conditions that increase the AOA, the speed  of the airflow over the wing, and chances of Mach buffet are:  AOA. In this situation, without reliable AOA information,  a nose-down pitch attitude with an increasing airspeed is no  guarantee that recovery has been affected, and up-elevator  movement at this stage may merely keep the aircraft stalled.  It is a characteristic of T-tail aircraft to pitch up viciously  when stalled in extreme nose-high attitudes, making  recovery difficult or violent. The stick pusher inhibits this  type of stall. At approximately one knot above stall speed,  pre-programmed stick forces automatically move the stick  forward, preventing the stall from developing. A G-limiter  may also be incorporated into the system to prevent the pitch  down generated by the stick pusher from imposing excessive  loads on the aircraft. A “stick shaker,” on the other hand,  provides stall warning when the airspeed is five to seven  percent above stall speed.  Mach buffet is a function of the speed of the airflow over the  wing—not necessarily the speed of the aircraft. Any time that  too great a lift demand is made on the wing, whether from too  fast an airspeed or from too high an AOA near the M$_{MO}$, the  “high-speed” buffet occurs. There are also occasions when  the buffet can be experienced at much lower speeds known  as the “low-speed Mach buffet.”  An aircraft flown at a speed too slow for its weight and  altitude necessitating a high AOA is the most likely situation  When used for roll control, the spoiler on the side of the  up-going aileron extends and reduces the lift on that side,  causing the wing to drop. If the spoilers are extended as speed  brakes, they can still be used for roll control. If they are the  Some jet transports have two sets of ailerons, a pair of outboard  low-speed ailerons and a pair of high-speed inboard ailerons.  When the flaps are fully retracted after takeoff, the outboard  ailerons are automatically locked out in the faired position.  Jet transport aircraft have small ailerons. The space for  ailerons is limited because as much of the wing trailing  edge as possible is needed for flaps. Also, a conventional  size aileron would cause wing twist at high speed. For that  reason, spoilers are used in unison with ailerons to provide  additional roll control.  Spoilers are used on the upper surface of the wing to spoil or  reduce lift. High speed aircraft, due to their clean low drag  design, use spoilers as speed brakes to slow them down.  Spoilers are extended immediately after touchdown to dump  lift and thus transfer the weight of the aircraft from the wings  onto the wheels for better braking performance.  [Figure 5-71]  On high-speed aircraft, flight controls are divided into  primary flight controls and secondary or auxiliary flight  controls. The primary flight controls maneuver the aircraft  about the pitch, roll, and yaw axes. They include the ailerons,  elevator, and rudder. Secondary or auxiliary flight controls  include tabs, leading edge flaps, trailing edge flaps, spoilers,  and slats.  High Speed Flight Controls  Mach Buffet Boundaries  • High altitudes—the higher an aircraft flies, the thinner  the air and the greater the AOA required to produce  the lift needed to maintain level flight.  • Heavy weights—the heavier the aircraft, the greater  the lift required of the wing, and all other factors being  equal, the greater the AOA.  • G loading—an increase in the G loading on the aircraft  has the same effect as increasing the weight of the  aircraft. Whether the increase in G forces is caused  by turns, rough control usage, or turbulence, the effect  of increasing the wing’s AOA is the same.  5-49 