example, the horizontal tail surfaces may be attached near  the lower part of the vertical stabilizer, at the midpoint, or  at the high point, as in the T-tail design.  similar recovery problems are also found with conventional  tail aircraft with an aft CG.  [Figure 6-11]  Deep stalls can  occur on any aircraft but are more likely to occur on aircraft  with “T” tails as a high AOA may be more likely to place  the wings separated airflow into the path of the horizontal  surface of the tail. Additionally, the distance between the  wings and the tail, the position of the engines (such as being  mounted on the tail) may increase the susceptibility of deep  stall events. Therefore a deep stall may be more prevalent  on transport versus general aviation aircraft.  In a T-tail configuration, the elevator is above most of the  effects of downwash from the propeller, as well as airflow  around the fuselage and/or wings during normal flight  conditions. Operation of the elevators in this undisturbed air  allows control movements that are consistent throughout most  flight regimes. T-tail designs have become popular on many  light and large aircraft, especially those with aft fuselage- mounted engines because the T-tail configuration removes  the tail from the exhaust blast of the engines. Seaplanes and  amphibians often have T-tails in order to keep the horizontal  surfaces as far from the water as possible. An additional  benefit is reduced noise and vibration inside the aircraft.  In comparison with conventional-tail aircraft, the elevator on a  T-tail aircraft must be moved a greater distance to raise the nose  a given amount when traveling at slow speeds. This is because  the conventional-tail aircraft has the downwash from the  propeller pushing down on the tail to assist in raising the nose.  Aircraft controls are rigged so that an increase in control force  is required to increase control travel. The forces required to  raise the nose of a T-tail aircraft are greater than the forces  required to raise the nose of a conventional-tail aircraft.  Longitudinal stability of a trimmed aircraft is the same for  both types of configuration, but the pilot must be aware that  the required control forces are greater at slow speeds during  takeoffs, landings, or stalls than for similar size aircraft  equipped with conventional tails.  T-tail aircraft also require additional design considerations  to counter the problem of flutter. Since the weight of the  horizontal surfaces is at the top of the vertical stabilizer, the  moment arm created causes high loads on the vertical stabilizer  that can result in flutter. Engineers must compensate for this by  increasing the design stiffness of the vertical stabilizer, usually  resulting in a weight penalty over conventional tail designs.  When flying at a very high AOA with a low airspeed and  an aft CG, the T-tail aircraft may be more susceptible to a  deep stall. In this condition, the wake of the wing impinges  on the tail surface and renders it almost ineffective. The  wing, if fully stalled, allows its airflow to separate right after  the leading edge. The wide wake of decelerated, turbulent  air blankets the horizontal tail and hence its effectiveness  diminished significantly. In these circumstances, elevator or  stabilator control is reduced (or perhaps eliminated) making  it difficult to recover from the stall. It should be noted that an  aft CG is often a contributing factor in these incidents, since  The elevator must also have sufficient authority to hold the  nose of the aircraft up during the roundout for a landing. In  this case, a forward CG may cause a problem. During the  landing flare, power is usually reduced, which decreases the  The elevator down spring produces a mechanical load on the  elevator, causing it to move toward the nose-down position if not  otherwise balanced. The elevator trim tab balances the elevator  down spring to position the elevator in a trimmed position.  When the trim tab becomes ineffective, the down spring drives  the elevator to a nose-down position. The nose of the aircraft  lowers, speed builds up, and a stall is prevented.  [Figure 6-12]  Since flight at a high AOA with a low airspeed and an aft  CG position can be dangerous, many aircraft have systems to  compensate for this situation. The systems range from control  stops to elevator down springs. On transport category jets, stick  pushers are commonly used. An elevator down spring assists in  lowering the nose of the aircraft to prevent a stall caused by the  aft CG position. The stall occurs because the properly trimmed  airplane is flying with the elevator in a trailing edge down  position, forcing the tail up and the nose down. In this unstable  condition, if the aircraft encounters turbulence and slows down  further, the trim tab no longer positions the elevator in the nose- down position. The elevator then streamlines, and the nose of  the aircraft pitches upward, possibly resulting in a stall.  T-Tail  Figure 6-11.  Aircraft with a T-tail design at a high AOA and an aft CG.  6-6  CG 