Since stalls are the cause of much concern among student pilots and the non-flying public, we will discuss them here. We mentioned that an airplane must attain flying speed in order to take off. Sufficient airspeed must be maintained in flight to produce enough lift to support the airplane without requiring too large an angle of attack. At a specific angle of attack, called the critical angle of attack, air going over a wing will separate from the wing or "burble" (see figure 1 ), causing the wing to lose its lift (stall). The airspeed at which the wing will not support the airplane without exceeding this critical angle of attack is called the stalling speed. This speed will vary with changes in wing configuration (flap position). Excessive load factors caused by sudden manoeuvres, steep banks, and wind gusts can also cause the aircraft to exceed the critical angle of attack and thus stall at any airspeed and any attitude. Speeds permitting smooth flow of air over the airfoil and control surfaces must be maintained to control the airplane.

Flying an airplane, like other skills that are learned, requires practice to remain proficient. Professional pilots for the major airlines, military pilots, and flight instructors all return to the classroom periodically for updating their skills. Good judgment must be exercised by all pilots to ensure the safe and skilful operation of the airplanes they fly.

fig 1 airfoil approaching and entering a stall

Types Of Stalls

Stalls can be practised both with and without power. Stalls should be practised to familiarize the student with the aircraft’s particular stall characteristics without putting the aircraft into a potentially dangerous condition. A description of some different types of stalls follows:

• Departure Stalls (can be classified as power-on stalls) are practised to simulate takeoff and climb-out conditions and configuration. Many stall/spin accidents have occurred during these phases of flight, particularly during overshoots. A causal factor in such accidents has been the pilot’s failure to maintain positive pitch control due to a nose-high trim setting or premature flap retraction. Failure to maintain positive control during short field takeoffs has also contributed towards accidents.

• Arrival Stalls (can be classified as power-off stalls or reduced power stalls) are practised to simulate normal approach-to-landing conditions and configuration. Simulations should also be practised at reduced power settings consistent with the approach requirements of the particular training aircraft. Many stall/spin accidents have occurred in situations, such as crossed control turns from base leg to final approach (resulting in a skidding or slipping turn); attempting to recover from a high sink rate on final approach by using only an increased pitch attitude; and improper airspeed control on final approach or in other segments of the traffic pattern.

• Accelerated Stalls can occur at higher-than-normal airspeeds due to abrupt and/or excessive control applications. These stalls may occur in steep turns, pull-ups, or other abrupt changes in flight path. For these reasons, accelerated stalls usually are more severe than un-accelerated stalls and are often unexpected.

Stall Recovery

The key factor in recovery from a stall is regaining positive control of the aircraft by reducing the angle of attack. At the first indication of a stall, the wing angle of attack must be decreased to allow the wings to regain lift. Every aircraft in upright flight may require a different amount of forward pressure to regain lift. It should be noted that too much forward pressure could hinder recovery by imposing a negative load on the wing. The next step in recovering from a stall is to smoothly apply maximum allowable power to increase the airspeed and minimize the loss of altitude. As airspeed increases and the recovery is completed, power should be adjusted to return the aeroplane to the desired flight condition. Straight and level flight should then be established with full co-ordinated use of the controls. The airspeed indicator or tachometer, if installed, should never be allowed to reach their high-speed red lines at anytime during a practice stall.