A stall occurs when the smooth airflow over the aircraft’s wing root is disrupted and the lift degenerates rapidly. This is caused when the wing root exceeds its critical angle of attack. This can occur at any airspeed in any attitude with any power setting.
The practice of stall recovery and the development of awareness of stalls are of primary importance in pilot training. The objectives in performing intentional stalls are to familiarize the pilot with the conditions that produce stalls, to assist in recognizing an approaching stall, and to develop the habit of taking prompt preventive or corrective action.
Pilots must recognize the flight conditions that are conducive to stalls and know how to apply the necessary corrective action. They should learn to recognize an approaching stall by sight, sound, and feel. The following cues may be useful in recognizing the approaching stall:
- Positioning the control bar toward the front tube
- Detecting a stall condition by visually noting the attitude of the aircraft for the power setting
- Hearing the wind decrease on the structure and pilot
- Feeling the wind decrease against the pilot
- Sensing changes in direction or speed of motion, or kinesthesia—probably the most important and best indicator to the trained and experienced pilot. If this sensitivity is properly developed, it warns of a decrease in speed or the beginning of a settling or mushing of the aircraft.
During the practice of intentional stalls, the real objective is not to learn how to stall an aircraft, but to learn how to recognize an approaching stall and take prompt corrective action. Though the recovery actions must be taken in a coordinated manner, they are broken down into the following three actions for explanation purposes.
First, at the indication of a stall, the pitch attitude and angle of attack must be decreased positively and immediately. Since the basic cause of a stall is always an excessive angle of attack, the cause must first be eliminated by releasing the control bar forward pressure that was necessary to attain that angle of attack or by moving the control bar backwards. This lowers the nose and returns the wing to an effective angle of attack.
The amount of movement used depends on the design of the wing, the severity of the stall, and the proximity of the ground. In some WSC aircraft, the bar can be left out and as the nose stalls, the wing lowers to an angle of attack and keeps flying since the tips do not stall. However, even though WSC aircraft generally have gentle stall characteristics, higher performance wings may not be as forgiving. Therefore during a stall, the control bar should be moved back to reduce the angle of attack and properly recover from the stall. The object for all WSC aircraft is to reduce the angle of attack but only enough to allow the wing to regain lift as quickly as possible and obtain the appropriate airspeed for the situation with the minimum loss in altitude.
Power application in a stall is different than an airplane. Since power application in a WSC aircraft produces a nose-up moment after a stall has occurred and the pitch has decreased from the control bar movement, power should be applied. The flight instructor should emphasize, however, that power is not essential for a safe stall recovery if sufficient altitude is available. Reducing the angle of attack is the only way of recovering from a stall regardless of the amount of power used. Stall recoveries should be practiced with and without the use of power. Usually, the greater the power applied during the stall recovery, the less the loss of altitude.
Third, straight-and-level flight should be regained with coordinated use of all controls. Practice of power-on stalls should be avoided due to potential danger of whipstalls, tucks, and tumbles, as detailed later in this chapter.
Power-off (at idle) turning stalls are practiced to show what could happen if the controls are improperly used during a turn from the base leg to the final approach. The power-off straight-ahead stall simulates the attitude and flight characteristics of a particular aircraft during the final approach and landing.
Usually, the first few practices should include only approaches to stalls with recovery initiated as soon as the first buffeting or partial loss of control is noted. Once the pilot becomes comfortable with this power-off procedure, the aircraft should use some power and be slowed in such a manner that it stalls in as near a level pitch attitude as is possible. The student pilot must not be allowed to form the impression that in all circumstances a high pitch attitude is necessary to exceed the critical angle of attack, or that in all circumstances a level or near level pitch attitude is indicative of a low angle of attack. Recovery should be practiced first without the addition of power by merely relieving enough control bar forward pressure that the stall is broken and the aircraft assumes a normal glide attitude. Stall recoveries should then be practiced with the addition of power during the recovery to determine how effective power is in executing a safe recovery and minimizing altitude loss.
Stall accidents usually result from an inadvertent stall at a low altitude in which a recovery was not accomplished prior to contact with the surface. As a preventive measure, stalls should be practiced at a minimum altitude of 1,500 feet AGL or that which allows recovery no lower than 1,000 feet AGL. Recovery with a minimum loss of altitude requires a reduction in the angle of attack (lowering the aircraft’s pitch attitude), application of power, and termination of the descent without accelerating to a high airspeed and unnecessary altitude loss.
The factors that affect the stalling characteristics of the aircraft are wing design, trim, bank, pitch attitude, coordination, drag, and power. The pilot should learn the effect of the stall characteristics of the aircraft being flown. It should be reemphasized that a stall can occur at any airspeed, in any attitude, or at any power setting, depending on the total number of factors affecting the particular aircraft.
Whenever practicing turning stalls, a constant pitch and bank attitude should be maintained until the stall occurs. In a banked stall or if the wing rolls as it stalls, side to side control bar movement is required to level the wings as well as pull the bar back to reduce the angle of attack.
Power-Off Stall Manuever
The practice of power-off stalls is usually performed with normal landing approach conditions in simulation of an accidental stall occurring during landing approaches. Aircraft equipped with trim should be trimmed to the approach configuration. Initially, airspeed in excess of the normal approach speed should not be carried into a stall entry since it could result in an abnormally nose-high attitude. Before executing these practice stalls, the pilot must be sure the area is clear of other air traffic.
To start the power-off stall maneuver, reduce the throttle to idle (or normal approach power). Increase airspeed to the normal approach speed and maintain that airspeed. When the approach attitude and airspeed have stabilized, the aircraft’s nose should be smoothly raised to an attitude that induces a stall. If the aircraft’s attitude is raised too slowly, the WSC aircraft may slow only to minimum controlled airspeed and not be able to reach an angle of attack that is high enough to stall. The position of the control bar at which the WSC stalls can vary greatly for different manufacturers and makes/ models. Some can stall abruptly when the control bar is inches from the front tube.
If the aircraft’s attitude is raised too quickly, the pitch attitude could rise above the manufacturer’s limitation. A good rule of thumb is 3 to 4 seconds from stabilized approach speed to pull the control bar full forward. The wings should be kept level and a constant pitch attitude maintained until the stall occurs. The stall is recognized by clues, such as buffeting, increasing descent rate, and nose down pitching.
Recovering from the stall should be accomplished by reducing the angle of attack by pulling the bar back and accelerating only to the trim speed while simultaneously increasing the throttle to minimize altitude loss if needed. Once the WSC accelerated to trim speed, the control bar can be pushed out to return back to normal trim attitude and speed. If there is any rolling during the stall or the stall recovery the control bar should be moved side to side to maintain a straight heading.
It is not necessary to go into a steep dive in a WSC aircraft to recover from a stall. This only loses more altitude than required and should be discouraged. The nose should be lowered as necessary to regain flying speed and returned to a normal flight attitude as soon as possible. [Figure 6-22]
Recovery from power-off stalls should also be practiced from shallow banked turns to simulate an inadvertent stall during a turn from base leg to final approach. During the practice of these stalls, care should be taken that the turn continues at a uniform rate until the complete stall occurs. When stalling in a turn, it does not affect the recovery procedure. The angle of attack is reduced and the wings leveled simultaneously with power applied if needed for altitude control. In the practice of turning stalls, no attempt should be made to stall the aircraft on a predetermined heading. However, to simulate a turn from base to final approach, the stall normally should be made to occur within a heading change of approximately 90°. After the stall occurs, the recovery should be made straight ahead with minimum loss of altitude, and accomplished in accordance with the recovery procedure discussed earlier.