Stall Recognition and Recovery
All pilots must be proficient in stall recognition and recovery. A stall can occur at any airspeed and at any attitude. In the case of the self-launching glider under power, a stall can also occur with any power setting. A stall occurs when the smooth airflow over the glider’s wing is disrupted and the wings stop producing enough lift. This occurs when the wing exceeds its critical AOA.
The practice of stall recovery and the development of stall awareness 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.
Intentional stalls should be performed so the maneuver is completed by 1,500 feet above the ground with a landing area within gliding distance, in the event lift cannot be found. Although it depends on the degree to which a stall has progressed, most stalls require some loss of altitude during recovery. The longer it takes to recognize the approaching stall, the more complete the stall is likely to become, and the greater the loss of altitude to be expected.
Pilots must recognize the flight conditions that are conducive to stalls and know how to apply the necessary corrective action since most gliders do not have an electrical or mechanical stall warning device. Pilots should learn to recognize an approaching stall by sight, sound, and feel. The following cues may be useful in recognizing the approaching stall.
- Vision—useful in detecting a stall condition by noting the attitude of the glider versus the horizon.
- Hearing—also helpful in sensing a stall condition. In the case of a glider, a change in sound due to loss of airspeed is particularly noticeable. The lessening of the noise made by the air flowing along the glider structure as airspeed decreases is quite noticeable, and when the stall is almost complete, the pilot starts to feel airframe buffeting or aerodynamic vibration as the stall occurs.
- Kinesthesia, or the sensing of changes in direction or speed of motion, is an important intuitive 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 glider.
- The feel of control pressures is also very important. As speed is reduced, the resistance to pressure on the controls becomes progressively less. Pressures exerted on the controls tend to become movements of the control surfaces. As the airflow slows and stalls, the aerodynamic controls (ailerons, elevator, and rudder) have significantly less authority and require much more movement to create the same amount of directional change as compared to the normal flight regime responses. As the wing airflow stalls and the stalling strongly affects the controls, the controllability of the glider can become questionable. Properly designed and certificated gliders should retain marginal control authority when the wing is stalled. The lag between these movements and the response of the glider becomes greater until in a complete stall.
Signs of an impending stall include the following:
- Nose-high attitude for higher wing loading with possible increasing trend.
- Low airspeed indication with a decreasing trend.
- Low airflow noise and decreasing.
- Back pressure increasing, requiring more elevator trimming and/or not having anymore aft trim.
- Poor control responses from the glider and decreasing feedback pressures from control movements.
- Wing (airframe) buffeting as stalling begins.
- Yaw string (if equipped) movement from normal flight position.
Always make clearing turns before performing stalls. During the practice of intentional stalls, the real objective is not to learn how to stall a glider, but to learn how to recognize an approaching stall and take prompt corrective action. The recovery actions must be taken in a coordinated manner.
First, at the indication of a stall, the pitch attitude and AOA must be decreased positively and immediately. Since the basic cause of a stall is always an excessive AOA, the cause must first be eliminated by releasing the back-elevator pressure that was necessary to attain that AOA or by moving the elevator control forward. This lowers the nose and returns the wing to an effective AOA. The amount of elevator control pressure or movement to use depends on the design of the glider, the severity of the stall, and the proximity of the ground. In some gliders, a moderate movement of the elevator control—perhaps slightly forward of neutral—is enough, while others may require a forcible push to the full forward position. An excessive negative load on the wings caused by excessive forward movement of the elevator may impede, rather than hasten, the stall recovery. The object is to reduce the AOA, but only enough to allow the wing to regain lift. [Figure 7-33]
If stalls are practiced or encountered in a self-launching glider, the maximum allowable power should be applied during the stall recovery to increase the self-launching glider’s speed and assist in reducing the wing’s AOA. Generally, the throttle should be promptly, but smoothly, advanced to the maximum allowable power. Although stall recoveries should be practiced with and without power, in self-launching gliders during actual stalls, the application of power is an integral part of the stall recovery. Usually, the greater the applied power is, the less the loss of altitude is. Maximum allowable power applied at the instant of a stall usually does not cause overspeeding of an engine equipped with a fixed-pitch propeller, due to the heavy air load imposed on the propeller at low airspeeds. However, it is necessary to reduce the power as airspeed is gained after the stall recovery so the airspeed does not become excessive.
When performing intentional stalls, pilots should never allow the engine to exceed its maximum designed rpm limitation. The maximum rpm is marked by a red line on the engine tachometer gauge. Exceeding rpm limitations can cause damage to engine components.
Whether in a towed glider or self-launching glider, stall recovery is accomplished by leveling the wings and returning to straight flight using coordinated flight controls. The first few practice sessions should consist of approaches to stalls with recovery initiated at the first airframe buffet or when partial loss of control is noted. Using this method, pilots become familiar with the initial indications of an approaching stall without fully stalling the glider.
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 an altitude that allows recovery at no lower than 1,500 feet AGL and within gliding distance of a landing area.
Different types of gliders have different stall characteristics. Most gliders are designed so the wings stall progressively outward from the wing roots (where the wing attaches to the fuselage) to the wingtips. This is the result of designing the wings so the wingtips have a smaller angle of incidence than the wing roots. When exceeding the critical angle of attack results in a stall, the inner wing does not support normal aerodynamic flight, but the outer part of the wing does retain some aerodynamic effectiveness. Wings are designed in this manner so aileron control is available at high AOA (low airspeed) and to give the glider more stable stalling characteristics. When the glider is in a stalled condition, the wingtips continue to provide some degree of lift, and the ailerons still have some control effect. During recovery from a stall, the return of lift begins at the tips and progresses toward the roots. Thus, the ailerons can be used to level the wings.
Using the ailerons requires finesse to avoid an aggravated stall condition. For example, if the right wing drops during the stall and excessive aileron control is applied to the left to raise the wing, the aileron that deflects downward (right wing) would produce a greater AOA (and drag). Possibly a more complete stall would occur at the tip, because the critical AOA would be exceeded. The increase in drag created by the high AOA on that wing might cause the airplane to yaw in that direction. This adverse yaw could result in a spin unless directional control were maintained by rudder and/or aileron control is sufficiently reduced.
Even though excessive aileron pressure may have been applied, a spin does not occur if directional (yaw) control is maintained by timely application of coordinated rudder pressure. Therefore, it is important that the rudder be used properly during both entry and recovery from a stall. The primary use of the rudder in stall recovery is to counteract any tendency of the glider to yaw. The correct recovery technique would be to decrease the pitch attitude by applying forward elevator pressure to reduce the AOA while simultaneously maintaining directional control with coordinated use of the aileron and rudder.
Due to engineering design variations, the stall characteristics for all gliders cannot be specifically described; however, the similarities found in gliders are noteworthy enough to be considered. The factors that affect the stalling characteristics of the glider are weight and balance, bank and pitch attitude, coordination, and drag. The pilot should learn the stall characteristics of the glider being flown and the proper correction procedures. It should be reemphasized that a stall can occur at any airspeed, in any attitude, or at any power setting in the case of a self-launching or sustaining glider, depending on the total value of factors affecting the particular glider.
Whenever practicing stalls while turning, a constant bank angle should be maintained until the stall occurs. After the stall occurs, coordinated control inputs should be made to return the glider to wings-level flight.
Advanced stalls include secondary, accelerated, and crossed-control stalls. These stalls are extremely useful for pilots to expand their knowledge of stall/spin awareness.