A stall occurs when the smooth airflow over the airplane’s wing is disrupted, and the lift degenerates rapidly. This is caused when the wing exceeds its critical angle of attack (AOA). This can occur at any airspeed, in any attitude, with any power setting. If recovery from a stall is not achieved in a timely and appropriate manner by reducing the AOA, a secondary stall and/or a spin may result. All spins are preceded by a stall on at least part of the wing. The angle of the relative wind is determined primarily by the aircraft's airspeed and attitude. Factors such as aircraft weight, center of gravity (CG), configuration, and the amount of acceleration used in a turn are also considered. The speed at which the critical angle of the relative wind is exceeded is the stall speed. Stall speeds are listed in the Airplane Flight Manual (AFM) or the pilot’s operating handbook (POH) and pertain to certain conditions or aircraft configurations, e.g., landing configuration. Other specific operational speeds are calculated based upon the aircraft's stall speed in the landing configuration. Airspeed values specified in the AFM or POH may vary under different circumstances. Factors such as weight, CG, altitude, temperature, turbulence, and the presence of snow, ice, or frost on the wings will affect an aircraft’s stall speed. To thoroughly understand the stall/spin phenomenon, some basic factors affecting aircraft aerodynamics and flight should be reviewed with particular emphasis on their relation to stall speeds. Much of the information in this AC is also applicable to gliders. The following terms are defined as they relate to stalls/spins.

AOA is the angle at which the chord line of the wing meets the relative wind. The chord line is a straight line drawn through the profile of the wing connecting the extremities of the leading edge and trailing edge. The AOA must be small enough to allow attached airflow over and under the airfoil to produce lift. AOA is an element of lift. Change in AOA will affect the amount of lift that is produced. An excessive AOA will disrupt the flow of air over the airfoil. If the AOA is not reduced, a section of the airfoil will reach its critical AOA, lose lift, and stall. Exceeding the critical AOA for a particular airfoil section will always result in a stall of that section.

Airspeed is controlled primarily by the elevator or longitudinal control position for a given configuration and power. Conversely, airspeed is controlled by power at a given configuration and AOA. If an airplane’s speed is too slow, the AOA required for level flight will be so large that the air can no longer follow the upper curvature of the wing. The result is a separation of airflow from the wing, loss of lift, a large increase in drag, and eventually a stall if the AOA is not reduced. The stall is the result of excessive AOA—not insufficient airspeed. For example, at a 60° banked turn in level coordinated flight, the load factor is 2 G’s and the stall speed increases 40 percent over the straight and level stall speed. A stall can occur at any airspeed, in any attitude, at any power setting.

Flaps, landing gear, and other configuring devices can affect an airplane’s stall speed. Extension of flaps and/or landing gear in flight will increase drag. Flap extension will generally increase the lifting ability of the wings, thus reducing the airplane’s stall speed. The effect of flaps on an airplane’s stall speed can be seen by markings on the airplane’s airspeed indicator, where the lower airspeed limit of the white arc (power-off stall speed with gear and flaps in the landing configuration) is less than the lower airspeed limit of the green arc (power-off stall speed in the clean configuration).

VSO is the stall speed or the minimum steady flight speed in the landing configuration.

VS1 is the stall speed or the minimum steady flight speed obtained in a specified configuration.

VA is the design maneuvering speed. Do not use full or abrupt control movements at or above this speed. It is possible to exceed the airplane structural limits at or above VA. Rapid and large alternating control inputs, especially in combination with large changes in pitch, roll, or yaw (e.g., large side slip angles) may result in structural failures at any speed, even below VA.

Load factor is the ratio of the lifting force produced by the wings to the actual weight of the airplane and its contents. Load factors are usually expressed in terms of “G.” The aircraft’s stall speed increases in proportion to the square root of the load factor. For example, an airplane that has a normal unaccelerated stall speed of 45 knots can be stalled at 90 knots when subjected to a load factor of 4 G’s. The possibility of inadvertently stalling the airplane by increasing the load factor (i.e., by putting the airplane in a steep turn or spiral) is much greater than in normal cruise flight. When an airplane stalls at a higher indicated air speed due to excessive maneuvering loads, it is called an accelerated maneuver stall. A stall entered from straight and level flight or from an unaccelerated straight climb will not produce additional load factors. In a constant rate turn, increased load factors will cause an airplane's stall speed to increase as the angle of bank increases. Excessively steep banks should be avoided because the airplane will stall at a much higher speed. If the aircraft exceeds maneuvering speed, structural damage to the aircraft may result before it stalls. If the nose falls during a steep turn, the pilot might attempt to raise it to the level flight attitude without shallowing the bank. This situation tightens the turn and can lead to a diving spiral. A feeling of weightlessness will result if a stall recovery is performed by abruptly pushing the elevator control forward, which will reduce the up load on the wings. Recoveries from stalls and spins involve a tradeoff between loss of altitude (and an increase in airspeed) and an increase in load factor in the pullup. However, recovery from the dive following spin recovery generally causes higher airspeeds and consequently higher load factors than stall recoveries due to the much lower position of the nose. Significant load factor increases are sometimes induced during pullup after recovery from a stall or spin. It should be noted that structural damage can result from the high load factors that could be imposed on the aircraft by intentional stalls practiced above the airplane’s design maneuvering speed. Large, aggressive control reversals can also lead to loads that can exceed the structural design limits, even at speeds below the airplane’s design maneuvering speed.

The CG location has a direct effect on the effective lift and AOA of the wing, the amount and direction of force on the tail, and the degree of stabilizer deflection needed to supply the proper tail force for equilibrium. The CG position, therefore, has a significant effect on stability and stall/spin recovery. As the CG is moved aft, the amount of elevator deflection needed to stall the airplane at a given load factor will be reduced. An increased AOA will be achieved with less elevator control force. This could make the entry into inadvertent stalls easier, and during the subsequent recovery, it would be easier to generate higher load factors due to the reduced elevator control forces. In an airplane with an extremely aft CG, very light back elevator control forces may lead to inadvertent stall entries and if a spin is entered, the balance of forces on the airplane may result in a flat spin. Recovery from a flat spin is often impossible. A forward CG location will often cause the stalling AOA to be reached at a higher airspeed. Increased back elevator control force is generally required with a forward CG location.