Stalls

Angle of Attack

An aircraft stall results from a rapid decrease in lift caused by the separation of airflow from the wing’s surface brought on by exceeding the critical angle of attack. A stall can occur at any pitch attitude or airspeed.

The angle of attack (AOA) is the angle at which the chord of the wing meets the relative wind. The chord is a straight line from the leading edge to the trailing edge. At low angles of attack, the airflow over the top of the wing flows smoothly and produces lift with a relatively small amount of drag. As the AOA increases, lift as well as drag increases; however, above a wing’s critical AOA, the flow of air separates from the upper surface, creating turbulent air which reduces lift and increases drag. This condition is a stall, which can lead to loss of control if the AOA is not reduced. It is important for the pilot to understand that a stall is the result of exceeding the critical AOA, not of insufficient airspeed. The term “stalling speed” can be misleading, as this speed is often discussed when assuming 1G flight at a particular weight and configuration. Increased load factor directly affects stall speed (as well as do other factors such as gross weight, centre of gravity, and flap setting). Therefore, it is possible to stall the wing at any airspeed, at any flight attitude, and at any power setting. For example, if a pilot maintains airspeed and rolls into a coordinated, level 60° banked turn, the load factor is 2Gs, and the airplane will stall at a speed that is 40 percent higher than the straight-and-level stall speed. In that 2G level turn, the pilot has to increase AOA to increase the lift required to maintain altitude. At this condition, the pilot is closer to the critical AOA than during level flight and therefore closer to the higher speed that the airplane will stall at. Because “stalling speed” is not a constant number, pilots must understand the underlying factors that affect it in order to maintain aircraft control in all circumstances.

**Angle of Attack**
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As discussed earlier, the lift coefficient is generally expressed in relation to the angle of attack. As the angle of attack increases, so too does the lift coefficient until reaching a particular angle of attack in which the trend reverses and lift begins to decrease. This point is known as the critical angle of attack. At angles of attack beyond the critical angle of attack, the airfoil is stalled and lift can only be recovered with a decrease in angle of attack.

Stall

A stall is an aerodynamic condition which occurs when smooth airflow over the airplane’s wings is disrupted, resulting in loss of lift. Specifically, a stall occurs when the AOA—the angle between the chord line of the wing and the relative wind—exceeds the wing’s critical AOA. It is possible to exceed the critical AOA at any airspeed, at any attitude, and at any power setting. For these reasons, it is important to understand factors and situations that can lead to a stall, and develop proficiency in stall recognition and recovery. Performing intentional stalls will familiarize the pilot with the conditions that result in a stall, assist in recognition of an impending stall, and develop the proper corrective response if a stall occurs.

Stalls are practiced to two different levels:

Impending Stall: An impending stall occurs when the AOA causes a stall warning, but has not yet reached the critical AOA. Indications of an impending stall can include buffeting, stick shaker, or aural warning.
Full Stall: A full stall occurs when the critical AOA is exceeded.

A usual indication of a full stall is an uncommanded nose-down pitch which may be accompanied by an uncommanded rolling motion. For airplanes equipped with stick pushers, its activation is also a full stall indication. Although it depends on the degree to which a stall has progressed, some loss of altitude is expected during recovery. The longer it takes for the pilot to recognize an impending stall, the more likely it is that a full stall will result. Intentional stalls should therefore be performed at an altitude that provides adequate height above the ground for recovery and return to normal level flight.

The stalling speed of a particular aircraft is not a fixed value for all flight situations, but a given aircraft always stalls at the same angle of attack regardless of airspeed, weight, load factor, or density altitude.

Stall Recognition

A pilot must recognize the flight conditions that are conducive to stalls and know how to apply the necessary corrective action. This level of proficiency requires learning to recognize an impending stall by sight, sound, and feel. Stalls are usually accompanied by a continuous stall warning for airplanes equipped with stall warning devices. These devices may include an aural alert, lights, or a stick shaker all which alert the pilot when approaching the critical AOA. Certification standards permit manufacturers to provide the required stall warning either through the inherent aerodynamic qualities of the airplane or through a stall warning device that gives a clear indication of the impending stall. However, most vintage airplanes, and many types of light sport and experimental airplanes, do not have stall warning devices installed.

Pilots in training must remember that a level-flight 1G stalling speed is valid only:

In unaccelerated 1G flight
In coordinated flight (slip-skid indicator centred)
At one weight (typically maximum gross weight)
At a particular center of gravity (CG) (typically maximum forward CG)

Stall Characteristics

Different airplane designs can result in different stall characteristics. The pilot should know the stall characteristics of the airplane being flown and the manufacturer’s recommended recovery procedures. Factors that can affect the stall characteristics of an airplane include its geometry, CG, wing design, and the use of high-lift devices. Engineering design variations make it impossible to specifically describe the stall characteristics for all airplanes; however, there are enough similarities in small general aviation training-type airplanes to offer broad guidelines.

Most training airplanes are designed so that the wings stall progressively outward from the wing roots (where the wing attaches to the fuselage) to the wingtips. Some wings are manufactured with a certain amount of twist, known as washout, resulting in the outboard portion of the wings having a slightly lower AOA than the wing roots. This design feature causes the wingtips to have a smaller AOA during flight than the wing roots. Thus, the wing roots of an airplane exceed the critical AOA before the wingtips, meaning the wing roots stall first. Therefore, when the airplane is in a stalled condition, the ailerons should still have a degree of control effectiveness until/unless stalled airflow migrates outward along the wings. Although airflow may still be attached at the wingtips, a pilot should exercise caution using the ailerons prior to the reduction of the AOA because it can exacerbate the stalled condition. For example, if the airplane rolls left at the stall and the pilot applies right aileron to try to level the wing, the downward-deflected aileron on the left wing produces a greater AOA (and more induced drag), and a more complete stall at the tip as the critical AOA is exceeded. This can cause the wing to roll even more to the left, which is why it is important to first reduce the AOA before attempting to roll the airplane.