Aircraft Stall and How to Prevent It
An airplane stall is an aerodynamic condition in which an aircraft exceeds its given critical angle of attack and is no longer able to produce the required lift for normal flight. This type of stall should not be confused with an engine stall, familiar to anyone who has driven an automobile. When flying an airplane, a stall has nothing to do with the engine or another mechanical part. In piloting, a stall is only defined as the aerodynamic loss of lift that occurs when an airfoil (i.e., the wing of the airplane) exceeds its critical angle of attack.
Angle of Attack
The angle of attack on an airfoil is measured by the angle between the chord line (i.e., the imaginary line from the leading edge to the trailing edge of the wing) and the relative wind. It is dependent on the shape of the airfoil, including its platform and aspect ratio. At a high angle of attack, the airflow over the wing is disrupted.
At the critical angle of attack, the airflow over the wing is disrupted enough to inhibit lift, resulting in the nose of the aircraft to fall. The critical angle of attack for an airfoil never changes. However, factors such as weight, configuration (e.g., flaps and gear changes, or other conditions, like airframe icing), and load factor, can change the airspeed at which an airplane will stall.
Characteristics of a stall include a distinctive decrease in lift, which is usually noted by a sudden (if sometimes gradual) pitch down of the nose of the aircraft. While this can feel like the plane is falling and has no lift, in reality, it's only a decrease in lift and a change in the plane's level. Additionally, a stall may be accompanied by a roll or yaw to one side if the aircraft is uncoordinated. If this happens and recovery procedures are not initiated right away, an aircraft may enter a spin.
In a stable airplane, the drop in the nose at the beginning of a stall often is enough to regain the proper amount of lift for the airfoil. If this happens, the airplane is easily recoverable just by lowering its pitch attitude and increasing airspeed. However, in an unstable airplane, a stall that is not corrected can further develop into a spin, which can be difficult or impossible to recover from.
Stalls commonly occur at slow airspeeds. For this reason, slow-speed flight, such as during approach and departure, are critical phases of flight, and pilots must be particularly cognizant at these times to prevent stalling the aircraft. A stall at cruise altitude offers the pilot enough space to recover. A stall during landing with limited space doesn't offer the same envelope of security from which to recover.
While stalling may be most common at slow speeds, a stall can happen at any airspeed, regardless of the attitude. Therefore, a pilot should not rule out the possibility of a stall based on airspeed or attitude. For example, when pulling out of a dive, the airspeed is high, but the angle of attack can be higher than you think because the plane is still dropping in altitude even though its nose is raised. If the angle of attack exceeds about 17 percent, the plane can stall.
Tailplane stalls often indicate that something is happening to the aircraft wings, but the plane's horizontal stabilizer can also stall. While this tailplane stall is also dangerous, it is a much less common aerodynamic condition.
Practicing Stalls and Recovery
Stall recovery procedures are different for each aircraft, but in general, a pilot can initiate a stall recovery by increasing airflow over the wing. This is usually accomplished by lowering the pitch attitude, leveling the wings, and increasing power or thrust. When a wing has stalled, it is usually best to use the rudder to raise the wing, rather than the ailerons.
Pilots practice stalls and recovery as part of their training, and they must perform a stall and recovery to earn a private or commercial certificate. However, routine flight reviews often do not involve stalls, and as a result, pilots may forget how to recognize the indications that an airplane is going into a stall. Practicing stalls and recovery at slow speeds—and at sufficient altitude to recover, of course—helps pilots recognize the early signs of a stall condition so that they can make the proper corrections.