What is an Aircraft Stall?

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Image: USAF

An aircraft stall is an aerodynamic condition in which an airplane 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. In an airplane, a stall has nothing to do with the engine, and 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 the angle between the chord line (an 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 planform 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, and the aircraft's nose will fall. The critical angle of attack for an airfoil never changes, but factors such as weight, configuration (flaps and gear changes, or other conditions like airframe icing), and load factor can change the airspeed at which an airplane will stall. 

Stability

In a stable airplane, often times the drop in the nose is enough to regain the proper amount of lift for the airfoil, and the airplane is easily recoverable just by lowering the pitch attitude of the airplane and increasing airspeed.

In an unstable airplane, a stall allowed to fully develop without taking action to recover can further develop into a spin, which can sometimes be difficult or impossible to recover from.

Stalls commonly occur at slow airspeeds. For this reason, slow speed flight, during approach and departures for example, are critical phases of flight and pilots must be cognizant to avoid stalling the aircraft during these times.

A stall at cruise altitude offers the pilot enough space to recover; a stall during landing doesn't offer the same envelope of security for which to recover.  

Stall Characteristics

Characteristics of a stall include a distinguished loss of lift, which is usually noted by a sudden pitch down of the nose of the aircraft, and may be accompanied by a roll or yaw to one side or another, if the aircraft is uncoordinated. If recovery procedures are not initiated right away, an aircraft may enter a spin.  

Tailplane Stalls

This type of stall of often referenced as happening to the aircraft wings, but the horizontal stabilizer can also stall. We call this a tailplane stall, and it's also dangerous, although less common, aerodynamic condition. 

Stall 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 commonly accomplished by lowering the pitch attitude, leveling the wings and increasing power or thrust.