What is a stall?

In fluid dynamics, a stall is a reduction in the lift coefficient generated by a foil as the angle of attack increases. This occurs when the critical angle of attack of the foil is exceeded. The critical angle of attack is typically about 15°, but it may vary significantly depending on the fluid, foil, and Reynolds number.

Put simply, a stall is a reduction of lift experienced by an aircraft. It occurs when the angle of attack of the wing is increased too much. This is known as the critical angle of attack and is typically around 15 degrees (but there are variations). In normal flight, the airflow over the shaped wings creates lift. The airfoil shape changes the airflow direction, and the downward deflection of the air causes an upward force (lift) to be exerted on the airfoil.

Increasing the angle of attack causes flow separation, where the air no longer flows cleanly over the upper surface of the wing. If this angle reaches the critical angle, the airflow is disrupted to the point where the lift generated begins to decrease.

It is the angle of attack of the wing exceeding its critical angle that causes a stall, but airspeed is also important. A 'stall speed' will be defined for an aircraft, rather than an angle of attack. How does this work?

If an aircraft flies slower, it required a greater angle of attack to generate sufficient lift. If the speed decreases to a certain level, this angle will reach the critical angle. At this speed, an aircraft cannot climb without causing a stall. This speed is affected by several factors, including weight, altitude, and configuration, and different stall speeds are set based on this (such as a minimum speed in landing configuration with fully extended flaps).

Unintentional stalls are deadly, resulting in fatalities almost 50% more often than non-stall accidents. Pilots are taught to recognize, avoid, and recover from stalls early in flight training, yet they still account for almost 25% of fatal accidents. The overwhelming majority of unintended stalls occur on personal flights in day visual meteorological conditions (VMC) under light winds. Perhaps surprisingly, more stalls occur during the departure phases of flight (takeoff, climb, and go-around) than in the arrival phases (approach, pattern, and landing). The stubbornly high percentage of stalls associated with personal flying (more than two-thirds) may indicate a weakness in typical pilot training. Most pilots are taught to recognize and recover from stalls in a controlled, predictable, and stable environment, with a focus on recognition of aircraft response followed by proper recovery technique. Outside the training environment, though, pilots continue to maneuver into the stall envelope unexpectedly with little time to recover. Seemingly, some pilots fly closer to the critical angle of attack than they realize. Adding a little more bank, G-force, or both can trigger an accelerated stall without the slow, predictable performance indicators pilots are taught to recognize.

What happens in a stall, and why is it dangerous?

An uncorrected stall will cause the aircraft to fall. The first sign for a pilot is sluggish flight controls, which become much less responsive due to the changes in airflow, and possible buffeting. Pilots will train to recognize this, but this is more relevant in smaller aircraft that are flown manually.

An early stall is easily corrected by pushing the aircraft's nose down to reduce the angle of attack. This is, of course, much more serious at low altitude when taking off or landing. If not corrected, the wing loses lift, and the aircraft will start to fall.

A spin is another situation that can occur. This occurs when the aircraft has sufficient yaw at the point of the stall. In this situation, one wing stalls before the other, and the difference in lift causes the aircraft to roll. This is much harder for a pilot to recover from. Training is sometimes given on smaller aircraft as part of pilot training, but in general, the focus is on preventing a spin from ever happening. Commercial airliners are not designed or tested in this area.