What causes an airplane to stall?

An airplane stalls primarily due to exceeding its critical angle of attack. The angle of attack is the angle between the chord line of the wing and the oncoming air (relative wind). When this angle becomes too steep, typically beyond 14 to 20 degrees for most aircraft, the airflow can no longer smoothly attach to the wing's upper surface, resulting in a significant loss of lift.

In this scenario, an excessive angle of attack leads to the airflow separating from the wing, causing rapid degradation of lift and potential loss of control. This critical concept is fundamental in understanding aircraft performance and flight safety, as pilots must be aware of the conditions that can lead to a stall and learn to avoid them through proper control inputs and awareness of the aircraft's flight envelope.

Other factors, such as excessive bank angles or insufficient thrust, can contribute to a stall or worsen its effects, particularly when combined with high angles of attack, but they do not directly cause a stall on their own. Similarly, sudden changes in altitude do not inherently lead to a stall unless they affect the angle of attack or load factor excessively.