What happens during a stall in an aircraft?

During a stall in an aircraft, airflow separates from the wing's upper surface. This phenomenon occurs when the angle of attack—the angle between the chord line of the wing and the oncoming air—exceeds a critical limit. When this critical angle is surpassed, the smooth flow of air over the wing is disrupted, leading to a significant loss of lift.

This separation of airflow is critical to understand, as it results in a decrease in lift and can cause the aircraft to begin descending rather than maintaining or gaining altitude. Recognizing the conditions that lead to a stall is essential for safe flying and helps pilots manage their approach and maneuvers effectively.

The other options reflect misunderstandings about what happens during a stall. For instance, lift does not increase during a stall; it decreases significantly due to the airflow separation. The aircraft also does not gain altitude uncontrollably, as a stall typically results in a loss of altitude. Similarly, engine thrust is not directly affected in this scenario; while the engines may be operating normally, it is the aerodynamic properties of the wing that have changed. Thus, the correct understanding of stalls centers on airflow separation from the wing’s surface, directly affecting lift and the aircraft's ability to maintain flight.