Which of the following is NOT a primary flight control?

Flaps are not classified as primary flight controls; instead, they are secondary or auxiliary flight controls. Primary flight controls include ailerons, rudder, and elevators, which are essential for controlling the aircraft's pitch, roll, and yaw, directly affecting its flight path and stability.

Ailerons control roll by adjusting the lift on the wings; when one aileron goes up, the other goes down, causing the aircraft to tilt. The rudder controls yaw, allowing the aircraft to turn left or right. Elevators manage pitch, determining whether the nose of the aircraft rises or drops.

Flaps, on the other hand, are used mainly during takeoff and landing to increase lift and drag, enhancing low-speed performance, but they do not play a fundamental role in the primary control of the aircraft during standard flight maneuvers. Thus, recognizing flaps as secondary to the primary controls is crucial for understanding aircraft operations and aerodynamics.