Which control surface is not considered a primary control surface?

The correct choice refers to flaps, which are not considered primary control surfaces but rather secondary control surfaces. Primary control surfaces are those directly responsible for controlling the aircraft's movement about its three axes: roll, pitch, and yaw.

Ailerons are located on the wings and control roll by manipulating the lift on opposite wings. The elevator, found on the tail, affects pitch by controlling the angle of the aircraft's nose in relation to the horizon. The rudder, also on the tail, manages yaw by controlling the aircraft’s direction left and right.

Flaps, on the other hand, are primarily used during takeoff and landing to increase lift and drag, allowing the aircraft to operate safely at lower speeds. They don’t directly control the aircraft's attitude or movement in the same way the primary controls do, which is why they are classified as secondary surfaces. This distinction is essential for understanding how pilots manage an aircraft's aerodynamics throughout various phases of flight.