The expression "secondary flight control" applies to which of the following components?

Secondary flight controls are those components of an aircraft that assist in enhancing the aircraft's performance and handling characteristics but are not primarily responsible for controlling the flight itself. They typically include systems that support the primary control surfaces, ensuring better stability, comfort, or aerodynamic efficiency.

In this context, the trimmable stabilizer and speed brake fall under secondary flight controls. The trimmable stabilizer helps in maintaining a desired pitch attitude without continuous pilot input, while the speed brake assists in reducing airspeed or increasing descent rate during descent or approach for landing without changing the angle of the wings. Their functions do not directly control the aircraft in flight, but instead, they support the overall flight dynamics and efficiency.

The other components mentioned in the choices, like ailerons and rudder, are classified as primary flight controls as they are directly involved in maneuvering the aircraft. Ailerons control roll, and the rudder controls yaw, making them essential for the fundamental movement of the aircraft. Therefore, the correct association of secondary flight controls with the trimmable stabilizer and speed brake accurately reflects their roles in enhancing the flying experience without being the main control surfaces.