Definition
A secondary flight control surface used to refine the aircraft's handling, performance, or trim, but not used as the primary means of maneuvering the aircraft about its three axes. Auxiliary flight controls include flaps, slats, spoilers, speed brakes, and trim tabs.
Plain English
A flight control that helps the airplane fly better in specific situations, but isn't one of the main controls used to steer it. The main controls are the ailerons, elevator, and rudder. Everything else that moves on the wing or tail to assist with lift, drag, or balance is auxiliary.
Context Anchor
Seen in aircraft systems, preflight checks, and discussions of how flaps, trim, slats, and spoilers affect aircraft handling.
Derivation
Auxiliary comes from the Latin auxilium, meaning 'help' or 'aid.' An auxiliary flight control is exactly that — a control that aids the primary controls rather than replacing them.
Why Pilots Care
They let the pilot adjust performance for takeoff, approach, landing, and cruise without overloading the primary controls.
Intuition Check
Do not read auxiliary as meaning emergency or backup. Here it means helping or supporting the main flight controls.
Example Sentence 1
The flaps and trim tabs on this trainer are auxiliary flight controls, used to assist with lift during landing and to relieve control pressures in cruise.
Example Sentence 2
Spoilers, an auxiliary flight control, were deployed after touchdown to reduce lift and keep the wheels firmly on the runway.