Definition
The act of adjusting an aircraft's trim controls so that the flight controls require little or no sustained pressure from the pilot to maintain a desired flight attitude, airspeed, or configuration. Trimming relieves the pilot of holding control pressures by repositioning a small auxiliary surface (or equivalent system) so that the primary control surface stays in the position the pilot wants without continuous input.
Plain English
Trimming is the process of fine-tuning the airplane so it flies hands-off in the attitude and speed you want, instead of you having to push or pull on the controls the whole time.
Context Anchor
Seen in instrument flying when holding altitude, changing airspeed, leveling off, climbing, descending, or any time the pilot needs the aircraft to stay steady without constant control pressure.
Derivation
From the nautical term 'to trim,' meaning to balance a ship by adjusting its load or sails. Aviation borrowed the word because the idea is the same — balancing the aircraft so it sits properly in the air without constant correction.
Why Pilots Care
Proper trimming reduces fatigue and control errors, allowing precise instrument flight with less physical and mental strain.
Grounding Statement
If you must keep steady pressure on a control to hold the aircraft where you want it, the aircraft is not yet trimmed for that condition.
Intuition Check
Trimming does not mean decorating the aircraft or cutting something off. In flying, it means balancing the control forces so the aircraft holds the selected condition with little or no steady hand pressure.
Example Sentence 1
After establishing the climb at 90 knots, the pilot finished trimming so the airplane held the pitch attitude on its own.
Example Sentence 2
During the instrument approach the pilot continued trimming to maintain the correct descent rate hands-off.