Definition
A small metal tab attached to the trailing edge of a flight control surface (typically an aileron or rudder) that can only be bent or repositioned while the aircraft is on the ground. It is used to correct minor flight imbalances, such as a wing that is heavy or a tendency to yaw, by being bent slightly in the direction opposite to the unwanted movement.
Plain English
A small bendable metal tab on the back edge of a control surface that a mechanic adjusts on the ground to fix a small flying imbalance. The pilot cannot move it from the cockpit -- it stays in whatever position the mechanic set it to.
Context Anchor
Seen during preflight inspection, aircraft maintenance, and discussions of how an airplane is trimmed when no cockpit trim control is provided for that surface.
Derivation
"Trim" comes from the Old English trymman, meaning to set in order or arrange properly. In aviation, trimming means adjusting a control surface so the aircraft flies straight and level without the pilot holding pressure on the controls. "Ground adjustable" tells you when the adjustment happens -- on the ground, not in flight.
Why Pilots Care
Reduces pilot workload by minimizing the steady control force needed to maintain straight-and-level flight.
Intuition Check
Do not read “ground adjustable” as “adjusted while taxiing” or “adjusted by the pilot whenever desired.” It means the tab is set on the ground before flight and then stays in that position during flight.
Example Sentence 1
After noticing the airplane consistently flew left-wing-low in cruise, the pilot asked the mechanic to bend the ground adjustable trim tab on the right aileron.
Example Sentence 2
During the preflight walk-around the pilot checked that the ground adjustable trim tab had not been damaged or moved since the last maintenance.