Definition
The capability of an airplane to respond to the pilot's control inputs, especially with regard to flightpath and attitude. A controllable airplane changes its attitude and direction predictably and promptly when the pilot moves the flight controls.
Plain English
How well the airplane does what the pilot tells it to do through the controls. When you move the stick, rudder, or yoke, the airplane responds the way you expect.
Context Anchor
Seen during the takeoff roll, when the airplane is accelerating along the runway and the pilot must keep it straight and stable before liftoff.
Derivation
From 'control' (to direct or command) plus '-ability' (the capacity to do something). Literally: the airplane's capacity to be controlled.
Why Pilots Care
Loss of controllability during the takeoff roll can cause the aircraft to leave the runway surface before flying speed is reached.
Intuition Check
Controllability does not mean the airplane is automatically easy to fly. It means the pilot still has enough response from the airplane to direct it where it needs to go.
Example Sentence 1
During the early part of the takeoff roll, the airplane has limited controllability until airspeed builds and the flight controls become effective.
Example Sentence 2
As speed increased, controllability improved and smaller rudder corrections were needed to stay aligned with the runway.