Definition
The minimum airspeed at which the airplane's flight controls remain effective enough to maintain directional control of the aircraft. Below this airspeed, the rudder, ailerons, and elevator no longer produce sufficient aerodynamic force to counter yawing, rolling, or pitching tendencies, and the pilot loses the ability to control the airplane's path through the air.
Plain English
The slowest speed at which the controls still 'bite' the air enough to steer the airplane. Go slower than that and the controls stop working well enough to keep the airplane pointed where you want it.
Context Anchor
Seen in emergency procedures and maneuver discussions, especially when the pilot must lower the nose or adjust pitch to keep the airplane flying under control.
Derivation
Controllable' comes from 'control' — to direct or steer. The phrase literally means 'a speed at which steering still works.' The point isn't whether the airplane is flying; it's whether the pilot can still command where it goes.
Why Pilots Care
Flying slower than this speed with an engine failed causes immediate loss of directional control and possible roll into the dead engine.
Grounding Statement
If the nose is held too high and the airplane slows too much, the controls can feel weak because there is not enough air moving over the airplane to make them work normally.
Intuition Check
Controllable does not mean the airplane is simply pointed where you want it. It means the airplane has enough airspeed to respond normally when you move the controls.
Example Sentence 1
After the engine failed on takeoff, the pilot lowered the nose immediately to maintain a controllable airspeed before turning toward the field.
Example Sentence 2
Knowing the airplane's controllable airspeed helps the pilot avoid losing directional control after an engine failure on takeoff.