Definition
The horizontal distance from the center of a turn to the aircraft's flight path during that turn. Turn radius increases with the square of true airspeed and decreases as the bank angle steepens. For a given bank angle, a faster aircraft will sweep a wider arc through the sky.
Plain English
How wide a circle the airplane carves when it turns. Faster speed means a bigger circle. Steeper bank means a tighter circle.
Context Anchor
Seen when planning turns in the traffic pattern, maneuvering near terrain, or understanding how speed and bank affect the space needed to turn.
Derivation
Radius comes from a Latin word meaning a spoke of a wheel or a line from the center outward. That helps here because turn radius is measured from the center of the turn outward to the aircraft, like the distance from the hub of a wheel to its rim.
Why Pilots Care
It tells the pilot how much airspace is needed to complete a turn without drifting outside protected areas or into obstacles.
Analogy
Think of riding a bicycle around a circle. If you go faster, the circle usually gets wider; if you lean more, you can make a tighter circle.
Grounding Statement
Picture the aircraft tracing a circle over the ground; turn radius is the distance from the middle of that circle to the aircraft.
Intuition Check
Turn radius is not the same as how fast the airplane is turning. It is the size of the path the airplane makes through the turn.
Example Sentence 1
Because of the high approach speed, the jet's turn radius forced the pilot to begin the base-to-final turn well before reaching the runway centerline.
Example Sentence 2
A faster approach speed increases turn radius, so the pilot began the turn earlier to align with the runway centerline.