Definition
The horizontal distance from the center of a turn to the airplane's flight path during that turn. For a given true airspeed, radius of turn decreases as the angle of bank increases; for a given bank angle, radius of turn increases as true airspeed increases.
Plain English
How wide the circle is when an airplane turns. A steeper bank or a slower speed makes the circle smaller. A shallower bank or a faster speed makes it bigger.
Context Anchor
Seen when learning how bank angle and airspeed affect the space an airplane needs to complete a turn.
Derivation
Radius comes from the Latin radius, meaning the spoke of a wheel or the line from a center to the edge of a circle. A turning airplane traces part of a circle, so radius of turn is simply the distance from the center of that circle out to the airplane.
Why Pilots Care
Knowing the radius helps pilots judge whether a turn will fit within available airspace, avoid obstacles, or maintain proper spacing in the traffic pattern.
Analogy
Think of swinging a weight on a string. A short string makes a tight circle; a long string makes a wide one. Bank angle and speed change the effective length of the string.
Grounding Statement
At any given airspeed, a steeper bank reduces the radius; at any given bank, higher speed increases it.
Intuition Check
Radius of turn is not how fast the airplane is turning. It is the size of the curved path the airplane makes through the air.
Example Sentence 1
At 100 knots and 45 degrees of bank, the radius of turn is much smaller than at 140 knots and the same bank angle.
Example Sentence 2
At higher airspeed the radius of turn grew larger, so the pilot began the turn earlier to align with the final approach course.