Definition
A half-circle turn flown so that the airplane's flight path traces an arc of unchanging radius over the ground, regardless of wind. To achieve this, the pilot must continuously vary the bank angle to compensate for the changing relationship between the airplane's heading and the wind direction during the turn.
Plain English
A 180-degree turn flown so the aircraft's path over the ground forms a perfect half-circle of the same width all the way around. Because wind keeps trying to push the airplane off that perfect arc, the pilot has to keep adjusting how steeply the wings are banked.
Context Anchor
Used in ground reference maneuvers, especially S-turns across a road or other straight reference line.
Derivation
“180°” means 180 degrees, or half of a full 360-degree circle. “Radius” comes from a Latin word meaning a spoke or ray from the center of a circle, which helps here because the turn is judged by the airplane staying the same distance from the center of the curve.
Why Pilots Care
Teaches precise wind-drift correction so the aircraft maintains a symmetrical ground track instead of being blown off course.
Analogy
It is like drawing half of a circle with a compass. The pencil moves around, but it stays the same distance from the center the whole time.
Intuition Check
Do not assume “constant-radius” means holding one exact bank angle the whole time. It means the curve over the ground stays the same size, so the bank may need to change as the wind changes the airplane’s ground path.
Example Sentence 1
During S-turns, the student flew a 180° constant-radius turn on each side of the road, steepening the bank when the tailwind pushed the airplane outward.
Example Sentence 2
On the downwind leg of the S-turn the pilot starts with a steeper bank for the 180° constant-radius turn and reduces it steadily to hold the same radius.