Definition
A reversal of the airplane's direction of flight, in which the heading is changed by half a circle so the aircraft ends up flying the opposite way from which it started. In the context of preventing irreversible deceleration or sink rate, it is used as an early decision point to abandon an approach, exit deteriorating conditions, or return toward better terrain or weather before the situation becomes unrecoverable.
Plain English
A turn that takes you back the way you came. You end up pointing in the opposite direction from where you were going.
Context Anchor
You will see this term in emergency-after-takeoff discussions, especially when considering whether turning back toward the runway is safer than landing ahead.
Derivation
A degree is one of the equal parts used to measure a circle. A full circle is 360 degrees, so 180 degrees is half of that circle. That is why a 180 degree turn points the airplane in the opposite direction.
Why Pilots Care
Allows an attempt to return to the departure airport rather than landing straight ahead, but requires precise control of bank angle, airspeed, and altitude to avoid stalling or sinking.
Grounding Statement
If the airplane is climbing away from the runway and then makes a 180 degree turn, it is now pointed back toward where it departed.
Intuition Check
Do not assume a 180 degree turn after takeoff simply puts the airplane safely back on the runway. It only points the airplane back toward the departure direction; extra turning, distance, and altitude are usually still needed.
Example Sentence 1
When the ceiling dropped below the ridge line, the pilot made a 180 degree turn and flew back to clearer weather.
Example Sentence 2
A 180 degree turn performed with too much bank can quickly produce an unrecoverable sink rate.