Definition
A short length of yarn or string taped to the outside of the windshield or canopy that streams in the relative wind, providing a direct visual indication of whether the airplane is flying coordinated or sideslipping. When the string points straight back along the longitudinal axis, the airplane is in coordinated flight; when it points off to one side, the airplane is slipping or skidding in the opposite direction.
Plain English
A piece of yarn stuck to the outside of the windshield. The way it blows in the airflow tells the pilot whether the airplane is flying straight through the air or is angled slightly sideways.
Context Anchor
Seen on some training airplanes, gliders, and multiengine airplanes as a simple visual aid for keeping the airplane aligned with the airflow, especially during one-engine-inoperative flight.
Derivation
Yaw refers to rotation of the airplane around its vertical axis (nose left or right). The string yaws with the airflow, hence the name.
Why Pilots Care
Keeping the yaw string centered produces zero sideslip, which reduces drag and improves climb performance after an engine failure.
Analogy
Like the telltale streamers sailors put on the rigging of a sailboat to show wind direction. A glance tells you exactly how the air is flowing past you.
Intuition Check
A yaw string is not a cockpit control and it does not show bank angle. It is a simple airflow indicator that shows sideways alignment with the air moving over the airplane.
Example Sentence 1
After losing the right engine, the pilot adjusted bank and rudder until the yaw string pointed straight back, restoring best climb performance.
Example Sentence 2
With the yaw string centered, the airplane achieved its best rate of climb on the remaining engine.