A yaw damper (sometimes referred to as a stability augmentation system[1]) is a system used to reduce (or damp) the undesirable tendencies of an aircraft to oscillate in a repetitive rolling and yawing motion, a phenomenon known as the Dutch roll. A large number of modern aircraft, both jet-powered and propeller-driven, have been furnished with such systems.
The use of a yaw damper provides superior ride quality by automatically preventing uncomfortable yawing and rolling oscillations and reduces pilot workload. On some aircraft, it is mandatory for the yaw damper to be operational at all times during flight above a specified altitude; several airliners were deemed to be unsafe to fly without an active yaw damper.[1]