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A yaw damper (sometimes referred to as a stability augmentation system) 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.
Despite what may be implied by its name, the yaw damper does not inhibit or reduce intentional (e.g. commanded by the pilot) yaw, as this would interfere with conventional turns and other common maneuvers that an aircraft would be expected to perform. Rather, the system is intended to counteract incidental and undirected yawing motions, which can be characterised as skids or slips. On a single-engine aircraft, the system is particularly useful at addressing the tendency to 'fishtail', smoothing out the left–right movements of the vertical stabilizer (fin), increasing ride comfort. It is also particularly useful on swept wing aircraft, particularly those using a T-tail arrangement; without a yaw damper system, these types of aircraft are susceptible to the Dutch roll, where yawing motions can result in repetitive corkscrew-like oscillations that could potentially escalate to excessive levels if not counteracted.
The yaw damper is typically disengaged at ground level and turned on shortly after takeoff; an active yaw damper during the takeoff run could potentially mask serious issues such as engine failure. Equally, the system is commonly disengaged prior to landing, as it could inhibit the control authority to the pilot at the critical moment of touchdown. On several modern aircraft that are outfitted with a yaw damper, these systems become engaged automatically once the aircraft has surpassed a set altitude (e.g. 200 feet); older aircraft typically have this function manually selected by the flight crew. Pilots who are used to flying aircraft with yaw dampers need to be particularly aware when flying aircraft that lack them. It has become common for such systems to be interfaced with other elements of an aircraft's avionics, enabling it to work with other functions such as the autopilot. Yaw damping can be readily simulated by software packages, such as Matlab.
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