For example, a twin engine plane would not be allowed to fly if one engine was inoperative, in case the only working engine failed mid-flight.
Why then are planes such as the 737, 747, 757, and 767 allowed to fly if one yaw damper is inoperative?
Yaw Dampers are designed to counteract Dutch Roll. Dutch Roll is a continuous yawing and rolling movement that can be experienced in swept wing aircraft in varying degrees of severity.
Dutch Roll can be Stable, Neutral or Unstable. Only aircraft with unstable Dutch roll characteristics have the need for robust redundancy in their yaw damper systems as total failure could lead to loss of control in certain speed ranges.
Aircraft that have Stable or Neutral Dutch Roll characteristics can dispatch with only one yaw damper because if it failed the subsequent Dutch roll would not be a flight safety hazard.