Editing Autopilot (section)

== Stability augmentation systems ==
A stability augmentation system (SAS) is another type of automatic flight control system; however, instead of maintaining the aircraft required altitude or flight path, the SAS will move the aircraft control surfaces to damp unacceptable motions. SAS automatically stabilizes the aircraft in one or more axes. The most common type of SAS is the [[yaw damper]] which is used to reduce the [[Dutch roll]] tendency of swept-wing aircraft. Some yaw dampers are part of the autopilot system while others are stand-alone systems.<ref>Automatic Flight Control Fourth Edition, Pallett and Coyle, {{ISBN|978 1 4051 3541 2}}, p.79</ref>

[[Yaw damper]]s use a sensor to detect how fast the aircraft is rotating (either a gyroscope or a pair of accelerometers),<ref>Avionics Fundamentals, Aviation Technician Training Series, {{ISBN|0 89100 293 6}}, p.287</ref> a computer/amplifier and an actuator. The sensor detects when the aircraft begins the yawing part of Dutch roll. A computer processes the signal from the sensor to determine the rudder deflection required to damp the motion. The computer tells the actuator to move the rudder in the opposite direction to the motion since the rudder has to oppose the motion to reduce it. The Dutch roll is damped and the aircraft becomes stable about the yaw axis. Because Dutch roll is an instability that is inherent in all swept-wing aircraft, most swept-wing aircraft need some sort of yaw damper.

There are two types of yaw damper: the series yaw damper and the parallel yaw damper.<ref>Automatic Flight Control Fourth Edition, Pallett and Coyle, {{ISBN|978 1 4051 3541 2}}, p.204</ref> The actuator of a parallel yaw damper will move the rudder independently of the pilot's rudder pedals while the actuator of a series yaw damper is clutched to the rudder control quadrant, and will result in pedal movement when the rudder moves.

Some aircraft have stability augmentation systems that will stabilize the aircraft in more than a single axis. The [[Boeing B-52]], for example, requires both pitch and yaw SAS<ref>{{cite web |last1=Johnston |first1=D. E. |title=Flight control systems properties and problems. Volume 2: Block diagram compendium |url=https://ntrs.nasa.gov/citations/19750009300 |publisher=NASA |date=1 February 1975}}</ref> in order to provide a stable bombing platform. Many helicopters have pitch, roll and yaw SAS systems. Pitch and roll SAS systems operate much the same way as the yaw damper described above; however, instead of damping Dutch roll, they will damp pitch and roll oscillations to improve the overall stability of the aircraft.