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Autopilot

An autopilot is a system used to control the trajectory of an aircraft, marine craft or spacecraft without constant manual control by a human operator being required. Autopilots do not replace human operators, but instead they assist them in controlling the vehicle. This allows them to focus on broader aspects of operations such as monitoring the trajectory, weather and systems.
In the early days of aviation, aircraft required the continuous attention of a pilot to fly safely. As aircraft range increased, allowing flights of many hours, the constant attention led to serious fatigue. An autopilot is designed to perform some of the tasks of the pilot.
Lawrence Sperry (the son of famous inventor Elmer Sperry) demonstrated it in 1914 at an aviation safety contest held in Paris. Sperry demonstrated the credibility of the invention by flying the aircraft with his hands away from the controls and visible to onlookers. Elmer Sperry Jr., the son of Lawrence Sperry, and Capt Shiras continued work on the same autopilot after the war, and in 1930 they tested a more compact and reliable autopilot which kept a US Army Air Corps aircraft on a true heading and altitude for three hours.
Autopilots in modern complex aircraft are three-axis and generally divide a flight into taxi, takeoff, climb, cruise (level flight), descent, approach, and landing phases. Autopilots exist that automate all of these flight phases except taxi and takeoff. An autopilot-controlled landing on a runway and controlling the aircraft on rollout (i.e. keeping it on the centre of the runway) is known as a CAT IIIb landing or Autoland, available on many major airports' runways today, especially at airports subject to adverse weather phenomena such as fog. Landing, rollout, and taxi control to the aircraft parking position is known as CAT IIIc. This is not used to date, but may be used in the future. An autopilot is often an integral component of a Flight Management System.
Some autopilots also use design diversity. In this safety feature, critical software processes will not only run on separate computers and possibly even using different architectures, but each computer will run software created by different engineering teams, often being programmed in different programming languages. It is generally considered unlikely that different engineering teams will make the same mistakes. As the software becomes more expensive and complex, design diversity is becoming less common because fewer engineering companies can afford it. The flight control computers on the Space Shuttle used this design: there were five computers, four of which redundantly ran identical software, and a fifth backup running software that was developed independently. The software on the fifth system provided only the basic functions needed to fly the Shuttle, further reducing any possible commonality with the software running on the four primary systems.
Yaw dampers usually consist of a yaw rate sensor (either a gyroscope or angular accelerometer), a computer/amplifier and a servo actuator. The yaw damper uses yaw rate sensor to sense when the aircraft begins a Dutch roll. A computer processes the signals from the yaw rate sensor to determine the amount of rudder movement that is required to dampen out the Dutch roll. The computer then commands the servo actuator to move the rudder that amount. The Dutch roll is dampened out and the aircraft becomes stable about the yaw axis. Because Dutch roll is an instability that is inherent to all swept-wing aircraft, most swept-wing aircraft have some sort of yaw damper system installed.
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 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 dampening out Dutch roll, they will dampen pitch and roll oscillations or buffeting to improve the overall stability of the aircraft.