An autopilot system consists of three main elements: a computerized guidance program that plots the aircraft’s course and compares its real position to its virtual one, a series of motion and position sensors such as airspeed indicators and gyroscopes to deliver real-time feedback, and a selection of servomotors to actuate the craft’s engines and alter its flight-altering components when changes are necessary.
By syncing these three elements autopilots can not only stabilize an aircraft’s pitch, yaw and roll movements – greatly relieving pilots on long-haul flights – but also handle, in poor visibility conditions, automatic runway approaches and landings.
In modern, commercial jets, autopilots are the central hub of the craft’s flight management system, a grouping of sensors such as GPS and INS (inertial guidance system) that help calculate the current positioning and course of the aircraft without need for external reference. The feedback from these systems are equalized and, in the case of accuracy discrepancies, resolved through a multidimensional Kalman filter, a mathematical model for mitigating random variations in feedback values.
The most common filter is a six-dimensional one, receiving and processing information for the aircraft’s pitch, yaw and roll, but also its altitude, latitude and longitude. The latter values are specifically important for modern day passenger aircraft, as they must run to specific schedules and on courses with certain rated performance factors, with set minimum velocity, altitudes and time factors to be met.
Cutting-edge advanced autopilot systems may also include an autoland sub-system, however these are usually only available at major international airports, which have various ground-based systems to communicate with the autopilot flight management system on board to help alignment and ratify its current and potential approach and landing course.
The International Civil Aviation Organization categorizes autoland systems dependent on the visibility level at the runway and degree of automation capable by the autopilot. For example, a simple CAT I system requires pilots to have a decision height of 60 metres and a forward visibility range of 550m to engage an automatic landing, while a complex CAT IIIb system allows pilots to engage the system with a decision height of less than 15 metres and a forward visibility of just 75 metres.