A three-stage autopilot system has its functionality divided into many stages, the stages are taxi stage, take off stage, climb stage, cruise stage, descent stage, and landing stage. A pilot has the assistance of a sophisticated computer and sensors driven system in every stage of his work. It must be noted that autopilots are only designed to assist pilots in performing their duties and in no sense can replace completely any part of work that goes into flying an aircraft.
Sophisticated computer software and systems are installed onboard an aircraft which can be aware of its current position, altitude, bearing and velocity and even more information that might be required for a safe flight.
Modern autopilots can also control the thrusts of the aircraft to optimize the speed of the aircraft as required for a safe journey. The autopilot can also be guided by ground-based instrumental landing systems for an instrumental landing in case of harsh weather conditions. Modern autopilots and completely guide an aircraft from take off to landing at the destination a lot of stress away from pilots shoulders.
Modern autopilot systems are very complex pieces of machinery. Their working is a bit complex. The onboard computer of an aircraft is laden with sensors which continuously feed the autopilot system with information on engine rpm, altitude, bearing, heading and even climatic conditions along with distance to be traveled and available fuel. Modern autopilots are then using a complex algorithm compute the best way for the flight.
In case of poor weather conditions with extremely low or no visibility a specialized system in the autopilot called instrumental landing system or auto-land is used. Auto-land system relies heavily on radar altimeter as there could be zero error in calculating the distance to land from the aircraft.
A radio-altimeter coupled with airstrip ILS beacon has to be in perfect synergy in order to achieve an error-free ILS guided landing. The land beacon of ILS bounces off signals to the aircraft system verifying that the approach taken by the aircraft’s onboard system is correct. Once this is achieved the onboard system calculates rest of the perimeters and lands the aircraft.
However, this system does not eliminate the pilots role completely as its response rate to other conditions like wind speed and wind shear is limited and would require pilots attention for a safe landing.