The autopilot
An autopilot is a system used to control the path of an aircraft.
It is often an integral component of a Flight Management System. A primary function is in-flight management of the flight plan. Using various sensors (such as GPS and INS often backed up by radio navigation) to determine the aircraft’s position, the FMS can guide the aircraft along the flight plan.
According to UAV Navigation, modern autopilots use computer software to control the aircraft. The software reads the aircraft’s current position, and then controls a flight control system to guide the aircraft. Autopilot software, which is integrated with the navigation systems, is capable of providing control of the aircraft throughout each phase of flight. In such a system, besides classic flight controls, many autopilots incorporate thrust control capabilities that can control throttles to optimize the airspeed.
The pilot can intervene by making appropriate inputs to the autopilot or the FMS, at any stage of the flight.
But what happens if the autopilot fails?
DC-8 flight engineer hand-flies the aircraft after the autopilot fails
Mark Hilsen, former DC-8 pilot, recalls on Quora;
‘I was a flight engineer on a DC-8, newly minted and living the good life. The jet was seriously long in the tooth, but, hey! Four engined goodness and on my way to a Maui, Hawaii 26 hour layover. It never got better than that.
‘Los Angeles runway 25R takeoff, pointed west and only one slight turn to the left then straight ahead for just under 5 hours. Copilot hand flying, time to put the autopilot on at about 18,000’ because he’d had his fun.
‘Copilot: “Shit, autopilot’s not working.” Tried several more times, resetting circuit breakers, until copilot shrugs and looks at Captain. “Time to engage the alternate autoflight system, sir?”
‘Captain, turning to look at me: “Feel like hand flying for four hours, Mark? Better than sitting at that panel and continuously eating until we get there, right?”
‘Me: “YEAH! GET OUT OF MY SEAT, BILL (copilot)!”’
Hilsen concludes;
‘I, officially a second officer and flight engineer, was that day a REAL PILOT for FOUR GLORIOUS HOURS at FL310 the whole way. Awesome.
‘Slept like a dog when we got there.’
Photo by Douglas via San Diego Air and Space Museum