News reports about the crashes of EgyptAir Flight 990 and the private plane carrying golfer Payne Stewart have focused on the use of the airplanes' autopilots. What exactly do autopilots do?
The autopilot is an electronic system that manipulates the three "control surfaces" that determine an airplane's course: the movable panels, called ailerons, on the back of each wing that allow the plane to bank right or left; the tail rudder, which turns the aircraft's nose; and the elevators, which point the plane up or down. The autothrottle, which determines the airplane's speed, is controlled by a separate system.
On commercial aircraft, the autopilot and autothrottle are controlled by an advanced onboard navigational computer called the flight management system (FMS). Typically, a pilot programs the FMS before takeoff, entering landmarks, altitude, and desired speed; during flight, the FMS uses instrument readings and radio signals from fixed points on the ground to figure out what adjustments are needed to meet the flight plan. If a human is in command, he or she must make the changes by hand; with the autopilot engaged, these adjustments are made automatically.
Autopilots have several advantages. Primarily, they help keep the crew from getting tired, leaving them free to alter the flight plan, scout for traffic, and monitor the plane's other systems (like hydraulics and air pressurization). Autopilots also improve fuel efficiency and passenger comfort, since the adjustments made by an autopilot are more subtle and accurate than those made by hand.
That's why autopilots are typically engaged on commercial aircraft throughout nearly the entire flight. When human pilots take control--usually during takeoff and landing, and occasionally in mid flight--it's largely because they need to stay in practice, not because the autopilot would be unable to fly safely. (In fact, in bad weather, the FAA may require that pilots allow the autopilot and FMS, which don't rely on visual cues, to land the plane.) Pilots also take command in turbulence since an autopilot would waste fuel and possibly exacerbate the bumps by making many adjustments to keep the plane on a steady course. If a pilot doesn't disengage the autopilot before taking control, the FMS interprets his movements as outside forces and instructs the autopilot to work against them.