What Do Autopilots Do?
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.
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question?
Explainer thanks the Air Transport
Association and the Aircraft Owners and Pilots Association .
