How Does Autopilot Work?

Otto the Autopilot

At any given moment there are more than 5,000 airplanes ushering passengers across the states to one of 19,000 airports in the country. Travelers can climb into a flying metal tube and make their way from Los Angeles to New York in five hours, watching Everybody Loves Raymond and sipping from miniaturized cups all the while. Tens of thousands of hours are spent in the sky on any given day, and assisting each flight across the clouds is the plane’s autopilot system.

When air travel was first introduced, flying demanded the complete attention of the pilot. As the range of planes increased, the longer journeys led to fatigue for the captain. Autopilot systems were designed to automate some of the pilot’s tasks and make flying easier. In 1933, American aviator Wiley Post became the first person to fly solo around the world. The trip took over eight and a half days. Without the assistance of another person to be navigate or take control, the journey wouldn’t have been possible without an autopilot system. Sophisticated control systems now exist in all modern airplanes, which causes passengers to wonder who is really flying the plane. To answer that question, it’s best to look at how autopilot works.

Roll, Pitch, & YawPlanes are controlled three dimensionally by adjusting pitch, yaw, and roll. A change in pitch would point the nose of the plane up or down; yaw, left or right; and roll would rotate the length of the plane left or right. These adjustments are made by moving elevators, the rudder, and ailerons respectively.

A basic autopilot system can mechanize control of pitch, yaw, and roll based on paramaters given by a pilot. Because a pilot must give commands to the autopilot, it’s better to think of autopilot as an automatic flight control system (AFCS). A simple single-axis ACFS will only control one dimension of the plane’s movement. In this case, it’s usually roll, which would be used to level the wings at cruising heights. Two-axis AFCS would control pitch and roll; three-axis controls pitch, yaw, and roll.

The feedback system for keeping the wings level would go something like this:

  • The plane is at cruising height keeping a specific heading. The pilot wants to keep the wings perfectly level so he tells the AFCS to make corrections if the wings deflect.
  • Eventually the plane will start to roll in one direction or the other. Position sensors constantly send data to the AFCS, if the computer detects a deflection, it acts.
  • The AFCS calculates the proper correction and sends the result to servo motors controlling the planes ailerons.
  • Position sensors are still streaming data to the AFCS. When the computer sees the wings are back to level, it tells the servos to disengage and return the ailerons to a resting state.
  • When the wings drift again, the process repeats.

The auto flight system on modern planes can control much more than just ailerons. Patrick Smith, a veteran pilot and flight blogger, talked to National Geographic about the capabilities of these systems: “We’re actually talking about a collection of subsystems that help control various aspects of a flight: heading, altitude, course, speed, engine power, etc. Different components are used at different times, and can be used together or separately, depending.”

The autopilot systems that control pitch, yaw, and roll are just components of a greater suite of tools used to automate flight.

But the AFCS is only as useful as the directions it receives. The pilots are always giving it commands. When you turn the wheel in your car, you’re commanding the steering system to move the wheels. Just because you’re not moving them manually doesn’t mean you’re not driving the car. “Airplanes do not fly themselves,” said Smith. “The crew flies the airplane through the automation… The equipment makes things easier, but the operation itself is controlled by humans.”

Part of modern systems is a tool called Autoland, which can control the entire descent and landing of a plane. According to Smith, this isn’t as common as one might think. “More than 99 percent of landings are performed manually,” he told National Geographic. “There is such a thing as an autoland where you set up an auto approach and, to put it coarsely, the airplane will land “hands off.” But I see only a few of those a year—they’re only performed in extreme low-visibility conditions, and the airplane, pilot, and airport all have to be certified.”

Even in manual landings, an instrument landing system (ILS) is often used to guide the plane into a safe landing. The ILS uses a combination of radio signals and visible markers to determine the proper path for a plane to take and greatly simplifies the process. But even with ILS, the pilot remains in control. Many airports don’t have ILS systems in place requiring pilots to land using a completely manual visual approach.

Though a pilot may not have his hands fixed to the stick, the captain and first officer are in charge of providing directives to the systems. There is no inflatable pilot like Otto in the movie Airplane! who takes control, and getting from Atlanta to Paris isn’t as simple as plugging a destination into the GPS.


National Geographic
Instrument Landing System

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