what does autopilot actually do?
In: Engineering
What autopilot? There are various different kinds.
In general, the autopilot keeps the plane at a set altitude, speed, and direction based on what’s put into the GPS. When flying and once you’re at a cruising altitude, there’s a lot of minor adjustments you need to make to keep yourself going in the right direction at the right hight and speed, and it’ll take care of all those adjustments so you can focus on other minor tasks or take a second to appreciate the view. It’s not used (as far as I know) for taking off, ascending to cruising altitude, descending from cruising altitude, making large turns (which you might do if you change destinations or are circling something) or landing, and it’s turned off (in my experience) for all of those.
Edit: As someone pointed out in the comments, commercial (as in large aircraft) airliners having more complex autopilot computers which can take on more tasks than what I described. This answer is best applicable to smaller aircraft like Cessna models which you’d see private owners use
In a big plane like a 737?
The auto-pilot has the ability to actually take control of the flight controls… the yoke (pitch up/down, bank left/right), the rudder (yaw, or turn left and right), and the engine throttles, and some trim (semi-permanent adjustment of inputs). Depending on the controls, you can usually see them move as the autopilot controls them.
Autopilot has lots of modes. Generally they come down to keeping the plane on a specific course (direction and altitude) or following the route programmed into the flight computer before take-off or tweaked during flight as the situation changes. In full navigation mode it can fly pretty much from the departure airport (once in the air) to the destination (almost) with no human input whatsoever. The pilots can switch between modes freely, including options like altitude managed by the pilots, but direction managed by the flight plan/route. If something goes wrong, at the push of a button the plane will level off if it was climbing/descending.
They can also land the plane automatically, if the airport has the appropriate equipment on the ground at the runway edges. Though this is generally only used in poor visibility scenarios because the pilots need to keep up their practice of how to fly a plane.
And speed of course, though generally it operates separately. Just set a target speed and the engine power levers move back and forth to try and maintain it. Very similar to the cruise control in your car, doing nothing more than operating the gas pedal while watching your speed.
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