How does autopilot work on commercial aircraft?

I'm an aviation enthusiast and in my free time I watch videos of pilots flying from the cockpit. I know that autopilot can take speed, altitude, and heading commands from the pilot, but from time to time I notice the pilot's navigation screen shows a pre-planned route that the autopilot seems to follow

I'm an aviation enthusiast and in my free time I watch videos of pilots flying from the cockpit. I know that autopilot can take speed, altitude, and heading commands from the pilot, but from time to time I notice the pilot's navigation screen shows a pre-planned route that the autopilot seems to follow directly without pilot intervention. When during the flight does the autopilot take commands directly from the pilot, and when does it follow the route from the FMS? If the plane can fly the pre-planned route entirely on its own, why would it be necessary to receive vectors from ATC at all?

Other answer:

Vegasfan9:
The entire flight plan is input into the MCDU (FMS on Boeing), before the plane even leaves the gate. Waypoints are entered by the pilot and this draws the line on the map, from point-to-point. After the pilot chooses the cruising altitude, all the other altitudes are calculated automatically. As soon as the plane takes off, if the pilot presses the AP button the autopilot will engage and begin to follow the flight plan on the map. But sometimes, if there is a lot of traffic in the area, ATC may instruct the pilot to fly a different departure path. This is when it is necessary for the pilot to tell the autopilot that they will be flying a different route. He then uses the autopilot panel to change the pre-set speed, heading, altitude, etc. On an Airbus, this is done by pulling the desired knob out (which allows a manual selection) and turning it to change. Now the plane will be flying off the line on the map. When ATC tell the pilot to 'resume own navigation', he/she can then push the autopilot selection knobs in and this will change back the autopilot to flying the pre-set MCDU route (line on map). They will usually fly this route for the rest of the flight (as they are away from traffic). When it comes to descent and the arrival route the same applies as above, ATC may change the pilot's desired route, due to weather, traffic, etc. If the plane is asked to hold in a holding stack, then this can be inputted into the MCDU and then the plane will fly in circles, until action is taken.
But if there are no reasons for the pilots to take a slightly different departure or arrival route, then they will follow the pre-set route the entire flight, right down to the ground.
So overall, yes, the pilot can override the pre-set flight plan in the MCDU, whilst keeping the autopilot flying the plane. They prefer not to, as it is usually the quickest most efficient route, but have to, if instructed by ATC (particularly at busy airports/areas, where there is a lot of traffic in the vicinity at once)
Eric West:
An autopilot does what the pilot tells it to do, either by entering commands or by loading a flight plan prior to take off.

If you want it to do something else, to comply with ATC, avoid weather or simply because you want to, you tell it to do that and it does it.

There is nothing magical about it, it simply takes the work of actually flying the aircraft from the pilots, the pilots still decide where the aircraft will go and how it will get there.

A pre-loaded plan is simply the ideal when you set off, you amend it to suit the circumstances you find enroute.

Aviation:
Modern autopilots. Not all of the passenger aircraft flying today have an autopilot system. Older and smaller general aviation aircraft especially are still hand-flown, and even small airliners with fewer than twenty seats may also be without an autopilot as they are used on short-duration flights with two pilots.
John R:
ATC's function is to keep traffic separated, and to organize traffic flow into the airport area. They do this by vectoring aircraft.

You can preload a flight plan into the autopilot (even 4 seat Cessna's built in the last 15 years have an autopilot that can fly a preprogramed route), but you do not always end up flying the exact route you were cleared for, and you don't know the exact approach you will be flying into the airport until It is assigned.

Once an approach is assigned, most of the time you do not fly the entire published approach, ATC provides vectors to establish you on the final approach course.

And that preprogramed course would not know anything about that thunderstorm that popped up ahead of you, so you ask ATC for vectors around it, you don't let the autopilot fly you into it. (There were several problems with the crews performance on AF447, but what got them into trouble to begin with was not deviating around a storm)

♛ Nicolas ♛:
preplanned routes dont take into account rapidly changing weather conditions nor do they consider air traffic. ATC vectors do. despite most of the flight being done on own navigation, ATC is still important.
Skipper747:
Autopilot Modes –

Autopilot controls airplane with manual controls = pitch wheel,. turn knob –
Autopilot can be controlled by heading bug (on compass) airspeed and altitude selection –

Autopilot can be selected to follow VOR/ILS navigation, INS or FMS navigation –
On INS or FMS autopilots directs airplane from waypoint to waypoint along route selected –

Autopilot operation is often initiated during climb (flaps retraction after takeoff) –
INS or FMS may be selected at that time –
Airplanes with autoland capability can be selected – ILS/LAND mode during the approach –
Autopilots are controlling airplane after landing on the runway centerline –

Braking is selected by the pilot (minimum, medium, maximum) – separate selector –
Thrust reversers after landing must be controlled manually by pilot –

On the 747 – most of autopilot controls are located on the "P-10" panel (glareshield) –

Autopilots can control airplanes for the entire flight EXCEPT TAKEOFF –

Pilsner Man:
It is "coupled" to the navigation systems, so it follows the commands that are going to the nav instruments. It doesn't work any different than on non-commercial, military, or any other aircraft.
JetDoc:
Pilots follow Air Traffic Control directions and vectors when arriving and departing airports to blend in with the other air traffic in the vicinity and to maintain required separation between airplanes. Once ATC releases them to "resume their own navigation" they can switch the autopilot to follow their pre-planned route
Scott:
Google it

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