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I have a number of questions about auto-pilots but rather than deluge you with a long prose here let me break them down into separate questions over the next little while.

From playing around with simulators I know that the pilot has the ability to enter desired altitudes, climb rates, heading etc into the auto-pilot. However, and forgive me, whenever I see a pilot engage the auto-pilot in movies etc, it is simply a flip of the switch.

So that got me wondering how the transition is made in actuality.

Does the pilot need to enter the current altitude, heading etc before flicking the switch or does the AP take the current numbers and hold those automatically.

If it is the latter and the aircraft is in say a climbing turn when the switch is clicked.. then what? Does the aircraft continue in that pattern or what?

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    $\begingroup$ Depends on the aircraft/autopilot. For my aircraft, it is a three step process... First move the switch to the "TEST" position and wait for the self-test to complete. Then move it to the "ON" position. The autopilot will show "STB" or Standby. Then I can use the buttons to select a mode (ALT, HDG, APR, or NAV). The ALT will hold the current altitude, HDG will follow the heading bug (make sure to set it!), and APR/NAV will follow the localizer which can be driven by the GPS or nav radio. $\endgroup$
    – Ron Beyer
    Commented Oct 4, 2017 at 18:08

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“One Switch” to operate the Autopilot is not true. That’s just the movies.

Autopilot systems vary in each aircraft. Larger jets have it divided into two: Auto-Throttle and Auto-Pilot. Smaller aircraft/piston aircraft don’t really have auto-throttle capabilities.

The Autopilot is not just to “maintain” current flight path. It really can fly the entire flight from take-off to touchdown. This includes flying a departure procedure, en-route navigation, arrival procedure, an approach and touchdown.

Theoretically, it is possible for a pilot to not touch the aircraft controls (yoke/stick/throttle) after liftoff all the way until touchdown even if the mission is a complex IFR flight. Realistically, pilots love to fly and they will fly.

Many settings exist for each autopilot system, and these include (but are not limited to):

Lateral Navigation

  • Heading Mode (fly the “selected” heading”)
  • Nav Mode (fly the “selected” nav course)
  • Roll Mode

Vertical Navigation

  • Altitude Select Mode (Maintain the selected altitude)
  • Vertical Descent with a fixed speed
  • Vertical Descent with a fixed rate
  • Approch Mode (Vertical Descent with Glideslope)

Auto-Throttle

  • Maintain Selected Airspeed
  • Accelerate/Decelerate to Selected Airspeed

However, there is that master AP Switch that’ll disable all things related to the AP.

autopilot in B737

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  • $\begingroup$ I understand that but it does not answer my question. If I am cruising at close to 9000ft do I need to rotate that altitude knob to select 9000 before I engage.. or does it grab the 9150 I'm actually at. $\endgroup$
    – Trevor_G
    Commented Oct 4, 2017 at 18:24
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    $\begingroup$ This is a lot about what an auto-pilot can do, but only your first sentence is applicable to the question at hand, and as Trevor says, doesn't really answer the meat of what he is getting at. @Trevor It depends, with autopilots that have a settable altitude, it must be set, other ones just hold the current altitude (and don't have knobs/displays to show it). $\endgroup$
    – Ron Beyer
    Commented Oct 4, 2017 at 18:29
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    $\begingroup$ Understandably the answer says that auto-pilot must be configured before it can be engaged, so NO - there is more than one switch to get it active from scratch. There is the one switch to turn it off. $\endgroup$
    – h22
    Commented Oct 5, 2017 at 7:54
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Turning on the autopilot is generally a single button or switch, but with a caveat.

That big panel with the altitude selector, heading bug, nav options, etc programs the flight director, which is most likely enabled prior to even taking off and pre-programmed with your initial altitude leveloff, departure heading, etc.

While hand flying the pilot not flying who is operating the radios will update the flight director settings as your ATC instructions vary. This is generally updating altitude, heading and engaging nav.

When it is time to let Otto fly, at that point it is just a single button to slave the autopilot to the flight director.

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