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After a plane takes off the pilot usually sets the autopilot to the altitude of the flight level they are about to cruise at, e.g. 30,000 ft, right? When that is the case, how much thrust should be applied during further ascent and then at the cruise altitude? Or does the autopilot do this itself as well? How would the autopilot behave if you ascend at 100% of thrust or at a thrust too low?

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The autopilot commands the pitch and roll and uses elevator and pitch trim as well as ailerons and rollspoilers for this. An optional auto-throttle system changes the thrust or power of the engine.

Some aircraft, not all, have an auto-throttle system, for example: Boeing 737, 747, 757, 767, 777, 787, Airbus A320 family, A330, A340, A350, A380 all have this. Other aircraft such as the Dash-8 Q400, Cessna 172 and most turboprops and piston and general aviation aircraft do not have an auto-throttle system.

In aircraft with the auto-throttle, when the system is turned on, the A/THR changes the thrust between idle and climb thrust (not full 100% thrust). So during the climb the auto-throttle advances thrust to climb thrust and keeps it there until the new altitude target is captured. During the climb the autopilot pitches up and down to maintain the set target speed. Then, upon reaching the target altitude the auto-throttle switches back to maintaining the airspeed and the autopilot pitches up and down to maintain the altitude. During the descent the auto-throttle system reduces thrust to idle.

In aircraft without an auto-throttle system, the autopilot can only change the pitch of the aircraft and you as a pilot need to feed in power for climb or reduce power for the descent. Otherwise the aircraft wont climb or descent. These autopilots may still have a flight level change mode where the autopilot maintains a set airspeed target and pitches up and down as you increase or decrease power but you still need to control the engine manually. And once the target altitude has been reached you also need to change thrust or power to fly at the desired speed.

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    $\begingroup$ Does the auto-throttle have to be turned on seperately or does it turn on when you turn on the cruise altitude? If the former, what do you turn on first, the altitude or the auto-throttle? And at what altitude do you usually start ascending / end descending on autopilot? $\endgroup$
    – Giovanni
    Commented Nov 23, 2022 at 9:58
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    $\begingroup$ @Giovanni All of those questions depend heavily on the airplane type, the policies of the company, and the preferences of the pilot. The autothrottle is usually a separate control from the autopilot, though exceptions exist. You turn them on in whatever order the checklist for that specific plane says. For big commercial planes, the autopilot and autothrottle are typically turned on a few seconds after takeoff, and remain that way until a few seconds before landing, but they can always be turned off if needed. For smaller planes, the pilot decides. $\endgroup$ Commented Nov 23, 2022 at 15:11
  • $\begingroup$ There was an autothrottle STC add-on for the CRJ200 corporate version that I've used a few times. I didn't like it because the thrust lever friction controls were removed and the AT servo clutches remained engaged full time, the clutch slip force being your one-size-only friction level, which was too high for my liking, especially on approaches. $\endgroup$
    – John K
    Commented Nov 23, 2022 at 15:40
  • $\begingroup$ @HiddenWindshield I mean the usual passenger jet airliners. You put in the altitude and set the switch so that the autopilot goes there. Do you immediately afterwards have to turn on autothrottle or otherwise what throttle is best so that the plane stays at the autopilot altitude? $\endgroup$
    – Giovanni
    Commented Nov 23, 2022 at 15:42
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    $\begingroup$ @Giovanni There's no such thing as "usual passenger jet airliners" in this case. A 737 is going to have a different procedure than an A320. Delta Airlines has different procedures from Emirates Air. Also, what do you mean by "have to"? If you're taking a technical perspective, you don't "have to" use the autothrottle at all, any more than you "have to" use the cruise control of your car. If you're taking a procedural perspective, you "have to" turn it on when the manufacturer/company policy says you "have to". $\endgroup$ Commented Nov 23, 2022 at 16:08

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