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Let's say you are engaged a Boeing 777 with autopilot on and you bank or pitch. Will the plane respond?. If so. will the plane try to return to its parameters once you quit handling the controls?. Same question for any Airbus.

I read that in the case of a TCAS RA you need to disengage the autopilot and fly manually. But let's suppose your TCAS tells you to Climb and you pull your yoke without disengaging the AP. What would happen on a Boeing or on an Airbus?

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The autopilot on a modern airliner isn't a single system. Rather, it's a combination of systems which work together to control the plane.

I'm not particularly familiar with the 777 specifically, but in general, the autopilot of a modern airliner will be able to do four main different things:

  • Keep the wings level
  • Maintain or change altitude
  • Maintain or change course
  • Adjust engine power

These can also be used via the Flight Management Computer to fly a preprogrammed course at designated altitudes by way of a set of specified waypoints.

For example, if the plane is flying at a heading of 180°, and the pilot turns the heading knob to change this to 200°, the autopilot will make a turn to the right to come to the new heading. In order to maintain altitude during the turn, it will need to adjust engine power and/or pitch, because the bank to turn costs energy which ultimate must come from the engines.

However, it's possible for the pilot to disengage, say, the autothrottle, while leaving the rest of the autopilot turned on. If so, the pilot will be handling the engine power levers, while the autopilot might be controlling, say, the speed during a climb or descent so as to ensure that the airframe isn't overstressed. If the pilot then also pulls or pushes the stick or yoke, the altitude maintenance system might also disconnect, but the autopilot can still help the pilot stay on course. And so on.

Generally, the autopilot will disengage one axis of control when the pilot manipulates the controls that control that axis.

For example, if the pilot turns the yoke or pushes the sidestick to the side in order to bank into a turn, the autopilot will likely disengage the wings leveller, possibly after a delay where the pilot's inputs are considered a temporary override, but it might still very well be controlling the altitude and engine power. This provides a quick autopilot override; if you get a TCAS RA saying climb, you just pull the yoke or stick back, which overrides or disengages the part of the autopilot that maintains altitude, while still leaving the automatic course and engine power control active. Since the autopilot controls the throttle at that point, the pilot doesn't need to worry (much) about the speed dropping too far and getting close to a stall during the climb. (Correspondingly, if the RA was for a descent, the pilot wouldn't need to worry about overspeeding the plane.)

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The autopilot will always disengage when you try to override the controls. Manual control has priority over autopilot. For the Boeing 737, the control mode changes to CWS (Control wheel Steering), which is an autopilot mode but follows control column inputs. So if you manually set it on 12 degrees pitch down (please do not do this in real life) then the autopilot will keep it there.

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Most autopilots on conventional cable operated control systems use an electric servo that drives the cable run via a capstan (basically an electric bi-directional winch with its own little cable loop linked into the main cable run). There is one for the elevator and one for the ailerons (most modern autopilots are only 2-axis; the rudder is automatically operated by the yaw damper system which is a separate system).

There are two clutches between the motor drive and the capstan, a toothed on-off clutch and an always-engaged friction slip clutch. The tooth clutch is engaged/disengaged by the A/P. The friction clutch is there to allow the A/P servo to be overpowered by manually applied force if the tooth clutch doesn't disengage.

There are torque sensors in the servo drive system that sense the force the servo is working against and tell the A/P computer. If you apply control input without disengaging the A/P, the A/P will initially resist your input but if you apply enough force you will break out the slip clutch to overpower the servo and move the control. After a short delay the A/P computer will sense the unusually high sustained torque being applied as the servo struggles against your own input (it can't win because of the slip clutch), and kick the A/P off.

On a TCAS RA you don't have to go that far because in addition to several other disconnect triggers in the cockpit, there will be an A/P disconnect button on the control wheel near your flying-hand thumb someplace. If you get a TCAS RA, you immediately jab the AP disconnect to kick it off as soon as you grab the column. This is something you train for in the sim during initial and recurrent training.

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