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In particular, does the Autopilot in a B737NG control the pitch only via trim adjustment, or first via the elevators and then trim?

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    $\begingroup$ In most airplanes, the AP flies the same way the pilot would. It makes elevator inputs to achieve a condition, and if it has to hold the input to keep that condition, it works the trim so that it can relax the input. So, elevator, followed up with trim where required to zero the elevator servo load. $\endgroup$ – John K Oct 19 '20 at 13:11
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This is from the B737NG airplane manual. Boeing doesn't offer the technical drawings in the pilot manuals, one would have to have access to the maintenance manuals. Boeing is very sticky about their material being published on the internet.

Autopilot Flight Director System (AFDS)

The AFDS is a dual system consisting of two individual flight control computers (FCCs) and a single mode control panel. The two FCCs are identified as A and B. For A/P operation, they send control commands to their respective pitch and roll hydraulic servos, which operate the flight controls through two separate hydraulic systems.

This is a brief explanation of an autopilot system such as on the B737 from this website.

Aircraft with hydraulically actuated flight control systems have autopilot servos that are electro-hydraulic. They are control valves that direct fluid pressure as needed to move the control surfaces via the control surface actuators. They are powered by signals from the autopilot computer. When the autopilot is not engaged, the servos allow hydraulic fluid to flow unrestricted in the flight control system for normal operation. The servo valves can incorporate feedback transducers to update the autopilot of progress during error correction.

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  • $\begingroup$ So it is not clear if the B737 AP works only through the horizontal stabilizer (trim) or elevators AND horizontal stabilizer. "Flight controls" could be meaning both. I guess someone who flies it would know though, simply by watching if the yoke moves back and forth during AP operation :-) $\endgroup$ – Stamatis Vellis Oct 20 '20 at 8:31
  • $\begingroup$ It is quite clear. The 737 NG autopilot controls the elevator and pitch trim, just like a pilot would. $\endgroup$ – Raffles Oct 20 '20 at 8:47
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    $\begingroup$ @StamatisVellis The hydraulic servo described in this answer is acting on the elevator (for pitch) and ailerons (for roll). Additionally, the autopilot has control over the electric motor driving the jackscrew of the THS. And yes, the yoke does move as a result of autopilot inputs (based on FCC signal to the elevator feel and centering unit). $\endgroup$ – Bianfable Oct 20 '20 at 8:59

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