What are the modes and architecture of a typical autopilot system?

Autopilot modes include: Inner loop - Attitude hold (pitch, roll stabilization); Outer loop - Altitude hold, heading hold, vertical speed, airspeed hold; Guidance - Lateral navigation (VOR, LNAV), Vertical navigation (glideslope, VNAV), Autothrottle (speed control); and Autoland - Automatic flare and rollout. Architecture: Flight control computers process sensor data and mode commands, Servos drive control surfaces, Mode control panel provides pilot interface, and Flight director displays commanded attitude. Redundancy levels vary from simplex (GA) to triplex (transport autoland). Autopilot relieves pilot workload, improves precision (especially approach), and enables all-weather operations. Certification requires extensive testing of normal and failure modes.