Final answer:
The PTU operates with pb in AUTO mode when the aircraft is in flight and the autopilot is engaged. It adjusts the pitch trim of the aircraft to maintain the desired pitch attitude.
Step-by-step explanation:
The PTU (Pitch Trim Unit) operates with pb (the longitudinal control input) in AUTO mode when the aircraft is in flight and the autopilot is engaged. In this mode, the PTU adjusts the pitch trim of the aircraft to maintain the desired pitch attitude based on inputs from the autopilot and various sensors.
For example, if the aircraft starts to pitch up, the autopilot sends a command to the PTU to trim the aircraft nose down to return to the desired pitch attitude. The PTU then adjusts the pitch trim using a motor or hydraulic actuator.
It is important to note that different aircraft may have different operating procedures and designations for their control systems, so it is always best to consult the specific aircraft's documentation for accurate and detailed information.