Control System Servos

At FL400, the autopilot started porpoising and was turned off. Afterward, the aircraft would not trim properly. The crew diverted; it was difficult to keep it pitched down while descending. During the final phase of flight, the yoke was very difficult to input pitch changes, but was okay in the roll axis. After landing, troubleshooting duplicated the problem. Elevator servo (p/n 4006719914) was replaced with serviceable unit.

Key Takeaways:

• Several aircraft models experienced diverse autopilot and flight control servo malfunctions, predominantly affecting pitch control.
• Reported incidents included overheated servos causing smoke, uncommanded pitch inputs, binding, and autopilot porpoising or tripping off.
• Multiple aircraft encountered recurring problems with the same components or issues, indicating persistent or repeated failures.

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