What happened
The aircraft experienced severe oscillations along the pitch axis during flight operations. These vibrations were directly linked to airspeed, increasing in intensity as speed changed. The control column moved fore and aft by approximately one to three inches, indicating significant instability in the longitudinal control system.
The investigation
Initial examination of the aircraft did not reveal any out-of-limit conditions that could explain the oscillations during the first occurrence. However, following a second incident, mechanics inspected the elevator trim tab assembly. They discovered that the hinge had suffered from wear, resulting in measurable play of five-thirty-seconds of an inch.
Findings
The excessive clearance in the elevator trim tab hinge was identified as the source of the instability. After the worn hinge was replaced, the aircraft no longer experienced the pitch oscillations. This confirms that mechanical looseness in the control surface linkage contributed to the flight characteristics observed.