What happened
While climbing through approximately 5,000 feet MSL with the autopilot engaged, the pilot adjusted the heading bug 30 degrees to the right. This maneuver caused the Cessna (type not specified in source) to snap with significant yaw into a 60-degree right bank. During this event, the aircraft lost approximately 1,800 feet of altitude. Following the recovery from the bank, the pilot continued the flight to the intended destination.
The investigation
The aircraft sustained substantial damage during the incident. Investigations into the aircraft's history noted that the autopilot system had been installed roughly five months before the accident. Since that installation, the pilot had reported worsening operational issues involving a yaw problem, though maintenance personnel were unable to duplicate the issue prior to this event.
A post-accident examination of the autopilot and heading system showed no anomalies. However, inspection of the aft fuselage and tailcone revealed that the yaw damper was contaminated with oil. The source of the contamination was traced to an oil streak originating from under the engine cowling, running along the belly of the aircraft, and traveling up through the air conditioning exhaust vent.
Findings
- The yaw damper system was contaminated with oil.
- Oil traveled from the engine cowling area through the air conditioning exhaust vent to the tailcone.
- Similar oil contamination of yaw damper amplifiers was identified in two other aircraft that had experienced similar yaw events.