The pilot reported that, while climbing to 15,000 feet mean sea level, the autopilot disengaged and the autopilot disconnect chime sounded accompanied by an “AP fail A-B” message. At this point, he decided to return to the departure airport near Wheeling, Ill.
He noted that, while trying to slow the climb, “extra” force was required to push the Cessna 680’s nose down. He attempted to apply nose-down trim but felt no change and then noted that the elevator trim tab indicator was showing full nose-down trim. He subsequently landed without incident.
Following the incident, a mechanic tested the stabilizer trim through its full range of travel and noted no discrepancy with the system or the elevator trim tab indicator. He contacted the manufacturer to assist in troubleshooting the problem.
They found that the connecting pin linking the drive shaft to the left elevator trim tab actuator had become dislodged, which likely caused the left elevator trim driveshaft to disconnect from the trim actuator.
The elevator trim tab drive shaft pin on the same make and model airplane had dislodged on two previous occasions. The manufacturer is in the process of proposing changes to the drive pin dimensions and the method of securing the pin to prevent the elevator trim tab drive pin from dislodging.
The NTSB determined the probable cause as the disconnection of the left elevator trim driveshaft from the trim actuator due to the elevator trim tab drive pin becoming dislodged, which resulted in full nose-down elevator trim.
NTSB Identification: CEN14IA121
This January 2014 accident report is provided by the National Transportation Safety Board. Published as an educational tool, it is intended to help pilots learn from the misfortunes of others.