Cessna S550 N550AJ incident near Tampa, Florida

Probable cause

The lack of flat roller bearings in the two elevator trim tab actuators and maintenance personnel’s inadequate lubrication of the left elevator trim tab actuator, both of which resulted in the overstress fracture of the left internal screw and subsequent in-flight failure of the pitch trim system.

NTSB analysis

The airline transport pilot and commercial-rated copilot, who was the pilot flying, were conducting a positioning flight. The copilot reported that, during takeoff when the airplane was at 400 ft, he retracted the flaps and turned right for the published departure procedure. He noted that, during the turn, the controls were stiff, and he was feeling pushback. Both pilots then began pushing on the control yoke, and the copilot tried to use both the electric and manual pitch trim to reduce the control forces without success. He then engaged the trim disconnect and pulled the pitch trim circuit breaker, but the secondary pitch trim was still frozen. He again attempted to use the manual pitch trim without success. The copilot subsequently declared an emergency with air traffic control, returned to the airport, and landed uneventfully.

During postincident troubleshooting of the pitch trim system anomaly, the left screw of the left elevator trim tab actuator was removed, cleaned, regreased, and then reinstalled. Subsequently, the primary and secondary sprockets were found separated from the left internal screw, which was fractured due to overstress; no evidence of a preexisting fracture was noted. The two actuator shaft assemblies were examined, and both assemblies were missing flat roller bearings. Maintenance records indicated that the actuator was newly installed over 24 years, or 6,560 flight hours, before the accident and had not been disassembled since that time. Given that fresh grease was found in the actuator in an area that normal maintenance is not performed, it is likely that undocumented maintenance was conducted on the actuator at some point and that the missing bearings were not reinstalled at that time.

The maintenance records also revealed that the pitch trim actuator was last lubricated as part of a phase 2 inspection about 1 year 7 months, or 197 flight hours, before the accident, which was within limits; the process did not require the removal of the left internal screw from the actuator housing. Although postincident troubleshooting precluded the determination of the amount and condition of the grease on the left internal screw, it is likely that maintenance personnel did not adequately lubricate the left elevator trim tab actuator during this inspection. Therefore, it is likely that excessive play caused by the missing bearings, which resulted in wear, and the inadequate lubrication of the left elevator trim tab actuator caused the overstress fracture of the left internal screw and the subsequent failure of the pitch trim system.

Text from the NTSB. Open the NTSB docket for ERA15IA008 for the full record.