Primary finding
Probable cause
The pilot's inability to maintain control due to the mechanic's inadvertent switching of two cyclic control tubes of different lengths during maintenance.
Investigator assessment
Analysis narrative
While in a 3-foot hover the helicopter began an uncommanded roll to the right. The pilot unsuccessfully attempted to arrest the roll by applying full left cyclic. Moments later the right landing skid made contact with the ground, followed by the main rotor blades. The helicopter came to rest in a right-side-low, upright position and the pilot was able to exit unassisted. The purpose of the flight was to perform an operational flight check of maintenance performed to the cyclic control system. The investigation revealed that the mechanic who had performed the maintenance had inadvertently switched two cyclic control tubes, of different lengths, during reassembly. This resulted in an out-of-rig condition in the cyclic control system. No other preimpact anomalies were noted with the helicopter's flight control system.
Source record
Factual narrative
On September 6, 2009, about 1400 central daylight time, a Messerschmitt-Boelkow-Blohm BO-105S helicopter, N131EH, was substantially damaged during a post maintenance flight near Thibodaux, Louisiana. The pilot, the sole occupant, sustained minor injuries. The helicopter was registered to and operated by ERA Helicopters LLC., of Lake Charles, Louisiana. Visual meteorological conditions prevailed and a company flight plan was filed for the 14 Code of Federal Regulations Part 91 local flight. According to the pilot, while in a 3-foot hover, the helicopter began an uncommanded roll to the right. The pilot attempted to arrest the roll by applying full left cyclic; however, the helicopter's roll could not be arrested. Moments later the right landing skid made contact with the ground followed by the main rotor blades. The helicopter came to rest in a right side low upright position and the pilot was able to exit unassisted. According to a Federal Aviation Administration (FAA) inspector who responded to the accident site, the purpose of the flight was to perform an operational flight check of maintenance performed to the cyclic control system. Prior to the accident flight, a company mechanic had removed both the lower lateral and longitudinal cyclic control tubes to replace the rod ends on both of the tubes. Although the lateral and longitudinal control tubes have the same part number, they are adjusted to different lengths. The rod ends were replaced with new ones and reinstalled in the helicopter; however, the investigation revealed that the mechanic had inadvertently switched the two control tubes during reassembly. This resulted in an out of rig condition in the cyclic control system. In addition, a bench check and a teardown were performed on the hydraulic control pack. There were no anomalies noted with the hydraulic system. Continuity was established in the "non-boosted" cyclic control system, and no anomalies were noted. The cyclic system above the hydraulic pack was damaged as result of the accident, but was functional. There also were no anomalies noted with the swashplate system or the main rotor system.