NTSB Identification: WPR10FA112
14 CFR Part 91: General Aviation
Accident occurred Sunday, January 17, 2010 in Reno, NV
Probable Cause Approval Date: 03/16/2011
Aircraft: EUROCOPTER AS350 B3, registration: N904CF
Injuries: 3 Uninjured.

NTSB investigators either traveled in support of this investigation or conducted a significant amount of investigative work without any travel, and used data obtained from various sources to prepare this aircraft accident report.

The pilot reported that he lifted the single-engine helicopter from the helicopter pad for the emergency medical service positioning flight to pick up a patient. The pilot maneuvered the helicopter into a 25-foot hover and, just as he was beginning the transition to forward flight, he heard a loud bang. The helicopter experienced a power loss and the pilot lowered the collective slightly, resulting in a hard landing on the pad. A surveillance camera that captured the accident sequence indicated that after the helicopter lifted into a hover over the pad, the nose abruptly rotated right 90 degrees and the helicopter descended vertically in a slight nose-down attitude, landing hard on the helipad.

The postaccident airframe examination revealed that the nuts that attach the engine-to-main gear box flex coupling were not present on their respective bolts. The nuts and associated washers were located loose and clumped together just forward of the gimbal ring in the transmission input housing. An examination of the bolts and flex coupling by the Safety Board Materials Laboratory concluded that the nuts most likely had been hand tightened and that cotter pins had not been installed on the bolts. The improper installation lead to the failure of the flex coupling and resulted in a loss of power to the rotor system.

Maintenance records showed that 59 flight hours before the accident the engine had been removed, the helicopter painted, and then the engine was reinstalled. The time between the engine removal and the reinstallation was 88 days. The mechanic who removed the engine stated that he removed the bolts to the engine-to-main gear box flex coupling, and then partially reassembled the flex coupling bolts. This action was not in accordance with the AS350 maintenance manual engine removal procedure. The mechanic who installed the engine 88 days later stated that he did not check the flex coupling bolts because the removal of those bolts is not specified in the maintenance manual as part of the engine removal or replacement procedure. The overall maintenance activity involved a 100-hour inspection, which included a visual inspection of the engine-to-main gear box flex coupling. Although a visual inspection of the engine-to-main gear box flex coupling is a required action, the Quality Assurance inspector signed off the maintenance without performing the visual inspection of the flex coupling.

The National Transportation Safety Board determines the probable cause(s) of this accident to be:

The improper installation of the engine-to-main gear box flex coupling, which resulted in the failure of the flex coupling and a loss of power to the rotor system during takeoff. Contributing to the accident was the mechanic who removed the engine's failure to follow the operator’s maintenance procedures. Also contributing was the Quality Assurance inspector's failure to follow the operator’s post-maintenance inspection requirements.

Full narrative available

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