What happened
At approximately 17:35 EDT, a Bell 206B-III helicopter departed from Beloeil Airport, Quebec, heading northeast. The flight, conducted under visual flight rules, was intended to verify the functionality of the aircraft's transponder. On board were the pilot and an aircraft maintenance engineer. Roughly five minutes into the flight, while the pilot was notifying the area control centre of an intention to return to the airport, the main rotor separated from the mast. The rotor blades subsequently penetrated the cockpit. The aircraft crashed on its back in a plowed field located 1.2 nautical miles northeast of the departure point. A post-impact fire destroyed the helicopter. The two fatalities involved were the pilot and the engineer.
The investigation
Investigators examined the rotor head and its components to determine the cause of the structural failure. The inspection revealed that the mast nut was absent from the assembly. While investigators considered whether a different nut had been installed and subsequently vibrated loose, they found all other inventory nuts remained in the maintenance hangar. This led to the conclusion that the aircraft departed without the component.
Further investigation into the maintenance environment at S.C.G. Hélicoptère inc. revealed that the aircraft was undergoing various repairs and cosmetic modifications. Although the maintenance personnel involved in the removal of the nut were present at the time of takeoff, there was no formal documentation or maintenance log entry indicating that the work on the mast nut was incomplete. The investigation also noted that the pilot and the engineer had worked closely together in the hours leading up to the flight, which may have contributed to a lack of scrutiny regarding the aircraft's airworthiness.
Findings
- The primary cause of the rotor separation was that the mast nut was not installed on the aircraft.
- The helicopter departed the aerodrome without the necessary mast nut.
- The pilot failed to perform a pre-flight inspection of the rotor head as required by the aircraft flight manual.
- There was a lack of maintenance documentation to alert personnel that the mast nut had been removed for painting.
- No warning signs or cockpit indicators were present to signal that the aircraft was out of service.
- The maintenance personnel involved in the removal of the part failed to remember its absence during the pre-flight period.