What happened
During an Operator Proficiency Check (OPC), a helicopter was performing an autorotation landing. Immediately after the aircraft touched down, intense vibrations began, accompanied by a heavy scraping noise. These vibrations fluctupect in frequency, initially matching the rotor speed before increasing sharply. This was followed by the complete separation of the main rotor from the helicopter, with the rotor assembly coming to rest approximately 10 meters to the left of the fuselage. All occupants on board remained uninjured and were able to exit the aircraft without assistance.
The investigation
The technical investigation focused on the structural failure of the rotor mast and the lubrication integrity of the freewheel assembly. Investigators discovered a contaminant lodged within a designed restrictor in the oil system, which provides lubrication to the freewheel. This blockage led to a malfunction in the lubrication of the assembly.
Analysis of the mechanical sequence suggested that while the freewheel had likely functioned correctly during previous autorotations, it failed to engage properly when the free turbine speed was meant to synchronize with the rotor speed during the gear-down phase. It is believed that the free turbine speed was significantly higher than the rotor speed at the moment of engagement. This discrepancy created an additional dynamic moment; the inertia of the main rotor acted to brake the energy stored within the engine and transmission, creating a torque load that exceeded the mast's fracture strength.
Findings
- The primary cause of the accident was a torsional overload of the mast.
- A contaminant, which could reasonably be introduced during standard Part-145 maintenance procedures, blocked the oil flow to the freewheel assembly.
- The resulting lubrication failure caused the freewheel to malfunction during the critical transition of the autorotation landing.
Safety action
Following the investigation, recommendations were issued to EASA and Transport Canada to address the following:
- Implement measures to reduce the sensitivity of the oil system to contaminants.
- Ensure helicopter operators are provided with information and preventive measures to mitigate the risk of contamination within the freewheel lubrication system.