What happened
On 17 February 2023, an Airbus Helicopters EC175 B, registration G-MCSH, was operating a commercial flight to the Elgin PUQ2 offshore platform in the North Sea. During the flight, the crew received updated weather information indicating high winds due to the approaching Storm Otto. After landing, a tail rotor gearbox chip warning prompted the crew to shut down the engines.
Following the shutdown, the crew attempted to secure the aircraft to the helideck. However, a failure of the rotor brake meant the main rotors continued to rotate slowly. As the storm progressed, the wind direction shifted, subjecting the aircraft to side winds. While the crew and platform personnel were attempting to attach tie-down straps, the wind caused the rotating blades to 'sail.' This aerodynamic force led to the sudden failure and detachment of four rotor blades, with one blade narrowly missing a crew member on the deck. The incident resulted in damage to the helicopter fuselage and no injuries to the personnel involved.
The investigation
The AAIB investigation examined the mechanical failure of the rotor brake, the impact of the weather conditions, and the performance of the aircraft's recording systems. Investigators reviewed CCTV footage from the platform, which captured the moment the blades fractured.
Technical analysis of the flight data recorder (CVFDR) revealed significant anomalies. The investigation found that the FA5000 model recorder experienced a loss of chronological flight data recording due to rapid cycling of input power during startup. Additionally, the investigation identified that the ARINC 717 loopback signal was out of specification, causing intermittent data loss. The investigation also looked into the maintenance history, noting that a previous report of the rotor brake failing to stop the blades during a ground move at Aberdeen Airport had not been addressed by maintenance engineers.
Findings
- The primary cause of the blade detachment was the aerodynamic lifting and 'sailing' of the rotating blades caused by vertical airflow and shifting winds.
- A failure of the rotor brake prevented the main rotors from coming to a complete stop, making it impossible to apply the rotor blade tie-down straps.
- The presence of an accommodation block on the platform created a 'cliff edge' effect, generating vertical airflow that exacerbated the blade movement.
- The flight data recorder (FA5000) failed to record chronological data due to power fluctuations during startup.
- There was a lack of follow-up regarding a previously reported issue with the rotor brake's inability to stop the blades during a recent ground movement.