What happened
In two separate incidents in 2012, EC225 LP Super Puma helicopters experienced critical mechanical failures while operating over the North Sea. On 10 May 2012, G-REDW, operated by Bond Offshore Helicopters Ltd, was 34 nm east of Aberdeen when the crew experienced a loss of main rotor gearbox oil pressure. Following the activation of the emergency lubrication system, a warning light indicated the system had failed, necessitating an immediate ditching.
On 2/22 October 2012, G-CHCN, operated by CHC Scotia Ltd, experienced a nearly identical sequence 32 nm southwest of the Shetland Islands. In both instances, the aircraft successfully ditched, and the crew and passengers safely evacuated into liferafts. There were no serious injuries in either accident.
The investigation
The AAIB combined the investigations due to the striking similarities between the two events. Investigators examined the mechanical integrity of the main rotor gearbox and the performance of the emergency lubrication and survival equipment. The investigation involved international cooperation with the BEA and EASA.
Technical analysis focused on the bevel gear vertical shaft, which drives the oil pumps. The investigation found that the shafts had developed 360º circumferential fatigue cracks. In the case of G-REDW, the crack originated from a corrosion pit caused by moisture trapped near a manufacturing hole. For G-CHCN, the crack started at a corrosion pit formed by iron oxide deposits.
Furthermore, the investigation identified that the emergency lubrication system warning light illuminated erroneously due to an electrical incompatibility between the aircraft wiring and the pressure switches. The investigation also reviewed the reliability of the Crash Position Indicator (CPI) and the deployment of the Type 18R MK3 liferafts.
Findings
- The primary cause of the oil pressure loss was a high-cycle fatigue crack in the bevel gear vertical shaft.
- The manufacturer's design model underestimated the maximum stress at the weld area and failed to account for residual stresses from the welding process.
- Corrosion pits, caused by trapped moisture and debris, acted as initiation points for the fatigue cracks.
- An electrical mismatch caused the emergency lubrication system warning to trigger incorrectly after activation.
- The safety factor for the shaft was found to be lower than originally predicted during certification when accounting for real-world stresses and surface defects.