The Air Accidents Investigation Branch (AAIB) was notified at 1112 hrs on 10 May 2012 that an EC225 LP Super Puma, G-REDW, was preparing to ditch in the North Sea approximately 32 nm east of Aberdeen.
On 22 October 2012 the AAIB was notified at 1428 hrs that an EC225 LP Super Puma, G-CHCN, had ditched in the North Sea approximately 32 nm southwest of Sumburgh, Shetland Islands.
In both cases the AAIB deployed a team to Aberdeen to commence an investigation. In accordance with established International arrangements the Bureau d’Enquêtes et d’Analyses pour la sécurité de l’aviation civile (BEA), representing the State of Manufacture of the helicopter, and the European Aviation Safety Agency (EASA), the Regulator responsible for the certification and continued airworthiness of the helicopter, were informed of the accidents. The BEA appointed an Accredited Representative to lead a team of investigators from the BEA and Eurocopter (the helicopter manufacturer). The EASA, the helicopter operators and the UK Civil Aviation Authority (CAA) also provided assistance to the AAIB team.
Owing to the similarities of the circumstances that led to the two accidents, the Chief Inspector of Air Accidents ordered that the investigations be combined into a single report.
While operating over the North Sea, in daylight, the crews of G-REDW and G-CHCN experienced a loss of main rotor gearbox oil pressure, which required them to activate the emergency lubrication system. This system uses a mixture of glycol and water to provide 30 minutes of alternative cooling and lubrication. Both helicopters should have been able to fly to the nearest airport; however, shortly after the system had activated, a warning illuminated indicating that the emergency lubrication system had failed. This required the crews to ditch their helicopters immediately in the North Sea. Both ditchings were successful and the crew and passengers evacuated into the helicopter’s liferafts before being rescued. There were no serious injuries.
The loss of oil pressure on both helicopters was caused by a failure of the bevel gear vertical shaft in the main rotor gearbox, which drives the oil pumps. The shafts had failed as result of a circumferential fatigue crack in the area where the two parts of the shaft are welded together.
On G-REDW the crack initiated from a small corrosion pit on the countersink of the 4 mm manufacturing hole in the weld. The corrosion probably resulted from the presence of moisture within the gap between the PTFE plug and the countersink. The shaft on G-REDW had accumulated 167 flying hours since new.
On G-CHCN, the crack initiated from a small corrosion pit located on a feature on the shaft described as the inner radius. Debris that contained iron oxide and moisture had become trapped on the inner radius, which led to the formation of corrosion pits. The shaft fitted to G-CHCN had accumulated 3,845 flying hours; this was more than any other EC225 LP shaft.
The stress, in the areas where the cracks initiated, was found to be higher than that predicted during the certification of the shaft. However, the safety factor of the shaft was still adequate, providing there were no surface defects such as corrosion.
The emergency lubrication system operated in both cases, but the system warning light illuminated as a result of an incompatibility between the helicopter wiring and the pressure switches. This meant the warning light would always illuminate after the crew activated the emergency lubrication system.
A number of other safety issues were identified concerning emergency checklists, the crash position indicator and liferafts.
Ten safety recommendations have been made. In addition, the helicopter manufacturer carried out several safety actions and is redesigning the bevel gear vertical shaft taking into account the findings of the investigation. Other organisations have also initiated a number of safety actions as a result of this investigation.
The following causal factors were identified in the ditching of both helicopters:
a A 360º circumferential high-cycle fatigue crack led to the failure of the main gearbox bevel gear vertical shaft and loss of drive to the oil pumps.
b The incompatibility between the aircraft wiring and the internal configuration of the pressure switches in both the bleed-air and water/glycol (Hydrosafe 620) supplies resulted in the illumination of the MGB EMLUB caption.
The following factors contributed to the failure of the EC225 LP main gearbox bevel gear vertical shafts:
a The helicopter manufacturer’s Finite Element Model underestimated the maximum stress in the area of the weld.
b Residual stresses, introduced during the welding operation, were not fully taken into account during the design of the shaft.
c Corrosion pits were present on both shafts from which fatigue cracks initiated:
i On G-REDW the corrosion pit was located at the inner countersink in the 4.2 mm hole and probably resulted from the presence of moisture within the gap between the PTFE plug and the countersink.
ii On G-CHCN the corrosion pit was located at the inner radius and probably resulted from moisture trapped within an iron oxide deposit that had collected in this area.