What happened
On 22 October 2012, an EC225 LP Super Puma, registration G-CHCN, was conducting a commercial passenger flight from Aberdeen International Airport to the West Phoenix drilling rig. While cruising at 3,000 feet in the North Sea, approximately 32 nm southwest of Sumburgh, the crew observed a transmission warning on the Central Warning Panel. This was immediately followed by a drop in both main and standby oil pump pressure to zero.
In response to the loss of oil pressure, the crew activated the emergency lubrication system. However, within one minute, a warning indicated that this emergency system had failed. Following the established emergency procedures for a power-on landing, the crew successfully ditched the helicopter in the sea near a vessel. There were no injuries to the two crew members or the 17 passengers on board.
The investigation
Investigators examined the main gearbox and the bevel gear vertical shaft to determine the cause of the pressure loss. The investigation focused on a 360-degree circumferential crack found in the bevel gear vertical shaft. This component, made of 32CDV13 steel alloy, is part of the gearbox assembly.
Engineers compared the data from this incident with a previous accident involving a similar failure on aircraft G-REDW in May 2012. Analysis of the Health and Usage Monitoring System (HUMS) showed that trend indicators for the bevel gear meshing frequency had exceeded established thresholds prior to the loss of oil pressure. The investigation also looked into the functionality of the emergency lubrication system, finding no evidence of heat damage, which suggested the system had provided cooling during the short period before the ditching.
Findings
- The primary cause of the failure was high cycle fatigue cracking in the bevel gear vertical shaft.
- The crack originated from an area of corrosion on the inner surface of the shaft.
- The failure occurred in a region of the shaft that was not subject to the surface hardening process used on the gear teeth.
- The fatigue crack was thought to be a result of the shaft flexing as it rotated.
- The investigation noted a correlation between the failure patterns of both the G-CHCN and G-REDW incidents.