What happened
On 10 May 2012, an EC225 LP helicopter, registration G-REDW, was performing a scheduled passenger flight from Aberdeen Airport to an offshore platform in the North Sea. While cruising at 3,000 ft, the crew received indications of low oil pressure in both the main and standby lubrication systems of the main gearbox (MGB), accompanied by rising oil temperatures and a chip indication.
In response to the warnings, the commander took control, reduced speed, and initiated a descent toward the coast. The crew activated the emergency lubrication system and performed a controlled ditching. Following the impact, the helicopter remained upright due to its emergency flotation gear. The crew and passengers evacuated via life rafts, with six occupants rescued by a search and rescue helicopter and eight others by an RNLI lifeboat. There were no fatalities, though two passengers sustained minor injuries.
The investigation
The AAIB examined the main gearbox and the bevel gear vertical shaft, which had been installed approximately 167 flying hours prior to the accident. A strip examination of the MGB revealed that the vertical shaft had failed, causing the lower part of the shaft to move and damage the outer race retainer of a roller bearing. This movement caused the pinion to partially disengage from the oil pump drive gears, generating metal debris that triggered the magnetic chip detector.
Analysis of the shaft fracture surface identified three distinct fatigue cracks. These cracks originated from the 4.2 mm diameter hole used in the electron beam welding process. The investigation also noted that the surface of the hole contained tooling marks and small corrosion pits. Furthermore, a review of the Health and Usage Monitoring System (HUMS) data showed an increase in vibration levels for specific indicators in the six hours preceding the accident.
Findings
- The primary cause of the incident was a fatigue failure of the bevel gear vertical shaft.
- The fatigue cracks initiated at the 4.2 mm diameter hole within the weld area, likely triggered by small corrosion pits and surface irregularities.
- The failure of the shaft led to the disengagement of the pinion from the oil pump drive gears, resulting in the loss of lubrication pressure.
- Manufacturing variations, including tooling marks and non-conforming countersink geometries, contributed to the crack initiation.