What happened
On 15 July 2002, a Sikorsky S-61N helicopter, registration G-BBHM, was conducting search and rescue operations near Poole, Dorset. During the flight, the No 2 engine experienced a rapid deterioration of its No 5 free turbine bearing. This failure caused the adjacent carbon oil seal to fail, leading to mechanical interference between the Main Drive Shaft Thomas coupling and the Engine Mounting Rear Support Assembly tube. The resulting force was sufficient to completely sever the support tube.
As engine oil was released, it ignited—either due to the mechanical interference or contact with the hot engine exhaust duct—triggering a severe fire outside of the designated engine fire zone. The incident occurred while the aircraft was operating in its standard search and rescue capacity.
The investigation
The AAIB investigation focused on the mechanical failure of the engine components and the subsequent fire. Investigators examined the wreckage, including the engine strip examination, the Main Gear Box, and the Main Drive Shaft assemblies. Metallurgical tests and electron microscopy were performed to understand the bearing failure and the fracture of the oil jet assembly.
Data from the Health and Usage Monitoring System (HUMS) and the flight recorders were analyzed. The investigation looked into previous engine starts to determine if high torque rotor engagements had contributed to the instability of the drive shaft. The team also reviewed the maintenance and operational procedures regarding torque limits during engine start-up.
Findings
- The primary cause of the accident was the rapid deterioration of the No 2 engine's No 5 bearing, which led to the failure of the carbon oil seal and the severing of the engine mounting rear support tube.
- The bearing failure was driven by unusual and excessive cyclic loading conditions resulting from shaft vibration.
- This vibration was likely caused by damage or distortion sustained during previous engine starts that involved high torque rotor engagements.
- The bearing's degradation was further worsened by a reduction in oil supply, as the vibration caused the live oil jet to fracture.
- A severe fire occurred outside the engine fire zone because released engine oil ignited upon contact with hot components.
- At the time of the accident, the manufacturer's Flight Manual used by the operator did not contain a specific torque limitation for rotor engagement following an engine start.