Investigation Summary
The accident was initiated by the failure of a splined sleeve connecting the right-hand engine to the main gearbox. Fatigue cracks had developed in the sleeve over several days, eventually causing it to disintegrate. This led to the severance of the high-speed Bendix shaft. Without the load of the rotor system, the right-hand power turbine accelerated instantly to an extreme overspeed condition. The turbine disk burst, and the resulting high-energy fragments pierced the engine casing, destroying the left-hand engine and severing the mechanical control rods located in the deck area above the engines. The crew experienced intermittent overspeed warnings minutes before the final failure but, lacking specific procedures for such symptoms, continued the flight toward their destination. The wreckage was eventually recovered from a depth of 830 meters.
Final Conclusions
The Norwegian Accident Investigation Board (AIBN) concluded that the primary cause was a fatigue failure of the engine-to-gearbox coupling. The investigation found that the overspeed protection system was compromised by the disintegrating sleeve before it could effectively shut down the engine. Furthermore, the AIBN noted that a HUMS accelerometer meant to monitor this specific area was unserviceable; however, other recorded data showed vibration levels exceeding setpoints for several days prior. Had this data been systematically reviewed by maintenance personnel, the impending failure could have been detected. The final report (2001/47) issued 18 safety recommendations, primarily focusing on HUMS integration and turbine overspeed protection.
Video Analysis
No video analysis linked for this case file yet.
Airframe & Maintenance
Systems & Failure Modes
Bendix Shaft
A high-speed drive shaft connecting the engine's power turbine to the main gearbox. Its failure led to the turbine overspeed.
HUMS (Health and Usage Monitoring System)
A system designed to monitor vibrations in rotating components to detect early signs of mechanical wear or failure.
Overspeed Protection System
A safety system intended to shut down the engine if the turbine exceeds safe RPM limits; it failed to prevent the burst due to the nature of the mechanical disintegration.
Interesting Facts
- 01The fatigue cracks in the splined sleeve had been developing for several days prior to the accident.
- 02The wreckage was located on the seabed at a depth of 830 metres (2,720 ft).
- 03The right-hand engine turbine burst destroyed the left-hand engine and severed flight control rods.
- 04HUMS data showed vibration setpoints were exceeded before the accident flight, but the data was not analyzed in time.
- 05At the time of the accident, HUMS was not a regulatory requirement for offshore operations.
- 06The aircraft, MSN 2344, was manufactured in 1991.
Safety Actions & Advisories
Mandatory HUMS
The AIBN recommended that HUMS be made mandatory for all offshore helicopter operations to ensure early detection of mechanical fatigue.
Turbine Burst Protection
Recommendations were made to improve the containment of engine turbine fragments and protect critical flight control rods from catastrophic engine failures.