Investigation Summary
On July 13, 1988, the helicopter was transporting 19 oil workers from the Safe Felicia platform to Sumburgh. Approximately 43 minutes into the flight, the crew heard a muffled bang followed by a fire warning for the No. 2 engine. The bang was caused by the failure of a bearing in the No. 2 engine input drive, which led to the high-speed drive shaft bending and severing its support tube. Leaking engine oil was ignited by the heat of the friction. As the crew initiated an emergency descent, a second fire warning illuminated for the No. 1 engine, and passengers observed oil leaking from the cabin ceiling. Smoke quickly filled the cabin, forcing the pilot to execute an immediate ditching. The aircraft remained afloat long enough for a full evacuation, but a fierce fire, fueled by the magnesium construction of the main gearbox housing and the lack of a firewall between the engine and gearbox bays, eventually destroyed the upper fuselage. The wreckage was recovered in two stages during July and August 1988, allowing investigators to identify the gearbox as the seat of the fire.
Final Conclusions
The AAIB concluded that the fire originated in the main gearbox bay, likely triggered by a bearing failure in the No. 2 engine input drive. The investigation highlighted a critical design vulnerability: the S-61N gearbox bay was not equipped with fire detection or extinguishing systems, nor was it separated from the engine bays by adequate firewalls. This allowed a localized mechanical failure to escalate into a dual-engine fire warning and a life-threatening cabin fire. The formal report, Aircraft Accident Report 3/90, was published in 1990.
Video Analysis
No video analysis linked for this case file yet.
Systems & Failure Modes
Main Gearbox (MGB) Bay
The area housing the transmission; the housing was constructed of magnesium, which significantly fueled the fire once ignited by leaking oil. At the time, it lacked fire detection and suppression systems.
Firewall Integrity
The barriers intended to prevent fire spreading between the engines and the gearbox were found to be insufficient to contain the gearbox bay fire, particularly where the support tube passed through.
Emergency Flotation Gear
The floats were successfully deployed by the crew, allowing the aircraft to remain buoyant for the evacuation despite the fire.
No. 2 Engine Input Drive
A bearing failure in this component caused the high-speed drive shaft to bend and sever the support tube, initiating the oil leak and fire.
Interesting Facts
- 01All 21 people on board survived the ditching and subsequent fire.
- 02The fire was so intense that it consumed most of the floating helicopter before it sank.
- 03The recovery of the engines and transmission required two separate diving operations in July and August 1988.
- 04The investigation resulted in 27 safety recommendations to the Civil Aviation Authority (CAA).
- 05The aircraft (MSN 61223) was manufactured in 1964.
Safety Actions & Advisories
Gearbox Fire Protection
Recommendations were made to install fire detection and suppression systems in the main gearbox bays of transport helicopters.
Maintenance and Detection
The AAIB called for improvements in maintenance procedures and early detection methods for bearing failures in helicopter transmissions.