What happened
On a flight from Massat Airport to a fishing lodge on Langara Island, British Columbia, a Sikorsky S-76A helicopter, registration C-GHJT, experienced a sudden loss of power during its final approach. While the pilot was attempting to balance engine torques, a loud bang and an engine-out warning signaled that the No. 2 engine had failed. At the time of the failure, the aircraft was approximately 49 feet above the water and 75 feet from the intended floating helipad.
Realizing that a landing on the small floating pad was not possible under single-engine conditions, the pilot performed a controlled ditching in the water adjacent to the helipad. The emergency flotation system deployed successfully, and the pilot and passenger evacuated the aircraft into a nearby boat. No injuries were reported. However, shortly after the engines were shut down, the helicopter began to list to the starboard side. Approximately four minutes after the ditching, the aircraft rolled completely inverted in the water.
The investigation
Investigators examined the mechanical failure of the Allison/Rolls Royce 250-C30S engine and the subsequent failure of the flotation system. The investigation focused on why the engine failed in flight and why the emergency floats failed to maintain the aircraft's upright position.
Findings
- The primary cause of the power loss was the failure of the engine outer combustion case (OCC) due to a fatigue crack that went undetected by existing inspection protocols.
- The engine failure occurred after the point where a single-engine overshoot landing could have been attempted, leaving the pilot with no choice but to ditch.
- While the emergency flotation system deployed, the starboard nose float cells deflated because of aged, degraded bonding at a seam.
- The loss of buoyancy in the starboard section caused an imbalance, leading the helicopter to roll over.
- Existing manufacturer-recommended inspections for both the engine's outer combustion case and the emergency flotation system were insufficient to detect cracks or assess the integrity of the aged bonded seams.