What happened
At approximately 1200 Pacific standard time, a Sikorsky S-61N helicopter, registration C-FHHD, departed from a service landing area near Lake Errock, British Columbia. The flight, intended for engine performance adjustments, carried two pilots and an aircraft maintenance engineer. Shortly after takeoff, while climbing through 1,000 feet, the crew heard a loud bang and an intensifying whine. The number 1 engine immediately lost power, and the number 2 engine failed to compensate for the loss.
As the pilot-in-command initiated an autorotative descent, smoke and flames appeared in the cabin near the main rotor gearbox. The pilot managed a southwest landing on Highway 7. During the final moments before touchdown, the crew encountered powerlines, prompting an increase in collective to avoid them. While the helicopter landed at approximately 20 knots, it struck additional powerlines and a large tree. The impact caused the tail rotor and pylon to break away and triggered severe airframe oscillations. The three occupants sustained minor injuries, though the aircraft suffered substantial damage.
The investigation
The investigation focused on the mechanical failure of the main gearbox and the engine's inability to maintain power. Investigators examined the plain bearing in the main gearbox cover for the number 1 input pinion gear. Evidence suggested that the bearing failed due to a loss of lubrication, leading to overheating and massive wear. This caused the input pinion gear to fracture, resulting in the number 1 engine overspeeding and shutting down. Simultaneously, the disintegration of a carbon seal allowed gearbox oil to spray onto the overheated pinion, igniting a brief fire.
Regarding the number 2 engine, the investigation found that a restricted pressure regulating valve in the fuel control unit prevented the engine from rapidly assuming the load from the failed engine. The investigation also noted that while the pilots wore helmets and lap belts, they were not using available shoulder harnesses. The maintenance engineer was not seated, contributing to injuries during the ground oscillations.
Findings
- The failure of the plain bearing in the main gearbox cover led to rotational imbalance and the overheating of the input pinion gear.
- The resulting bending forces caused the input pinion gear to fracture, triggering the shutdown of the number 1 engine.
- The number 2 engine was unable to assume the total load due to a restricted pressure regulating valve in the fuel control unit.
- The collision with a tree during landing caused severe vibrations that broke the engine mounts and the FOD shield.
- The lack of shoulder harness use by the pilots increased the risk of head injuries during the ground impact.