Accident Overview
On the day of the accident, a Bell 407 helicopter (registration C-GNVI, serial number 53847), operated by VIH Helicopters Ltd., was transporting 5 skiers and 1 guide to a drop-off site at approximately 6000 feet above sea level, 8 nautical miles southeast of Blue River, British Columbia. At 1035 Pacific Standard Time, as the helicopter neared the site, a bang and shudder were heard and felt, immediately followed by low rotor rpm and engine-out warning horns. Moments later, the helicopter landed heavily. The pilot and the ski guide, seated in the right and left front seats respectively, sustained back injuries. The 5 skiers in the middle and rear seats were not injured. The helicopter was substantially damaged. The emergency locator transmitter activated automatically on impact and was manually shut off by the pilot once help arrived. There was no fire.
Sequence of Events
Immediately prior to the power loss, the helicopter was climbing at about 65 knots, roughly 200 feet above ground level over rising terrain. The intended landing site was about 100 feet vertically above and 700 feet laterally from the helicopter when the bang and shudder (later identified as a compressor stall) occurred. The engine sound had begun changing about 5 seconds prior to the bang and shudder. The main rotor low rpm warning, which activates when main rotor rpm falls below 95%, occurred 1 second after the bang and shudder. The engine-out warning, activating when gas producer rpm falls below 55%, occurred 4 seconds later. Main rotor rpm decayed from 100% to 66% by the time the helicopter hit the snow-covered terrain. The moment from the bang and shudder to impact was about 6 seconds. The helicopter remained upright but sustained structural damage consistent with low rotor rpm during a hard landing on uneven, snow-covered ground. There were no chip lights or abnormal instrument indications prior to the low rotor rpm warning. Weather at the time was suitable for VFR flight, and the pilot was certified and qualified for the flight.
Engine and Maintenance Examination
Examination of the Rolls Royce M250-C47B engine (serial number CAE-848126) determined that the No. 2 bearing, supporting the aft end of the compressor, had failed. When the bearing failed, the compressor rotor moved forward until it contacted the shroud, causing a surge (compressor stall), engine spool down, and power loss. Because the power loss occurred on final approach at low airspeed and low height above uneven ground, it resulted in a hard landing. The helicopter had been serviced and maintained in accordance with existing directives and operated within approved limits. The chip detection system was functional, the bearing had been correctly installed and adequately lubricated, and the bearing components met specified alloys and hardnesses with no pre-existing deficiencies. The maintenance history was unremarkable except that the power turbine section had been replaced about 437 hours prior due to a No. 5 bearing chip detection warning, which the engine manufacturer stated was unrelated to this accident.
Bearing Failure Analysis
The No. 2 bearing was manufactured with a tolerance stack-up that made it prone to large ball excursions at manoeuvre rates less than the design manoeuvre envelope. The ball excursions led to separator fatigue followed by rapid failure of the bearing and subsequent engine failure. This failure was unusual in its rapidity, lack of preceding chip detection warning, and distinctive damage patterns. On 11 April 2011, Rolls-Royce issued a Commercial Engine Bulletin requiring replacement of affected No. 2 bearings with an improved design (part number M250-10354) at the next engine or module overhaul. Transport Canada and the Federal Aviation Administration concluded that the probability of another similar bearing failure was too low to warrant additional safety action beyond the bulletin.
Conclusion
The Transportation Safety Board concluded its investigation, and the report was authorized for release on 12 October 2011.