Accident Sequence
On the morning of the accident, a Bell 212 helicopter (serial number 30525) was on approach to a prepared landing site in the Monashee Mountains near Mica Creek, British Columbia. The helicopter carried a pilot, 11 passengers, and a ski-guide. At approximately 200 feet from touchdown, the occupants heard a loud explosion and saw several cockpit warning lights illuminate; simultaneously, the number 1 engine stopped. The pilot continued the approach, but the low rotor rpm warning horn sounded and the helicopter began a gradual right turn, forcing the pilot to land in an unprepared area. The initial touchdown was not hard, but the helicopter then tipped over onto its left side in waist-deep snow, and the main rotor blades struck the surface. The main transmission was ripped out, and one rotor blade struck the cockpit roof, severely damaging the overhead electrical circuit breaker panels. The 11 passengers in the cabin evacuated without difficulty using the right-hand sliding door. The ski-guide, seated in the left pilot seat, broke open and escaped through the roof window, then returned with two passengers to extract the pilot through the same opening. After the rollover, the number 2 engine continued to run for 15 minutes until another loud explosion was heard, and it stopped. Two small engine bay fires began but quickly self-extinguished. The helicopter was substantially damaged. The emergency locator transmitter (ELT) activated, and its signal was received by the SARSAT network.
Investigation Findings
The pilot was certified, trained, and qualified for the flight in accordance with regulations. Weather at the accident site was reported as more than 4 miles visibility in very light snow, with an overcast or broken cloud layer at about 7,500 feet and a light south wind. The accident occurred at an elevation of 5,500 feet, about 50 feet from the intended landing site. Weight and center of gravity were within prescribed limits. Performance charts indicated the helicopter was capable of a single-engine rate of climb of 250 feet per minute under the existing conditions, but the pilot was committed to touchdown because the helicopter was in a slow, shallow descent when the number 1 engine failed, and was too slow and too close to the ground to arrest the rate of descent with one engine. Main rotor rpm decayed, forcing the landing at an unprepared site.
Examination revealed that both engines suffered uncontained power turbine failures. The number 1 engine power turbine wheel had shed about three-quarters of its blades, with the remaining quarter grouped on one section. The failures were traced to failures in the drive train from the engine to the final output shaft. The primary failure was the shearing of the number 1 engine clutch coupling shaft due to excessive loads when the worn clutch slipped and suddenly re-engaged. This caused the power turbine wheel to overspeed sufficiently for the blades to fracture. The resulting imbalance placed extreme loads on the number 5 bearing. The number 2 engine continued running while the helicopter was on its side and eventually failed due to loss of lubrication.
No single factor was identified that caused the clutch to wear. Bearing races likely wore while the clutch was engaged, with ball bearings wearing grooves into the races due to drive system vibrations. The grooves would have caused greater stress on the outer race during clutch freewheeling, accelerating bearing wear. Wear on clutch engagement surfaces may have been caused by contamination from bearing wear metals. Neither the tail rotor system torque overload experienced two weeks before the accident nor the C-box chip light warning eight days before led maintenance staff to suspect a clutch malfunction. The operational environment was not unusual, and no operational practice was identified that contributed to abnormal clutch wear. There is no significant history of worn or slipping clutches on the Bell 212 helicopter.
Cause and Contributing Factors
The number 1 engine clutch coupling shaft sheared because of excessive loads placed on it by the worn and slipping clutch. The power turbine then oversped, causing the engine to lose power and forcing the pilot to land at an unprepared landing site.
Safety Action Taken
After the accident, the operator introduced a cost-sharing program with its pilots to assist them in purchasing flight helmets.