Accident Sequence
On May 10, 2000, at approximately 0850 Pacific daylight time, a Bell 47 helicopter (serial number 2211) departed from the south ramp of Abbotsford Airport in visual meteorological conditions. The helicopter was piloted by a student pilot and a flight instructor. As the helicopter climbed through about 700 feet above ground level, still over the airport, it lost tail-rotor thrust and began to spin to the right. The nose then dropped and the spinning turned into a spiral. The helicopter struck the ground in a steep, nose-down attitude on the airport infield, broke apart, and a post-impact fire ensued. Both occupants were fatally injured by the impact forces.
Maintenance and Inspection
On the afternoon of May 9, 2000, maintenance personnel conducted a 100-hour inspection on the helicopter. The inspection required changing the tail-rotor gearbox oil. An apprentice aircraft maintenance engineer (AME) drained the oil, inspected it for metal particles, and installed and lock-wired the drain plug. He initialed the inspection check sheet item for draining and refilling. The AME signed the journey log book as having completed the inspection. However, the oil was not replaced.
On the morning of the accident, the student pilot conducted a pre-flight inspection, which included visually checking the oil level in the tail-rotor gearbox through a small sight gauge. The instructor was not involved but was aware that a 100-hour inspection had been completed and a control cable had been changed. The helicopter operated for about 15 minutes on the ground and about 2 minutes in the air before the loss of yaw control.
Investigation Findings
Inspection of the wreckage revealed damage to the tail rotor consistent with a tail rotor not turning on impact. The tail-rotor gearbox gears showed heat distortion and had uncoupled. No remnant of oil or burnt oil was found in the gearbox. The control system, including forward cables for the horizontal stabilizer, showed no anomalies. It was concluded that no oil was in the gearbox when the helicopter began operating. The lack of oil was not detected prior to flight by the apprentice AME, the AME, the student, or the instructor. The apprentice AME had experience as an aircraft technician and had just completed training for a commercial helicopter pilot's license. The AME had 23 years of helicopter maintenance experience. Both pilots were appropriately certificated; the student had about 50 hours of flight time, and the instructor had about 2100 hours, with 600 of instruction. Neither maintenance personnel nor pilots were on duty for extraordinary time.
Probable Cause
The Transportation Safety Board determined that maintenance was conducted (100-hour inspection) on the helicopter just before the accident flight, and oil was removed from the tail-rotor gearbox but not replaced; the inspection was signed as completed. The apprentice AME, the AME, the student pilot, and the instructor pilot did not detect the lack of oil. Due to lack of oil, the gears overheated, smeared, and disengaged, causing loss of tail-rotor thrust and yaw control. When yaw control was lost, the pilot did not immediately execute an autorotative descent, was unable to stop the rotation, and subsequently lost total control.