What happened
On the morning of May 10, 2000, a Bell 47 helicopter departed from the south ramp of Abbotsford Airport under visual meteorological conditions. Shortly after takeoff, while climbing through approximately 700 feet above ground level, the aircraft lost tail-rotor thrust and began a right-hand spin around its main-rotor mast. As the descent continued, the nose dropped, and the rotation transitioned into a steep spiral. The aircraft became completely uncontrollable, striking the airport infield in a nose-down attitude. The impact caused the helicopter to break apart and ignited a post-crash fire. The two fatalities resulted from the forces of the impact.
The investigation
Investigators examined the wreckage and discovered that the tail-rotor gearbox gears had suffered heat distortion and had uncoupled. Further analysis at specialized facilities confirmed that the gears had overheated and smeared due to a total lack of oil in the gearbox. No traces of oil or burnt oil residue were found within the unit.
Maintenance records showed that a 100-hour inspection had been performed on the afternoon of May 9, 2000. During this service, an apprentice engineer was tasked with changing the tail-rotor gearbox oil. While the oil was drained and the drain plug was secured, the oil was not actually replaced. Despite this, the aircraft journey log book was signed off as having completed the inspection.
On the morning of the accident, the student pilot performed a pre-flight inspection, which included a visual check of the oil level through a small sight gauge. However, the lack of oil went undetected by the student, the instructor, and the maintenance personnel. It was noted that reading the oil level through the gauge can be difficult due to the visibility of the window.
Findings
- The primary cause of the accident was the failure to replace the tail-rotor gearbox oil during a recent 100-hour inspection.
- The absence of lubrication caused the gearbox gears to overheat, smear, and disengage, resulting in the loss of tail-rotor thrust and yaw control.
- The pilot did not immediately initiate the autorotative descent required by the flight manual following the loss of yaw control.
- The pilot's attempt to use airspeed to stabilize the aircraft via the vertical stabilizer was unsuccessful, leading to a loss of attitude control and a subsequent spiral.