What happened
During a ferry flight from Tumbler Ridge, British Columbia, to Springbank, Alberta, the pilot of C-FHAG, an Aerospatile AS350BA, experienced escalating vibrations from the main rotor. While cruising at 5,500 feet, the pilot noticed the vibration increasing in intensity. Approximately 10 minutes later, the pilot attempted a precautionary landing at an abandoned well site in a wooded area. During the flare for landing, the pilot lost control of the Aerospatiale AS350BA, causing the aircraft to strike the ground and come to rest on its right side. The pilot sustained minor injuries, but the helicopter suffered substantial damage.
The investigation
The investigation focused on the failure of the main rotor head components and the maintenance history of the aircraft. Investigators examined the spherical thrust bearings (STBs) and found that the elastomeric center section of the red blade STB had debonded and detached from the rotor head. Evidence indicated that the bearing had failed due to fatigue.
Maintenance records revealed that approximately 81 flight hours prior to the accident, an aircraft maintenance engineer had identified a blister on one of the STBs. However, the engineer released the aircraft for service, intending to replace the bearing during a scheduled 500-hour inspection. The investigation also looked into the pilot's awareness of the vibration and the adequacy of the manufacturer's warnings and the flight manual's instructions regarding STB degradation.
Findings
- The primary cause of the loss of control was the fatigue failure of a spherical thrust bearing on the main rotor head.
- The aircraft was released for service despite the fact that the STB's condition exceeded the manufacturer's rejection criteria.
- The pilot believed the increasing vibrations were due to the rotor blades being out of track rather than a critical component failure.
- The flight manual lacked specific warnings or corrective actions for pilots to take when encountering sudden, abnormal vibration levels indicative of STB failure.
- While the manufacturer had issued urgent fleet telexes regarding STB inspections, this information was not integrated into the aircraft's flight manual, leaving the pilot potentially unaware of the risks.