What happened
On 13 October 2006, an Aerospatiale (Eurocopter) AS 332L Super Puma, registration G-PUMI, was performing a scheduled takeoff from Runway 14 at Aberdeen Airport. The aircraft was configured for an offshore mission to North Sea oil platforms, carrying 13 passengers.
Approximately five seconds after liftoff, the crew experienced a loud bang followed by intense, abnormal vibrations. The pilot immediately rejected the takeoff and returned the aircraft to the runway. As the helicopter attempted to taxi, the severe vibration persisted, prompting the commander to halt the aircraft and shut down the engines on the threshold of Runway 32. There were no fatalities or injuries during the incident.
The investigation
Technical examination of the wreckage revealed that a main rotor blade spindle had suffered a fracture. The failure occurred through the lower section of the attachment yoke on the leading side of the spindle. The force of the fracture caused significant plastic deformation, stretching the lug open by approximately 12 mm.
Investigators analyzed the maintenance history and found that the tie bolt had been torqued to its specified dry value approximately 175 hours prior to the failure, despite the presence of grease during reinstallation. Further research involving flight tests with strain gauge instrumentation established that the stresses acting on the failure site were significantly higher than those anticipated during original certification testing, particularly during turning maneuvers.
Findings
- Wear on the flapping hinge inner race contributed to the instability.
- Excessive clamping pre-load was created because the tie bolt was torqued using dry values while the assembly was lubricated with grease.
- Significant hoop stresses were present in the yoke bore due to adverse tolerance stacking and the interference fit of the bush.
- Increased flight loads were generated by the action of the lead-lag dampers, with stresses during turns being notably higher than those used in certification models.
- A minor deviation in the corner radius profile of the yoke bore created a localized area of stress concentration.