What happened
On 13 October 2006, an Aerospatiale (Eurocopter) AS 332L Super Puma, registration G-PUMI, was departing from Runway 14 at Aberdeen Airport, Scotland. The aircraft, operated by Bristow Helicopters Limited, was performing a flight to the Britannia Platform carrying 13 passengers.
Approximately five seconds after liftoff, the flight crew experienced a loud bang followed by intense, abnormal vibrations. The crew immediately initiated a rejected takeoff, returning the aircraft to Runway 14. The severe vibration persisted as the helicopter taxied, prompting the commander to stop and shut down the engines at the threshold of Runway 3 and allow the passengers to disembark. There were no fatalities or injuries during the incident.
The investigation
The investigation focused on the structural failure of the main rotor blade spindle. Inspectors examined the aircraft's components and determined that the lower section of the attachment yoke on the leading side of the spindle had fractured.
Analysis of the fracture surfaces revealed that the failure was a fatigue-related event. The crack had originated at the corner radius of the inner end of the bore in the yoke. This crack propagated through approximately 90% of the cross-section before the remaining material could no longer support the load, leading to the final rupture. The investigation also looked into the assembly history of the component, noting that the tie bolt had been torqued to a specified dry value approximately 175 hours prior to the failure, despite the presence of grease.
Findings
- The primary cause of the yoke failure was excessive clamping pre-load caused by the tie bolt being torqued to a dry value while in the presence of grease.
- Wear on the flapping hinge inner race contributed to the failure.
- Significant hoop stresses were present in the bore of the yoke due to the interference fit of the bush and adverse tolerance stacking.
- The failure was a fatigue fracture that had progressed through several dozen rotor starts prior to the final rupture.
- Flight loads during the operation were slightly higher than those experienced during standard level flight cruise conditions.
- Trace amounts of grease likely contaminated the tie bolt during reinstallation, which was not detectable by the maintenance personnel.