What happened
On 21 April 2000, an Aerospatelle (Eurocopter) AS355F2, registration G-SAEW, was performing a public transport mission for the South Wales and Gwent Police forces. While hovering at an altitude of 500 to 600 feet over a residential area near the M4 motorway in Cardiff, the aircraft experienced a sudden, uncommanded yaw to the left of approximately 180 degrees.
Despite the pilot's attempts to counter the rotation using full right pedal and adjusting the collective to maintain control, the helicopter entered a steep, spiralling descent. The pilot, unable to recover the aircraft, focused on maintaining a level attitude and prepared for impact. The helicopter ultimately struck the pitched roof of a semi-detached house, embedding itself in the structure. The three crew members on board, all wearing protective helmets, escaped the wreckage without injury.
The investigation
The AAIB investigation focused on the tail rotor pitch change unit (PCU) following evidence of its disruption. Examination of the PCU revealed that the bearing clamping nut had detached, caused by the failure of the threads on the aluminium alloy shaft. This detachment allowed the rotating spider to move axially, resulting in a total loss of tail rotor blade pitch control and subsequent loss of yaw control.
Investigators found that the assembly had likely been loose since its recent maintenance. Evidence showed that the required anti-corrosion chromate compound had not been applied to the threads and pressure face of the nut. Furthermore, the investigation noted that the anti-fretting varnish on the rotating spider was missing in the bearing location area, and the stationary spider housing showed signs that the bearing had not been installed squarely. The maintenance task, which had been performed approximately 20 flying hours prior, was carried out by engineers who had not previously replaced this specific component.
Findings
- The loss of yaw control was caused by the detachment of the PCU bearing clamping nut due to sheared threads on the aluminium alloy shaft.
- The threads were weakened by a fretting process that occurred because the assembly was insufficiently tight since installation.
- The clamping force was likely reduced because the required anti-corrosion chromate compound was missing from the nut threads and pressure face.
- The installation of the new bearing was complicated by the absence of appropriate anti-fretting varnish on the rotating spider shaft.
- The maintenance task was performed at a line maintenance level without the use of a calibrated oven or the specific technical manuals required for the task.