Airframe vibration Wollongong Aerodrome, New South Wales, on 11 June 2008, VH-UAH, Bell Helicopter Co. 412

On the morning of 11 June 2008, a Bell 412 helicopter, registered VH-UAH, was being used to conduct training operations from Wollongong Aerodrome, New South Wales. Shortly after landing on the runway, the helicopter developed severe vertical airframe vibrations that resulted in reduced pilot control. In an attempt to mitigate the vibrations, the pilot raised the helicopter into the hover, however, the vibrations continued to increase in severity. In response, the pilot lowered the collective to set the helicopter back down onto the runway. The resulting heavy landing caused serious damage to the helicopter, but the crew were not injured.

A subsequent examination of the helicopter's flight control system revealed an anomaly with the collective hydraulic actuator. Excessive free play was found to have developed in the bolted joint between the pivot bolt and the pilot input lever, which then allowed vertical vibrations and controllability issues to develop. It is likely that free play at the bolted joint was introduced when the collective actuator was last overhauled.

As a result of this occurrence, the collective actuator manufacturer revised the tensioning procedures and requirements for the pivot bolt assembly during the overhaul process. In addition, the helicopter operator changed its inspection regime of the collective servo-hydraulic actuator units in its fleet of Bell 412 helicopters and issued a 'flight staff instruction' to provide guidance to pilots on what actions to take if they experienced unusual or excessive vibrations during flight.