The investigation of an in-flight breakup that occurred near Clombinane, Victoria on 31 July 2007 has found that it most likely resulted from an encounter with localised and intense turbulence, from an elevator control input, or from a combination of both. The accident resulted in the death of the pilot and passenger on board the Rockwell International Aero Commander 500-S aircraft on a business flight from Essendon Airport to Shepparton.
As a result of its investigation, the Australian Transport Safety Bureau reissued the publication Mountain Wave Turbulence (available for download at www.atsb.gov.au), distributed the investigation report to all Australian operators of the Aero Commander aircraft, and issued a safety advisory notice to aircraft operators and pilots. That notice encouraged aircraft operators to review their procedures to ensure an appropriate awareness amongst operating personnel of the implications for aircraft performance of the combination of aircraft weights and speed, and of the ambient conditions; in particular, when flying in, or near areas of forecast severe turbulence.
The investigation found that some pilots operating the aircraft type were generally unaware of the applicability of the aircraft's manoeuvring speed during flight through turbulence, despite the inclusion of relevant advisory information in the operator's documentation. There was also a concern that pilots generally may not have been exercising as much caution in forecast severe turbulence conditions as they would for thunderstorms, even though the intensity of the turbulence could be similar.
At the time of the in-flight breakup, special weather reports for severe turbulence and severe mountain waves were current for the area. Wind speeds on the ground were reported to be 50 kts and calculations using the recorded radar data and forecast wind showed that the aircraft had been in cruise flight at 7,000 ft above mean sea level at speeds probably greater than its published manoeuvring speed, prior to it disappearing from radar. The wreckage and its distribution pattern were consistent with an in-flight breakup during cruise flight.
There was no evidence of any pre-existing defect, corrosion or fatigue found in the aircraft structure. An examination of the wreckage and fracture surfaces showed that the aircraft structure failed under symmetrical negative overstress.
A full report is available from the ATSB website Aviation Occurrence AO-2007-029