Pilots in the dark on risks of tackling turbulence at speed

OPERATORS of Rockwell Aero-Commander aircraft have been warned to review procedures about flying in or near severe turbulence after the in-flight break-up of a plane over Victoria.

OPERATORS of Rockwell Aero-Commander aircraft have been warned to review procedures about flying in or near severe turbulence after the in-flight break-up of a plane over Victoria.

An Australian Transport Safety Bureau investigation found that some pilots of the aircraft were unaware of the importance of aircraft's manoeuvering speed during flight through turbulence. Two people died when a Rockwell Aero-Commander 500S broke up in severe turbulence during a night business flight from Essendon to Shepparton on July 31, 2007. The aircraft was cruising at 7000ft in Class C controlled airspace about 46km north-northeast of Essendon when radar and radio contact were lost.

At the time, special weather reports were current for severe turbulence and severe mountain waves -- waves in atmosphere caused when wind moves over mountains -- and 50-knot wind speeds were reported on the ground.

ATSB calculations show the plane had been in cruise flight at speeds likely to have been greater than its published manoeuvering speed prior to the loss of the radar contact.

It said the wreckage and its distribution pattern were consistent with an in-flight break-up in straight flight and a reconstruction determined it happened after the separation of both the right and left outer wings at symmetrical locations. There was no evidence of any defect, corrosion or fatigue in the aircraft structure.

"The break-up of the aircraft was consistent with it being subjected to rapid and extreme aerodynamic forces during normal cruise flight at 7000ft," it said. The ATSB report cited a number of recent instances of Aero-Commander in-flight structural failures where fatigue, corrosion or intentional manoeuvering were not identified as contributing factors.

Investigators found that some pilots operating the aircraft were unaware of potential problems with the plane's manoeuvering speed during flight through turbulence.

There was also concern manufacturer documentation did not provide guidance on the issue. It said transient airspeed fluctuations could occur in turbulence, resulting in an aircraft travelling at, or above, its manoeuvering speed. Using a slower speed with a margin appropriate to the conditions could avoid the problem.

"There was also 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," the ATSB said.

The bureau has reissued its publication Mountain Wave Turbulence and distributed its investigation report to all Australian Aero-Commander operators.