An Australian soldier killed in a helicopter crash in Afghanistan should never have been on the aircraft, let alone riding on its open rear ramp, a defence inquiry has found.
It found the cause of the crash was not enemy action but the twin-rotor Chinook "porpoising", or oscillating violently.
The crew responded incorrectly and the aircraft crashed, killing Lieutenant Marcus Case, 26. He was the 26th Australian to die in Afghanistan.
The commission of inquiry report followed an earlier technical probe.
The accident happened on May 30, 2011, in Afghanistan's Zabul province.
Lt Case, a member of the Australian unit operating Heron unmanned surveillance aircraft, flew as a passenger on CH-47D Chinook known as Brahma 12.
The report says his presence on the aircraft was not essential to his own mission or that of Brahma 12.
Further, he was categorised as crew and allowed to ride on the aircraft's open rear ramp.
He was wearing a safety harness and attached to the aircraft by a 4.4m strap.
The inquiry found the Chinook was susceptible to "pitch oscillations during operations at high-density altitude" and that's what occurred on this flight.
Lt Case was thrown clear, dangling about two metres below the aircraft on the safety strap.
He died when the aircraft crashed. The aircraft caught fire and was destroyed but all other crew members survived.
The commission said the pilot and co-pilot believed the Chinook's electronic flight control system could correct pitch disturbances and the appropriate response was for them to do nothing with the controls.
That did not work and on the fourth oscillation, the aircraft exceeded 90 degrees nose-down.
Only then did the captain take the controls, with the aircraft flying near straight and level when it hit the ground.
Defence agreed to 19 recommendations from the inquiry and 35 from the accident investigation, including improvements in pilot training to properly respond to porpoising.
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