Test pilots of a Virgin Galactic passenger spaceship that crashed – killing the co-pilot – last year were unaware of the catastrophic consequences of unlocking the craft's moveable tail section too early because of poor training, investigators probing the fatal accident said on 28 July.

The premature unlocking of SpaceShipTwo's hinged tail triggered a mid-air break up of the ship during its fourth powered test flight on 31 October over California's Mojave Desert, the National Transportation Safety Board (NTSB) has determined.

"What we saw led us to explore the extent to which Scaled Composites considered human factors in this launch," chairman of NTSB Christopher Hart said in his opening statement.

Scaled Composites, a company the developed the craft and employed its test crew, provided inadequate training to co-pilot 39-year-old Michael Alsbury, who unlocked the tail, and died in the crash, and pilot Peter Siebold, the NTSB said.

Scaled Composites, a subsidiary of Northrop Grumman, should have informed the crew of what could happen to the craft when the tail section was unlocked prematurely. The company did not consider that a pilot would induce that kind of failure, lead investigator Lorenda Ward said during the hearing, monitored via webcast.

"I think that each step that you [Scaled Composites] add for a pilot to do it adds complexity and with the increased complexity comes the increased possibility of an error," NTSB member Robert Sumwalt said.

The company designed the two-pilot, six-passenger vehicle, for Virgin Galactic, a US offshoot of Richard Branson's London-based Virgin Group.

The Spaceship Company, a Virgin-owned operation based in Mojave, has taken over construction of SpaceShipTwo vehicles from Scaled Composites. Test flights of its new ship are scheduled to begin before the end of the year.

Siebold, 43, managed to parachute to safety.

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