The premature in-flight deployment of the feathered tail plane speed braking system was the primary cause of the destruction of the Virgin Galactic SpaceShipTwo (VSS Enterprise) on 31 October 2015, the formal investigation into the accident has concluded. The US National Transportation Safety Board (NTSB) investigation found that the co-pilot aboard, Michael Alsbury, pulled a lever to unlock the wing/tail-planes at the wrong point in the flight. The accident report noted that he was in a high workload situation at the time with accelerations/vibrations being experienced.
The wing should have come out in a controlled manner at the unpowered highpoint of the flight and at a velocity of Mach 1.4, as commanded to do so under electric motor power triggered by another set of levers. Instead the premature unlocking combined with transonic aerodynamic loads to cause the wing to come out at Mach 0.92 during the rocket-powered ascent. This caused the craft to pitch at 9g, breaking the craft apart.
The NTSB declared that that SpaceShipTwo’s manufacturer, Scaled Composites, had had a “failure to consider and protect against the possibility that a single human error could result in a catastrophic hazard”. For example, an inhibitor system could have been introduced to prevent the wing unlocking before the correct Mach number was achieved. This inhibitor system has, subsequent to the accident, been integrated into the SpaceShipTwo design by Scaled Composites.
This was not the only measure that could have prevented the tragedy, which killed Alsbury and seriously injured the pilot, Peter Siebold (Siebold managed to eject from the smashed up craft). The NTSB also criticised the lack of training and instruction in the rocket plane’s handbook to reinforce the pilots’ understanding of the hazard of premature unlocking of the wing.
Finally, the Federal Aviation Authority’s Office of Commercial Space Transportation also came under fire from the investigators for being too quick, admittedly while under political and managerial pressure, to pass the rocket plane for flight despite not properly examining all the safety issues, especially related to human factors. The NTSB noted that there was a lack of expertise in the Office of Commercial Space Transportation on human-factor issues related to accidents.