The United States National Transportation Safety Board (NTSB) released the conclusions of its accident investigation into the crash of Virgin Galactic's SpaceShipTwo last year over the Mojave Desert. According to the report, the accident was due to an error by the co-pilot, who prematurely released the spacecraft's feather system, placing too much stress on the fuselage and causing it to break up.

At about 10:07 PDT on October 31, 2014, SpaceShipTwo (SS2-001) broke up in midair 13 seconds after being dropped from the WhiteKnightTwo mothership, resulting in the death of the co-pilot Michael Alsbury and severe injury to the pilot, Peter Siebold, who was thrown clear as the craft disintegrated. It was the 55th test flight and fourth powered flight of the suborbital spacecraft built by Scaled Composites, a wholly-owned subsidiary of the Northrop Grumman Corporation, for Virgin Galactic.

The spacecraft uses a hybrid rocket system to propel it on a suborbital trajectory into outer space. Key to this design is a feather system for reentry, which uses a pair of tail boom structures that pivot upright, so the spacecraft spirals down into the Earth's atmosphere like a maple seed.

Pilot Pete Siebold as he parachutes safely down to earth after the accident, with his arm up in the air to show everybody that he is alive and well(Credit: Virgin Galactic)

The flight during which the accident occurred was supposed to see the spacecraft reach an altitude of 135,000 ft (41,000 m) and a speed of Mach 2. The planned program included testing of a new polyamide-based hybrid rocket motor fuel and the first supersonic deployment of the feathering system. After being dropped by WhiteKightTwo, SpaceShipTwo fired the engine and operations were normal until the feathering booms deployed prematurely. This produced high g-forces, which tore the craft into several pieces that scattered over a five-mile (8-km) area near Koehn Dry Lake, California.

The Board had access to video, on-board and telemetry data recorded during the flight, and on Tuesday released its findings and made its recommendations to the Federal Aviation Administration and the Commercial Spaceflight Federation.

According to the report (PDF), the co-pilot made an error under time pressure, vibrations and loads that he had not previously experienced. The tailboom was unlocked while SpaceShipTwo was still in the boost phase. The velocity was Mach 0.9, when the safe minimum speed for unlocking was Mach 1.4. The specifications state that, at speeds above Mach 1.4, the air pressure on the boom keeps it in place with the aid of the actuators. When the speed is lower, the boom can press up with too much force for the actuators to overcome.

The investigation found that the co-pilot unlocked the booms 14 seconds early when the craft was travelling at only Mach 0.92 and the low-velocity caused the boom to deploy without any action by the pilot, resulting in the destruction of the craft. This was "indisputably confirmed by telemetric, in-cockpit video and audio data." In addition, the Board found no indication of misleading display indicators and that all information in the cockpit was accurate.

The reason given for the error was that the co-pilot hadn't received sufficient training regarding how to safely unlock the boom, hadn't had enough simulation trainings, and that in flight had been confused by the time pressure and the unexpected vibration and pressure of the rocket firing.

Regarding responsibility, the Board found no mechanical shortcomings with the spacecraft. The engine performed above expectations and all systems operated as designed up until the spacecraft broke apart. The only major mechanical problem that the Board identified was the need for an automatic safety lock to prevent the booms from being unlocked below the safe speed.

The Board also blamed Scaled Composites for not doing enough to prevent pilot error. It said the pilots had inadequate training and experience in the operation of the craft and the danger of prematurely unlocking the booms had only been discussed once, three years before the accident.

The Board says that the FAA had noted some software and human error shortcomings, but had waived them in issuing a flight permit to them due to the experimental nature of the craft. It recommended a challenge/response routine by the crew for unlocking the booms and providing pilots with explicit warnings about improper procedures.

"Manned commercial spaceflight is a new frontier, with many unknown risks and hazards," says NTSB Chairman Christopher A. Hart. "In such an environment, safety margins around known hazards must be rigorously established and, where possible, expanded."

"Every new transformative technology requires risk, and we have seen the tragic and brave sacrifice of Mike and the recovery of injured surviving pilot Pete Siebold," said Virgin Galactic CEO Sir Richard Branson in the wake of the report. "Their tremendous efforts are not in vain and will serve to strengthen our resolve to make big dreams come true."

The video below is the statement by Sir Richard.


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