Credit: NTSB

WASHINGTON — The accident that destroyed the SpaceShipTwo suborbital vehicle and killed one its pilots last year was caused by the co-pilot’s premature unlocking of the vehicle’s feathering system and inability of its developer, Scaled Composites, to foresee such an event and take measures to prevent it, the National Transportation Safety Board (NTSB) concluded July 28.

The NTSB, during a public meeting at its headquarters here, accepted a report regarding the Oct. 31 accident that also criticized the Federal Aviation Administration’s Office of Commercial Space Transportation for rushing to approve the vehicle’s experimental permit application without properly scrutinizing its safety issues.

The probable cause of the accident, the NTSB concluded in a unanimous vote of its four-person board, “was Scaled Composites’ failure to consider and protect against the possibility that a single human error could result in a catastrophic hazard to the SpaceShipTwo vehicle.” That failure, the board concluded, “set the stage for the co-pilot’s premature unlocking of the feather system as a result of time pressure and vibration and loads that he had not recently experienced,” causing the vehicle to break up.

According to procedures, SpaceShipTwo’s co-pilot is supposed to unlock the feathering system, which raises its twin tail booms for stability during reentry, once the vehicle reaches a speed of Mach 1.4. Instead, the co-pilot on the Oct. 31 flight, Michael Alsbury, unlocked the feather at Mach 0.82. The forces the vehicle experienced as it flew through the sound barrier caused the feather to deploy on its own, leading to “aerodynamic overload and in-flight breakup of the vehicle,” according to the NTSB.

Credit: NTSB
Rocket motor and tail boom control console. Credit: NTSB
Rocket motor and tail boom control console. Credit: NTSB

Exactly why Alsbury, who was killed in the accident, unlocked the feather prematurely was not clear to investigators. However, the NTSB report found that the co-pilot was subjected to a number of “stressors,” including memorizing a more extensive task list than previous test flights and being subjected to accelerations that he had not experienced in the vehicle since flying the first powered SpaceShipTwo test flight 18 months earlier.

In particular, the report noted that Alsbury was responsible for stating when the vehicle reached a speed of Mach 0.8, to provide a warning for the “bobble” the vehicle experiences as it passed through Mach 1. Immediately after making that notification, he unlocked the feather, the NTSB found.

The NTSB found that while Scaled Composites considered a wide range of possible problems SpaceShipTwo could experience in flight, the company never considered the possibility of pilot error, specifically, that the pilot could prematurely unlock the feather. That scenario was not formally noted in company documentation, although Scaled engineers and pilots knew, at least informally, that such a premature unlocking would be “catastrophic” to the vehicle.

“They put all their eggs in the basket of the pilots doing it correctly,” said NTSB member Robert Sumwalt during the two-hour hearing. Such a “single point failure,” he said, would be unacceptable for mechanical systems. “So why would a single point human failure be acceptable?”

“The assumption was that these highly-trained test pilots would not make mistakes in those areas, but truth be told, humans are humans,” NTSB Chairman Christopher Hart said in an interview after the hearing. “Even the best-trained human on their best day will still make mistakes. That is one of the areas they did not adequately cover.”

“Pressure” At The FAA

Another factor that contributed to the accident was a lack of familiarity in human factors issues by the FAA’s Office of Commercial Space Transportation. In particular, the NTSB found that the office’s review of Scaled’s application for an experimental permit was “deficient” because it failed to catch that the company, in its application, had not identified hazards caused by human error.

The NTSB investigation also found problems with the overall review by the FAA of Scaled’s original permit application in 2012 and its renewal in 2013 and 2014. FAA staff said that agency management would filter questions they wanted to ask Scaled, limiting them to issues relating to public safety, the office’s mandate. They added they felt “political pressure” to complete reviews of permit applications within 120 days, the deadline set by law.

“Our interpretation of those statements made by staff was pressure of two types,” Hart said in the interview. One pressure was to meet the 120-day deadline, he said, while the other was “figuring out where the line is drawn between protecting the public and mission assurance.”

The NTSB approved 10 recommendations at the meeting, eight for the FAA. Those recommendations include developing human factors guidance for crewed vehicles, improved policies for communication between FAA staff and companies, and development of a lesson learned database of commercial spaceflight mishap data.

Credit: NTSB
SpaceShipTwo’s wreckage. Credit: NTSB
SpaceShipTwo’s wreckage. Credit: NTSB

Two other recommendations were for the Commercial Spaceflight Federation, the industry group whose members include a number of companies developing commercial human spaceflight systems. Those recommendations called for working with the FAA on human factors guidance and advising its members to work with local officials on emergency response planning.

The report included no specific recommendations for Virgin Galactic, who was the customer for the Scaled-developed SpaceShipTwo and has taken over construction of the second SpaceShipTwo vehicle. The lead NTSB investigator, Lorenda Ward, said at the hearing that Virgin Galactic has taken steps to address the specific technical cause of the failure, adding an inhibitor to prevent the feather from being unlocked prematurely.

Scaled Composites, which continues to have a contractual relationship with Virgin Galactic to support SpaceShipTwo development, accepted the report’s conclusions. “As part of our constant and continuing efforts to enhance our processes, we have already made changes in the wake of the accident to further enhance safety. We will continue to look for additional ways to do so,” the company said in a statement issued after the hearing.

“Contrary to some initial speculation, the NTSB made clear that the spaceship Scaled Composites had designed, built, and then flew for us was performing exactly as it should have,” Virgin Galactic founder Sir Richard Branson said in a video posted on the company’s website after the hearing. The company, though, did not disclose when it planned to begin flight tests of its second vehicle.

This video shows the tail booms rise up after the feather was prematurely unlocked at the 0:34 mark. 

Jeff Foust writes about space policy, commercial space, and related topics for SpaceNews. He earned a Ph.D. in planetary sciences from the Massachusetts Institute of Technology and a bachelor’s degree with honors in geophysics and planetary science...