National Transportation Safety Board (NTSB) team members view debris from SpaceShipTwo's tail booms where the parts fell back to Earth in California's Mojave desert in October 2014.(NTSB)
July 28, 2015
— The fatal in-flight loss of Virgin Galactic's first SpaceShipTwo spacecraft last October was likely the result of pilot error and the failure of the sub-orbital space plane's designer to protect against such occurring, federal investigators said on Tuesday (July 28).
The Oct. 31, 2014 accident, which claimed the life of co-pilot Mike Alsbury and seriously injured pilot Pete Siebold, took place during a test flight of the spacecraft, which was developed and built by Scaled Composites for Virgin. The private vehicle broke apart over California's Mojave Desert after its tail booms prematurely feathered into their upright position.
"The National Transportation Safety Board determines that the probable cause of this accident was Scaled's failure to consider and protect against the possibility that a single human error could result in a catastrophic hazard to the SpaceShipTwo vehicle," the investigators reported during the meeting on Tuesday. "This failure set the stage for the co-pilot's premature unlocking of the feather system."
SpaceShipTwo (SS2), which Virgin Galactic is planning to use to fly paying passengers to just beyond the boundary of space and back, utilizes the feather system to stabilize its attitude and increase drag during its re-entry back into the atmosphere. Following its normal flight procedures, the tail booms are not to be unlocked until the spacecraft has reached a velocity of Mach 1.4.
View of Virgin Galactic's SpaceShipTwo feathering its tail booms in a 2011 test flight.(Clay Center Observatory/Virgin Galactic)
"A forward-facing cockpit camera and flight data showed that the copilot unlocked the feather just after SS2 passed through a speed of 0.8 Mach," investigators described in a summary of the findings. "Afterward, the aerodynamic and inertial loads imposed on the feather flap assembly were sufficient to overcome [its] actuators."
The NTSB was not able to determine why Alsbury moved the feather lock handle to the unlock position 14 seconds before he should have, other than explaining that he was experiencing a high workload and was recalling tasks from memory while he was performing under time pressure and with vibration and loads he had not recently experienced. Alsbury's previous powered test flight was in April 2013.
"I don't mean this flippantly, because I have made plenty of mistakes, but humans will screw up anything if you give them enough opportunity," board member Robert Sumwalt said. "I do not mean that with any disrespect to the crew."
"The fact is, a mistake was made here, but the mistake is often a symptom of a flawed system," he stated.
That said, the NTSB investigators found that both Alsbury and Siebold were properly certified and qualified to fly SS2 and there were no fatigue, medical or pathological issues contributing to the accident. Furthermore, the spacecraft's recovered components showed no evidence of any type of structural, system or rocket motor failure before the break-up.
"Contrary to 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," Richard Branson, Virgin Galactic's founder, said in a video posted Tuesday. "We can therefore be certain that among other things, the rocket motor was working perfectly, the vehicle's airframe performed as it had been designed, and the cockpit displays were all fully functional."
Virgin, which is building its second SpaceShipTwo and is planning to resume test flights without the involvement of Scaled, has already designed an automatic mechanism to prevent the feather from being unlocked at the wrong time on its future flights. It also has made changes to its pilots' training and procedures to ensure that the consequences of releasing the feather lock early are clearly understood.
As part of its investigation, the NTSB found that the FAA (Federal Aviation Administration), which regulates all U.S. commercial spacecraft, had issued waivers after deciding that Scaled Composites' hazard analysis did not meet the software and human error requirements spelled out under the federal code. Scaled did not request or have input into those waivers.
As part of eight recommendations to the FAA, the NTSB advised that it develop and issue human factors guidance for spacecraft operators and that it refine its assessment of single flight crew tasks "that, if performed incorrectly or at the wrong time, could result in a catastrophic hazard." It also urged inspectors be assigned to individual operators to ensure they have the time needed to become familiar with the technical, operational, training, and management controls that they will inspect.
"We are thankful to the NTSB for conducting [its] thorough investigation, as well as for the clarity of their findings and recommendations, all of which will help make the fledgling commercial space industry safer," stated Branson. "Virgin Galactic can now focus fully on the future with a clean bill of health and a strengthened resolve to achieve its goals."