NTSB: Scaled's Failure To Protect Against Human Error Led to Loss of SpaceShipTwo

The National Transportation Safety Board (NTSB) voted today to adopt its final report on the October 31, 2014 SpaceShipTwo (SS2) accident that killed one of the spaceplane’s two pilots.  The Board agreed to 17 findings and 10 recommendations, along with a statement of probable cause that focused on the failure of Scaled Composites to “consider and protect against” the possibility that a single human error could doom the vehicle and its crew.

SS2 broke apart during a flight test over the Mojave Desert killing co-pilot Michael Alsbury.  The pilot, Peter Siebold, survived after being thrown clear of the spaceplane unconscious.  He regained consciousness during the fall to Earth and was able to detach himself from his seat and his parachute opened automatically. 

It was immediately evident from telemetry and cockpit video that Alsbury had prematurely moved one of two levers that activate a feathering system
intended to slow the spaceplane during descent, creating aerodynamic instability that tore the plane apart.  Why he did so and why
the feathering system deployed even though the second lever was not
activated were among the subjects of the investigation.

The findings and recommendations span a wide range of concerns about government and private sector actions, many of which were leveled at the FAA’s Office of Commercial Space Transportation (AST), but the statement of probable cause is aimed at Scaled, which built SS2 and was in charge of the test flight.

The NTSB staff’s version said the probable cause was that Alsbury prematurely unlocked the feathering system as the result of time pressure and vibration and loads he had not experienced recently.  It added as a contributing factor Scaled’s failure to consider the possibility that a single human error could cause the feathering system to deploy at the wrong time and to adequately warn pilots of that risk.

NTSB chairman Christopher Hart proposed a revised version that swapped those sentences, placing Scaled’s failure first and identifying Alsbury’s actions as a consequent result.   During a brief recess, the staff and Board members wrote a third version that was thereupon adopted:

The National Transportation Safety Board determines that the probable cause of this accident 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.  This failure set the stage for the copilot’s premature unlocking of the feather system as a result of time pressure and vibration and loads that he had not recently experienced, which led to uncommanded feather extension and the subsequent aerodynamic overload and in-flight break up of the vehicle.

Hart was acting NTSB chairman at the time of the accident and was on-site at Mojave Air and Space Port during the initial stages of the investigation and provided the press briefings.

One focus of the investigation was the training the pilots received including human factors and the information formally conveyed to them by Scaled about the dangers of premature deployment of the feathering system.  The NTSB found that the copilot (Alsbury) was experiencing high workload as a result of recalling tasks from memory while performing under time pressure and with vibration and loads that he had not recently experienced, which increased the possibility for errors.  NTSB found that Scaled “did not ensure that pilots correctly understood the risks of unlocking the feather early” and missed opportunities to mitigate against the consequences of human error in its design. 

The NTSB also found that AST’s evaluations of Scaled were “deficient” because they did not recognize that Scaled had not identified the potential human-error hazards.  The NTSB also found that a lack of direct communications between the AST and Scaled  technical staffs, pressure to approve experimental applications within 120 days, and other factors interfered with AST’s evaluation process.  Questions also arose about why AST granted a waiver to Scaled in July 2013, which the company did not request, regarding required hazard analysis that did not meet requirements for human and software errors.

The NTSB has posted a synopsis of its findings and recommendations.