SpaceShipTwo was a commercial space vehicle that Scaled Composites built for
Virgin Galactic. The vehicle broke up during a rocket-powered test flight,
seriously injuring the pilot and killing the co-pilot.
The feather system, which was designed to pivot the tailboom structures upward
to slow the vehicle during reentry into the earth’s atmosphere, was to be
unlocked during the boost phase of flight at a speed of 1.4 Mach. The copilot
unlocked the feather at 0.8 Mach; once unlocked, the loads imposed on the
feather were sufficient to overcome the feather actuators, allowing the feather
to deploy uncommanded, which resulted in the breakup of the vehicle.
The Board found that Scaled Composites failed to consider the possibility that
a test pilot could unlock the feather early or that this single-point human
error could cause the feather to deploy uncommanded. The Board also found that
Scaled Composites failed to ensure that test pilots adequately understood the
risks of unlocking the feather early. Investigators found that the only
documented discussion with the accident pilots about the loads on the feather
as the vehicle transitioned from subsonic to supersonic flight occurred more
than 3 years before the accident.
The FAA was responsible for evaluating Scaled Composites’ experimental permit
applications for test flights of the vehicle. After granting an initial permit
and renewing the permit once, the FAA recognized that Scaled Composites’ hazard
analysis did not meet the software and human error requirements in FAA
regulations for experimental permits. The FAA then waived the hazard analysis
requirements related to software and human errors based on mitigations included
in Scaled Composites’ experimental permit application; however, the FAA
subsequently failed to ensure the mitigations in the waiver were being
implemented by Scaled.
NTSB Chairman Christopher A. Hart emphasized that consideration of human
factors, which was not emphasized in the design, safety assessment, and
operation of SpaceShipTwo’s feather system, is critical to safe manned
spaceflight to mitigate the potential consequences of human error.
“Manned commercial spaceflight is a new frontier, with many unknown risks and
hazards,” Hart said. “In such an environment, safety margins around known
hazards must be rigorously established and, where possible, expanded.”
The Board made recommendations to the Federal Aviation Administration and the
Commercial Spaceflight Federation. If acted upon, the recommendations would
establish human factors guidance for commercial space operators and strengthen
the FAA’s evaluation process for experimental permit applications by promoting
stronger collaboration between FAA technical staff and operators of commercial
space vehicles.
“For commercial spaceflight to successfully mature, we must meticulously seek
out and mitigate known hazards, as a prerequisite to identifying and mitigating
new hazards,” Hart said.
