Investigation Summary
The flight, designated Powered Flight 04 (PF04), was the first to test a new nylon-based hybrid rocket motor. After being released from the WhiteKnightTwo carrier aircraft at approximately 46,400 feet, the crew ignited the rocket motor for a planned boost phase. The mission profile required the feathering system—a mechanism that rotates the tail booms 60 degrees to increase drag for reentry—to remain locked until the vehicle reached Mach 1.4. This lock was critical because, between Mach 0.8 and Mach 1.3, aerodynamic loads exert a powerful upward force on the tail booms. At Mach 0.92, approximately nine seconds after rocket ignition, the co-pilot moved the feather lock lever to the 'unlock' position. Without the mechanical locks engaged, the aerodynamic forces overcame the feather actuators, which were only designed to move the booms in thinner air or at different speeds. The booms deployed prematurely, leading to a massive increase in drag and structural loads that exceeded the airframe's limits, resulting in a total breakup within four seconds of the unlock command.
Final Conclusions
The NTSB determined the probable cause was Scaled Composites' failure to protect against a single-point human failure. The design lacked a fail-safe or mechanical inhibit to prevent the feather system from being unlocked during safety-critical transonic flight. The board also cited the co-pilot's premature action as a result of time pressure and intense vibration/acceleration loads he had not experienced in over a year. Furthermore, the NTSB criticized the FAA's Office of Commercial Space Transportation for insufficient oversight of human factors and for granting waivers without fully understanding the technical risks associated with the feathering system's design.
Photographic Evidence (1)
Video Analysis
No video analysis linked for this case file yet.
Systems & Failure Modes
Feathering System
A reentry stabilization system that rotates the tail booms upward by 60 degrees to increase drag and ensure a stable, 'carefree' reentry into the atmosphere.
Feather Lock Lever
A cockpit control used to mechanically lock the tail booms in the stowed position to prevent uncommanded deployment during high-speed ascent.
Hybrid Rocket Motor
The propulsion system using a combination of solid fuel (nylon) and a liquid oxidizer (nitrous oxide).
Interesting Facts
- 01The feathering system was designed to be unlocked at Mach 1.4, but was unlocked at Mach 0.92.
- 02The pilot, Peter Siebold, survived after being thrown from the aircraft and his parachute deployed automatically.
- 03The co-pilot, Michael Alsbury, was killed in the breakup.
- 04The NTSB found that Scaled Composites assumed test pilots would not make such basic errors and thus did not include a mechanical inhibit.
- 05The investigation cleared the new nylon-based rocket fuel of contributing to the accident.
Safety Actions & Advisories
Mechanical Inhibit Installation
Virgin Galactic modified the second SpaceShipTwo (VSS Unity) with an automatic mechanical inhibit to prevent the feather from being unlocked during safety-critical phases.
Enhanced CRM and Checklists
Formalized Crew Resource Management (CRM) protocols, including challenge/response call-outs and explicit warnings in the pilot's handbook regarding premature unlocking, were implemented.
Known Controversies & Unanswered Questions
FAA Oversight Criticism
Reference ↗The NTSB explicitly criticized the FAA's Office of Commercial Space Transportation for being understaffed and for failing to provide adequate human factors oversight, noting pressure to approve permits quickly.