Last year's crash of Virgin Galactic's SpaceShipTwo was caused by a premature repositioning of the vehicle's tail wings, an investigation by the National Transportation Safety Board found. The findings confirm a preliminary investigation made by the NTSB, which found that SpaceShipTwo's wings had shifted before the spaceplane broke apart.

The vehicle's two tail wings are supposed to move when the spacecraft hits a certain speed, a maneuver known as "feathering." This repositioning helps to increase drag in order to slow the vehicle down for reentry. The NTSB found that the co-pilot Michael Alsbury — who died in the crash — unlocked the feathering system early, doing so at Mach 0.92 instead of the intended speed of Mach 1.4. This triggered the feathering maneuver to occur prematurely, which ultimately caused the vehicle's "catastrophic structural failure," said Lorenda Ward, the NTSB investigator in charge of the SpaceShipTwo crash.

The co-pilot unlocked the feathering system early

The feathering system is controlled by a series of levers; the co-pilot must first unlock the system by pulling down a large lever, and then the pilot and co-pilot together pull two additional levers to initiate the motor that feathers the wings. Alsbury unlocked the system too early, and the feather then extended without the pilot and co-pilot pulling the needed levers. As the vehicle traveled through transonic speeds, aerodynamic and inertial loads overcame the feathering system’s motor, causing the wings to move from their normal position up to a 60-degree angle.

Feathering is supposed to occur just as SpaceShipTwo climbs to the peak of its parabolic flight path — a point known as apogee. Pilots are supposed to unlock the system at Mach 1.4 and then extend the feather after the rocket engines cut off while in space, just before apogee. Feathering brings the tail wings up to a 90-degree angle, creating enough atmospheric drag to slow the plane down as it reenters Earth’s atmosphere. It works because the spacecraft has already slowed down and is pointed down toward Earth. However, in this case, feathering occurred during this test flight’s ascent, when it was accelerating at transonic speeds. The wing repositioning placed a lot of force on SpaceShipTwo — 9Gs to be exact — which was too much for the plane to handle.

The reason Alsbury unlocked the feather early may be because he didn't want to run the risk of abort, suggests NTSB investigator Katherine Wilson. The system must be unlocked by Mach 1.8 at the latest or else the flight is aborted. Scaled Composites, the manufacturer of SpaceShipTwo, said that when building the vehicle, it did not consider the possibility that a pilot would unlock the feather prior to Mach 1.4. Virgin Galactic says it has added an inhibitor to SpaceShipTwo to prevent premature unlocking during future flights.

The reason Alsbury unlocked the feather early may be because he didn't want to run the risk of abort

The experimental SpaceShipTwo vehicle called the VSS Enterprise violently broke apart during a test flight over the Mojave Desert on October 31st. It was the vehicle's first powered test in nine months. The spacecraft had just been released from its carrier aircraft and had fired up its onboard rocket engine when an in-flight anomaly caused the vehicle to seemingly explode. The crash killed co-pilot Alsbury and seriously injured pilot Peter Siebold. Alsbury was described as a very professional pilot during an NTSB briefing about the crash.

Virgin's SpaceShipTwo is a suborbital spaceplane aimed at transporting tourists into Earth's upper atmosphere. A White Knight Two aircraft must carry the SpaceShipTwo up to an altitude of 50,000 feet, according to the plane’s flight plan. There, SpaceShipTwo will separate from the White Knight Two and fire its liquid / solid hybrid rocket engine. The thrust carries passengers up to a maximum height of 361,000 feet, allowing them to experience a few minutes of weightlessness.

This post has been updated to include more information about how SpaceShipTwo's feathering system works during flight.