Human Error in Test Flight Disasters

The first SpaceShipTwo during a glide flight over the Mojave desert. Credit: Virgin Galactic Human error played the main role in the midair spaceplane disaster that killed a Virgin Galactic test pilot in October. The simple act of unlocking a braking mechanism 14 seconds too early led to the spaceplane's destruction, according to the U.S. National Transportation Safety Board. But the NTSB's investigation also pointed to a chain of "human factors" related to time pressure and a lack of safeguards that contributed to the copilot's tragic mistake. Such human factors have played a big role in the history of test flight disasters involving experimental aircraft and military jets. The recently-concluded NTSB investigation found that Scaled Composites, the company that designed Virgin Galactic's SpaceShipTwo, failed to consider the possibility of a pilot deploying the "feather" braking mechanism too early. There were no safeguards in place to prevent that potentially catastrophic mistake. Scaled Composites also apparently failed to warn the pilots about the risks of premature feather deployment through either manuals or procedures. When copilot Michael Alsbury prematurely unlocked the feather at a speed of Mach 0.8, aerodynamic forces took over by moving the feather mechanism into its braking position too early. The resulting stress tore SpaceShipTwo apart. Pilot Peter Siebold survived the breakup of the spaceplane and parachuted to safety, but Alsbury died in the accident. "These two test pilots took on an uncommon challenge: testing technologies for manned commercial space flight, which is still in its infancy," said Christopher Hart, chairman of the NTSB, during a public meeting held on July 28. "Human space flight is subject to unique hazards, and test-pilots work in an environment in which unknown hazards might emerge." Human error is responsible for the majority of aviation disasters. But the cases tend to be less about "bad pilots" and more about the human brain's limitations in reacting to certain situations. Since World War II, a field known as human performance or human factors engineering has tried to identify the factors that can contribute to human mistakes. Such factors can include pilot fatigue from not getting enough sleep, distractions within or outside the cockpit, and simple design choices for the aircraft or spacecraft that make human mistakes more likely. Those mistakes become even more potentially deadly in the test flight disasters of experimental aircraft such as SpaceShipTwo. Many lessons from past test flight disasters have been collected in a NASA book titled "Breaking the Mishap Chain," which explains how complex human factors played different roles. Some examples that follow include an experimental rocket-plane disaster, the crash of a futuristic "lifting body" vehicle that inspired "The Six Million Dollar Man" TV show, and a series of human errors that destroyed a $325-million Air Force bomber.