Tag Archives: human error

A single human error resulted in the deadly SpaceShipTwo crash

By Kim Smiley

The National Transportation and Safety Board (NTSB) has issued a report on their investigation into the deadly SpaceShipTwo crash on October 31, 2014 during a test flight.  Investigators confirmed early suspicions that the space plane tore apart after the tail boom braking system was released too early, as discussed in a previous blog.  The tail boom is designed to feather to increase the drag and slow down the space plane, but when the drag was applied earlier than expected the additional aerodynamic forces ripped the space plane apart at both high altitude and velocity.  Amazingly, one of the two pilots survived the accident.

Information from the newly released report can be used to expand the Cause Map from the previous blog.  The investigation determined that the pilot pulled the lever that released the braking system too early.  Even though the pilots did not initiate a command to put the tail booms into the braking position, the forces on the tail booms forced them into the feathered position once they were unlocked.  The space plane could not withstand the additional aerodynamic forces created by the feathered tail booms while still accelerating and it tore apart around the pilots.

A Cause Map is built by asking “why” questions and documenting the answers in cause boxes to visually display the cause-and-effect relationships. So why did the pilot pull the lever too early?  A definitive answer to that may never be known since the pilot did not survive the crash, but it’s easy to understand how a mistake could be made in a high-stress environment while trying to recall multiple tasks from memory very quickly.  Additionally, the NTSB found that training did not emphasize the dangers of unlocking the tail booms too early so the pilot may not have been fully aware of the potential consequences of this particular error.

A more useful question to ask would be how a single mistake could result in a deadly crash.  The plane had to be designed so that it was possible for the pilot to pull a lever too early and create a dangerous situation.  Ideally, no single mistake could create a deadly accident and there would have been safeguards built into the design to prevent the tail booms from feathering prematurely.  The NTSB determined the probable cause of this accident to be “failure to consider and protect against the possibility that a single error could result in a catastrophic hazard to the SpaceShipTwo vehicle.”  The investigation found that the design of the space plane assumed that the pilots would perform the correct actions every time.  Test pilots are highly trained and the best at what they do, but assuming human perfection is generally a dangerous proposition.

The NSTB identified a few causes that contributed to the lack of safeguards in the SpaceShipTwo design.  Designing commercial space craft is a relatively new field; there is limited human factors guidance for commercial space operators and the flight database for commercial space mishaps is incomplete. Additionally, there was insufficient review during the design process because it was never identified that a single error could cause a catastrophic failure. To see the recommendations and more information on the investigation, view a synopsis from the NTSB’s report.

To see an updated Cause Map of this accident, click on “Download PDF” above.