A tourist helicopter that crashed in Nevada in December 2011, killing the pilot and two couples on a sightseeing tour over Hoover Dam, was brought down by an improperly fastened nut, the NTSB determined on Tuesday. The investigators found that a maintenance crew working on the Eurocopter AS350-B2 before the flight had probably reused a self-locking nut that should not have been reused. The nut failed in flight, making the aircraft uncontrollable. The mechanic and a supervisor both had been called in to work on their day off, the board said, and fatigue was cited as a factor. NTSB Chairman Deborah Hersman said the accident shows that mechanics need to use checklists in their work, just as pilots do.
Finding the cause of the crash required "old-fashioned investigative gumshoe work," Hersman said at the board meeting. The aircraft, operated by Sundance Helicopters, had no flight data recorder on board, there were no witnesses to the crash, weather was clear and calm, and there were not even any ATC tapes. Radar showed that the helicopter deviated from the usual tour route about one minute before the accident, first climbing and turning, then descending and turning again before impact. The helicopter was heavily damaged by the impact and a post-crash fire. A synopsis of the safety board's findings is posted online; the full final report will be posted in several weeks.