Editor's note: This report was originally published on June 15, 2010.
Evidence from the wreckage of the CareFlite helicopter that went down near Midlothian on June 2 indicates that mechanical failure, rather than human error, led to the crash.A preliminary report issued Monday by the National Transportation Safety Board offers no conclusions or opinions about the cause of the crash, which killed the pilot and a mechanic.But in a brief interview with the Star-Telegram, crash investigator Tom Latson said the entire rotor assembly of the Bell 222U helicopter was found largely intact, although damaged, 200 feet from the main fuselage wreckage.The so-called Jesus nut, a large retaining nut that holds the rotor hub and rotor assembly on the rotor mast, was still firmly in place, Latson said. Some aviation experts had suggested that the nut may not have been properly tightened during maintenance and had come loose during flight.The condition of the rotor assembly and mast, as well as the circumstances of the crash, also suggest that the crash was not caused by mast bumping, a major cause of accidents in many helicopters using two-bladed rotor systems, experts say.The rotor mast, which runs from the transmission to the rotor hub, was fractured in two places - up high, several inches below the rotor hub, and farther down, just above the transmission.Latson declined to offer possible explanations for the mast to have fractured in such a way. But he said the tailboom, which was also found intact and 400 feet from the fuselage, showed no evidence of having been struck by the main rotor blade.Dallas aviation attorney Jon Kettles, a former military helicopter pilot, said it would appear that something, perhaps damage or metal fatigue, may have led to the rotor mast shearing off near where it was attached to the transmission.In a typical mast-bumping scenario, Kettles said, a sudden control movement by the pilot causes the mast to shear off near the rotor hub and the blade frequently slices the tail rotor or even into the fuselage.Kettles said it seems likely that once the rotor assembly came off the aircraft, leaving it tumbling uncontrollably, the aerodynamic forces tore the tailboom off the helicopter."It's possibly some kind of fatigue or fracture of the rotor mast," Kettles said. "I'd say mast bumping is unlikely but it can't be ruled out."CareFlite bought the helicopter, which was made in the 1980s, in March from a for-profit air medical service, Omniflight Helicopters of Addison. It was being prepared for FAA certification to be put into service for air medical flights.A CareFlite official did not return a message Monday.The safety board is conducting metallurgical and other tests on the rotor mast and other components, Latson said. "This is going to be an extremely thorough investigation and could take six to nine months or up to a year," he said.Witnesses told the safety board that they saw the rotor and tail break away from the helicopter fuselage. The main rotor system and all the components, including the hub and blade, pitch change links and walking beam assembly, remained connected to the swash plate, which transmits the pilot's control commands to change the angle of the rotor blades.Loss of the main rotor in flight made it impossible for the pilot to control the aircraft.The purpose of a preliminary report is to release facts of the crash known to investigators at the time, not to find a cause. It may take 12 to 18 months more for investigators to determine why the helicopter came apart, said Keith Holloway, a safety board spokesman.Have more to add? News tip? Tell us
