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U.S. accident reports from 1974–1998 show that diversion of attention was a major factor in accidents during external-load operations, which accounted for more than half of the accidents. Powerplant malfunctions and powerplant failures were
involved in more than a third of the accidents. More than a quarter of the accidents occurred when helicopters struck obstacles, such as trees and wires, during fire fighting missions.
Accident reports for 1994 through 1998 also showed that each of the 48 accident aircraft was flown by one pilot and that all but one accident aircraft had turboshaft engines.
Oxygen System Installation Cited In Fire Aboard Bell 206L-3 4 pages. [PDF 33K]
The pilot was seriously injured, and the emergency medical services helicopter was destroyed, by a fire that occurred on the ground when the pilot repositioned a valve on a medical-oxygen cylinder. The accident investigation revealed deficiencies in the control, design, construction, installation and maintenance of medical-oxygen systems in helicopters operated in Australia.
The accident investigation concluded that gradual deterioration of the turbine-rotor-shroud sealing rings was not detected because the helicopter operator did not comply with the manufacturer’s engine-performance trend-monitoring procedures.
The U.K. Civil Aviation Authority has recommended a review of helicopter-certification regulations to define realistically pilot-reaction time. During simulated training sorties, pilots immediately detected failures involving variables within their focus of attention, but required more time to detect alerting cues outside their focus of attention.
Statistics for 1993 through 1997 show a relatively low accident rate for U.S.-registered twin-turbine helicopters. Nevertheless, almost half of the accidents were fatal, and 70 percent of the accidents involved pilot error.