NTSB Identification: SEA08LA043.
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Accident occurred Wednesday, December 05, 2007 in Ennis, MT
Probable Cause Approval Date: 05/28/2008
Aircraft: Bell UH-1B, registration: N46969
Injuries: 1 Uninjured.
NTSB investigators may not have traveled in support of this investigation and used data provided by various sources to prepare this aircraft accident report.
The pilot reported that he had flown the helicopter for five cycles throughout the day and landed at the landing zone to remove the long line. He was asked by ground personnel to reposition the helicopter from the landing zone to a nearby road in order for a snowplow to access the landing zone. Once the snowplow was done, the pilot began to reposition the helicopter back to the landing zone. As the helicopter lifted from the ground, it began to vibrate, and the vibration turned into a severe "hop or a bounce." The pilot lifted the helicopter to 15 feet; however, it continued to vibrate and subsequently began to nose over and become uncontrollable The pilot then decided to force land the helicopter and touched down hard in two feet of snow, substantially damaging the doorframe. Inspection of the helicopter revealed that the collective control absolute friction was less than the manufacturer's specified setting and the maintenance manual indicated that if the friction is not set properly, a collective bounce could be induced. No other mechanical anomalies were identified and the reason for the loss of friction could not be determined. The operator's manual indicates that the severity of this oscillation is such that effective control of the helicopter may become difficult to maintain.
The National Transportation Safety Board determines the probable cause(s) of this accident to be: The loss of collective control friction during takeoff that resulted in a collective bounce oscillation. Contributing to the accident was the pilot's failure to maintain aircraft control during the aborted takeoff and the snow covered terrain. Full narrative available
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