On December 5, 2007, a Bell UH-1B (registration N46969) was involved in an aviation accident near Ennis, MT. Investigators recorded the probable cause as: 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. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
A helicopter sustained substantial damage after the pilot executed a forced landing following a severe vibration and nose-over event. Investigation revealed the collective control absolute friction was below manufacturer's specification.
Incident Summary
The pilot had completed five flight cycles throughout the day and landed at the landing zone to remove the long line. Ground personnel requested that the pilot move the helicopter from the landing zone to a nearby road to allow a snowplow access. After the snowplow finished, the pilot began to reposition the helicopter back to the landing zone. As the helicopter lifted off, it started to vibrate. The vibration escalated into a severe hop or bounce. The pilot raised the helicopter to approximately 15 feet, but the vibration persisted and the helicopter began to nose over and became uncontrollable. The pilot then decided to force land the helicopter, touching down hard in two feet of snow. The hard landing substantially damaged the doorframe.
Inspection Findings
Post-incident inspection of the helicopter revealed that the collective control absolute friction was less than the manufacturer's specified setting. The maintenance manual indicated that if the friction is not set properly, a collective bounce could be induced. No other mechanical anomalies were identified. The reason for the loss of friction could not be determined.
Maintenance Information
The operator's manual states that the severity of a collective bounce oscillation is such that effective control of the helicopter may become difficult to maintain.