On March 21, 1988, a Bell 47G-3B1 (registration N7863S) was involved in an aviation accident near Gobernador, NM. Investigators recorded the probable cause as: The locking rollers in the freewheeling unit had worn beyond tolerance, allowing the main rotor to become and remain disengaged from the transmission. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
A commercial helicopter experienced a rotor RPM decay and hard landing after a loud bang during a hover. Post-accident inspection revealed worn locking rollers in the freewheeling unit caused main rotor disengagement.
Incident
A commercial helicopter was hovering out of ground effect at an altitude of 40 feet above ground level. During a maneuver, the pilot reported hearing a loud bang, after which the rotor rpm began to decay. The pilot performed a hovering autorotation, which resulted in a hard landing. During the landing, the main rotor struck the tail boom and severed it.
Findings
A post-accident teardown of the transmission revealed that the locking rollers in the freewheeling unit had worn beyond tolerance. This wear allowed the main rotor to become and remain disengaged from the transmission.
Probable cause
The locking rollers in the freewheeling unit had worn beyond tolerance, allowing the main rotor to become and remain disengaged from the transmission.