On April 30, 1992, a Bell 47-G2A (registration N52HB) operated by MC Dermott Enterprises, Inc. was involved in an aviation accident near Dousman, WI. Investigators recorded the probable cause as: THE IN-FLIGHT LOSS OF THE SWASH PLATE DRIVE LINK BOLT WHICH ALLOWED THE SWASH PLATE DRIVE ASSEMBLY TO DISCONNECT. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
During a dual instructional flight, a sudden violent vibration occurred. The CFI performed an autorotation to an off-airport site, striking trees. Inspection revealed a missing swash plate drive link bolt.
Incident Overview
A certified flight instructor (CFI) reported that a dual instructional flight proceeded without incident for the first hour. While returning to the airport in cruise flight, a sudden and very violent vibration occurred. The CFI took control of the flight controls and initiated an autorotation to an off-airport site. During the descent, the helicopter struck trees.
Post-Flight Inspection
Following the incident, an inspection of the rotor head discovered a missing swash plate drive link bolt. The absence of the bolt allowed the drive link to disconnect. Both the CFI and the student pilot recalled checking the bolt, associated nut, and cotter pin prior to the flight and found no anomalies. The bolt was never located.
Details
The flight was a dual instructional training session. No other specifics about the aircraft, location, or time were provided in the preliminary information. The investigation continues.