Gregory, MI, USA
N1955Z
Wolf Rotorway Exec-162F
While in cruise flight, the secondary drive shaft failed completely and power to the main rotor was lost. The pilot executed an autorotation to an open field. However, on landing, the forward speed was higher than planned and the helicopter flipped forward and rolled to the left. The pilot stated that on his return to the airport, the secondary shaft bearing temperature started to rise. Once the bearing temperature reached 150 degrees Fahrenheit, the "drive let go", causing the engine RPM to surge. A post-accident inspection revealed that the secondary shaft had failed at a point inside the upper bearing race. Due to a history of failures of this secondary shaft design, Rotorway had released a 35 mm diameter shaft configuration in April 2001. The preceeding shaft diameter was 30 mm. An advisory bulletin was issued by Rotorway in May 2002. The 30 mm configuration was installed on the accident aircraft. In addition, the accident aircraft incorporated an after-market cog belt drive system between the secondary shaft and the main rotor shaft. The basic Rotorway design utilizes a chain drive system.
On October 7, 2002, approximately 1830 eastern daylight time, an amateur-built Wolf Rotorway Exec-162F helicopter, N1955Z, owned and piloted by a student pilot, was substantially damaged when it lost power to the main rotor and executed an auto-rotation into an open field near Gregory, Michigan. The flight was being conducted under 14 CFR Part 91 and was not on a flight plan. Visual meteorological conditions prevailed at the time of the accident. The pilot reported no injuries. The flight departed the Livingston County Airport (OZW), Howell, Michigan, at 1800 edt for a local flight and was returning to the airport when the accident occurred. The pilot stated that the "secondary shaft bearing temperature started to rise." He reported that approximately one minute "elapsed from the time the [bearing] temperature increase was noted, to the time the temperature reached 150 degrees Fahrenheit," and the "drive let go", causing the engine RPM to surge. At this point, the pilot noted that all power to the main rotor was lost and he initiated an auto-rotation. He stated that, although the touchdown point was as planned, the forward speed was faster than intended and the helicopter flipped forward and rolled to the left. Following a post-accident examination, the owner/pilot reported that the secondary drive shaft had failed completely at a point inside the upper bearing race. He noted that the shaft had been in service for approximately 155 hours at the time it failed. Due to a history of secondary drive shaft failures, Rotorway had released a "re-designed configuration" in April 2001. This configuration increased the shaft diameter from 30 mm to 35 mm. The 30 mm shaft was installed in the accident aircraft. According to Rotorway, owners were notified of the availability of the new shaft design by an Advisory Bulletin in May 2002. In part, this bulletin states: "The larger [35mm] secondary shaft was supplied with new aircraft and as an upgrade to existing aircraft over a year ago. ... It is the suggestion of Rotorway International that all Rotorway owners consider upgrading to the 35mm secondary shaft." The owner/builder stated that he had planned to upgrade to the 35mm shaft over the coming winter. In addition, the accident aircraft included an after-market main rotor drive system which incorporated a cog belt in place of the chain drive.
Failure of the secondary drive shaft, which resulted in a complete loss of power to the main rotor, and a high forward airspeed on touchdown, causing the helicopter to flip forward.
Source: NTSB Aviation Accident Database
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