IAD96LA094

The pilot stated that about three minutes into the flight something in the rear of the helicopter popped very loud. The helicopter was about 150 agl, over open fields, when he heard the noise. The aircraft hit the ground on the skids, the main rotor blades hit the tailboom and helicopter came to rest about 180 degrees from the initial direction of travel. Examination of the wreckage found that the bearing assembly shaft insert had separated inside of the main rotor shaft. Examination of the maintenance records found that the main rotor shaft assembly had accumulated about 100 hours at the time of the accident and had been replaced in January 1995. The manufacturer stated that the new assemblies replacement interval should be every 350 hours.

History of flight

On June 10, 1996, at about 1503 eastern daylight time, a Rotorway Exec 152 homebuilt helicopter, N7182Z, was in straight and level flight approximately 150 feet above the ground at an estimated airspeed of 55 to 60 miles per hour when the pilot heard a loud noise from the rear of the helicopter. The helicopter was destroyed when it rolled over during the subsequent auto-rotational landing in a field near Gretna, Virginia. The pilot, the sole occupant, was not injured. Visual meteorological conditions prevailed at the time of the accident, and no flight plan was filed. The local flight was conducted under 14 CFR Part 91, and originated from Gretna at approximately 1500.

The pilot stated that about five hours before the accident flight he had worked on the dynamic balancing and inflight tracking of the rotor blades. He reported that because of the work performed, he did a preflight inspection which included a lead-lag RPM check before he took off. The pilot indicated that the purpose of the flight was to survey his tobacco fields for standing water. Approximately three minutes into the flight he heard "...something in the rear of the ship popped very loud." He said that he was approximately 150 feet above ground level (AGL) over open fields and decided to reduce the throttle to make a precautionary landing. When he reduced throttle, the helicopter "...begins to grab and jerk and sounds like it is coming apart behind me... ." The helicopter touched down hard on the skids and the main rotor blades hit the tailboom. The helicopter rolled over to the right and came to rest about 180 degrees from the initial direction of travel. The pilot stated that he escaped with no injuries.

Wreckage examination/documentation

On June 12, 1996, examination of the wreckage by Federal Aviation Administration (FAA) representatives examined the helicopter. They reported that the bearing assembly shaft insert separated inside of the main rotor shaft. In addition, the stub shaft fell out of the main rotor shaft, and the main shaft, sprocket and chain had pulled to the rear of the fiberglass oil bath pan. The sprocket and chain dug into the fuel tank. The top of the main bearing was also rotated to the rear and left. The collective scissors base support was broken and rotated up, and the 3-roller drive chain jumped up one set of rollers on the sprocket. The FAA Inspector stated that when the main rotor shaft separated, nothing was holding the lower bearing in place. He said that with the bearing missing from the lower end of the main rotor shaft, the shaft was allowed to wobble out of the tip path plane, and in turn broke one of the two bolts holding the collective adjustment to the swash plate. In addition, the FAA Inspector stated that the main chain drive had slipped off the sprockets and in turn jammed the main rotor blades in an autorotation pitch, reducing the pilots collective pull. The FAA said that had the second bolt on the collective failed, the pilot would not have had any collective pull in the autorotation during this emergency.

Examination of the maintenance records by the FAA, found that the main rotor shaft assembly had accumulated about 100 hours at the time of this accident and had been replaced by the owner in January 1995. The replacement assembly was manufactured by Liess engineering Company, Niland, California, and the replacement interval stated by the manufacturer was every 350 hours.