Events Leading to the Accident
The accident occurred during a flight where the pilot and passenger had noted a cracked greenhouse window above the pilot during a previous leg. According to the cockpit voice recorder transcript, about 29 minutes before the accident, the passenger remarked, "I think if I hadn't-a broke that off we wouldn't had any greenhouse by the time we got home." Approximately 18 minutes before impact, the passenger commented on catfish activity, attributing it to vibrations from the helicopter. The pilot agreed. Later, the pair discussed sighting deer.
About 1 minute 30 seconds before the accident, the pilot asked, "has this vertical just gotten in here or has it been here for a while?" The passenger replied they had not had any verticals. The pilot responded, "we do now." The passenger then suggested the condition started after a previous departure and speculated it might be due to the air. About 20 seconds later, the passenger mentioned that another person had tracked the helicopter's blades before departure, noting their smoothness. Approximately 40 seconds after that, the pilot stated, "this stuff is getting worse," and the recording ended.
Witness Observations
Witnesses near the accident site saw the helicopter flying from east to west just above treetops. They observed the helicopter begin to roll back and forth, then saw the main rotor blades contact the tail area, causing the aft tail boom and tail rotor to separate. Shortly thereafter, the main rotor separated, and the helicopter descended and crashed. A fire erupted upon ground impact. No witnesses reported any smoke from the helicopter before the accident.
Wreckage Examination
The entire main rotor system was accounted for except the pitch change horn for the "red" main rotor blade and most of the pitch change link that attaches to that horn. The "red" grip remained in place. The grip surface exhibited two boreholes matching two bushings that were part of the departed pitch change horn. A blackish residue was present near the boreholes, extending beyond them. Extensive searches of the crash site and surrounding area, including by Boy Scout parties, did not locate the missing pitch change horn or link.
Metallurgical Findings
Examination of the "red" grip by an NTSB metallurgist revealed extensive dark deposits and surface damage on both the upper and lower hole surfaces, consistent with fretting from repeated small relative movements. Visible fretting covered most of the bushing counterbores and portions of fractured thread surfaces. Energy dispersive X-ray analysis of black deposits from the upper hole showed mostly aluminum, iron, and significant oxygen, with minor amounts of other grip and bushing constituents. Samples from the lower hole contained mostly aluminum with large peaks for iron, cadmium, silicon, and oxygen. Both bushing counterbores showed localized material removal and diameter enlargement, with up to 0.020 to 0.025 inches of material removed in localized areas. The bores were deformed and distorted adjacent to the grip surface. Circumferential marks on the upper hole's bushing counterbore, lock ring diameter, and fractured threads indicated progressive incremental movement of mating objects. No other evidence of failure or malfunction of the helicopter structure, flight controls, engine, or rotor system was found. Aircraft records indicated the rotor head, including the "red" blade grip and pitch change horn, was last removed in March 1998, 162.2 flight hours before the accident, for blade retention strap replacement.