Accident Details
According to the flight instructor, the helicopter had been conducting hovering autorotations on the parallel taxiway of runway 26. The CFI demonstrated three maneuvers, and the pilot receiving instruction performed two. Approximately one second after the student pilot's second touchdown, the left skid collapsed, causing the helicopter to slowly roll onto its left side. The main rotor blades made contact with the ground and came to a stop.
The instructor reported that all hovering autorotations were performed into the wind from a stabilized hover of 3 to 5 feet above ground level. Each landing was terminated with the skids close to parallel to the taxiway, and no hard landings occurred.
Maintenance and Inspection
The cross tubes had accumulated about 1,500 hours since the last major overhaul. The aircraft had flown 9.6 hours since the most recent 100-hour inspection. An A&P mechanic stated that he examined the cross tubes during that inspection and measured the tail height at 35 inches. The manufacturer's criteria for replacing the cross tubes is a measured tail height of 34 inches or less.
NTSB Examination
The mating fractures from the left sides of the front and rear landing skid cross tubes were sent to the National Transportation Safety Board's Materials Laboratory for examination. The inspection revealed that magnified optical examinations of both fractures showed features typical of overstress separations in aluminum tubes. The orientations and paths of the fractures, along with associated deformation patterns, were consistent with large upward loads on the skid ends of both tubes at the time of fracture. Both tubes also displayed large radius upward bends along most of their lengths. No indications of preexisting fatigue or corrosion were apparent on the fractures or in the surrounding areas.
Damage
The helicopter sustained substantial damage as a result of the skid collapse and subsequent rollover. The main rotor blades contacted the ground, and the aircraft came to rest on its left side.