CHI02LA061 · Robinson R22

The helicopter sustained substantial damage when the left, rear landing skid cross tube collapsed during landing. The CFI reported they taxied to the parallel taxiway of runway 26 to practice hovering autorotations. He reported, "I demonstrated three of them and then [the pilot receiving instruction] performed two. About a second or so after the touchdown on [the pilot receiving instruction's] 2nd hovering auto, the left skid collapsed and the helicopter slowly and gently rolled onto it's left side. The main rotor blades made contact with the ground and came to a stop." He reported, "All hovering autos were performed into the wind from a stabilized hover of 3-5 feet agl. All were terminated with the skids close to parallel with the taxiway without any hard landings." The cross tubes had about 1,500 hours since the last major overhaul. The aircraft had flown 9.6 hours since the last 100 hour inspection. An A&P mechanic reported he had examined the cross tubes during the 100 hour inspection. He reported the height of the tail measured 35 inches. The manufacturer's criteria for replacing the cross tubes is a measured tail height of 34 inches or less. 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 (NTSB) Materials Laboratory for examination. The inspection revealed the following: "Magnified optical examinations of both fractures revealed features typical of overstress separations in aluminum tubes. In addition, 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. In addition to the local deformation at the fractures 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."

Verbatim NTSB analysis and probable cause from the NTSB dataset