CHI02LA061

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."

On January 5, 2002, at 0900 central standard time, a Robinson R22 helicopter, N215WW, operated by WW Helicopters, sustained substantial damage when the left, rear landing skid cross tube collapsed during landing. The 14 CFR Part 91 instructional flight departed East Troy Municipal Airport (57C), East Troy, Wisconsin, at 0820 on a local training flight. The Certificated Flight Instructor (CFI) and private pilot receiving instruction were practicing "Engine Failure in a Hover" maneuvers. The helicopter's left rear cross tube collapsed when the helicopter touched down during the fifth practice maneuver. The aircraft rolled counterclockwise and the main rotor blades impacted the asphalt pavement. The pilots were uninjured. Visual meteorological conditions prevailed and no flight plan was filed.

The CFI reported that they had practiced, "a couple of running takeoffs and run on landings. We then executed about 6 straight-in autorotations. All maneuvers were completed normally." He reported they taxied to the parallel taxiway of runway 26 to practice hovering autorotations. He reported, "I demonstrated three of them and then [pilot receiving instruction] performed two. About a second or so after the touchdown on [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. We turned off all electrical systems and the fuel cutoff valve." He reported, "All hovering autos were performed into the wind from a stabilized hover of 3-5 feet agl. All were terminated with the kids close to parallel with the taxiway without any hard landings."

The helicopter airframe had a total of about 7,510 hours, and about 1,500 hours since its last major overhaul. The cross tubes were replaced during the major overhaul. The last 100 hour inspection was conducted on January 3, 2002, and it had flown 9.6 hours since the 100 hour inspection.

An Airframe and Powerplant Mechanic (A&P) 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 fractured parts from the helicopter's landing gear were sent to the National Transportation Safety Board's (NTSB) Materials Laboratory for examination. The inspection revealed the following:

"Mating fractures from the left sides of the front and rear landing skid cross tubes were received. Both tubes were fractured adjacent to the skid tube fittings at the outboard end of each cross tube.

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.

Telephone conversations with an engineering representative of Robinson Helicopter established that the cross tubes were manufactured from 7075 T6 aluminum tubing, per WW-T-700/7, with a nominal outer diameter of 1.507 inches and a nominal wall thickness of 0.156 inches.

Energy dispersive x-ray spectra of a small piece removed from the rear cross tube was consistent with 7075 aluminum alloy. Hardness measurements on sanded areas of the outer diameters of the cross tubes averaged HRB 85.8 for the front tube and HRB 86.2 for the rear tube. Electrical conductivity in the sanded regions measured 33.2 and 33.4 % IACS. The conductivity and hardness are consistent with 7075 aluminum in the fully aged T6 condition.

Dimensional measurements on the tube ends away from the fractures established that the forward tube had an outer diameter of 1.517 inches with a wall thickness that ranged from 0.158 to 0.161 inches. The rear tube outer diameter was 1.517 with a wall thickness that ranged from 0.150 to 0.0164 inches. All measurements were performed with the exterior paint and interior primer in place and were consistent with the specified dimensions." (See NTSB Materials Laboratory Factual Report)