ANC12TA035

The pilot said he was landing a ski-equipped airplane on a remote, frozen and snow-covered lake. He said that as the skis contacted the flat and smooth snow-covered ice, the airplane pitched forward violently, then to the right, and then back to the left. A postaccident examination revealed that the right main landing gear had collapsed and subsequently folded under the airplane. An examination of the right main landing gear revealed a fractured suspension strut that had fractured laterally across a bored and threaded area of the suspension tube. The fractured tube was examined under magnification and there was no evidence of external corrosion. The tube had a long radius bend adjacent to the internal threads. The fracture had a clean, evenly colored fracture surface, and no evidence of repeated fracturing (beach marking). The fracture appearance is consistent with an overstress fracture. The pilot noted that, other than the fractured main landing suspension strut, there were no preaccident mechanical anomalies with the airplane.

On April 27, 2012, about 1620 Alaska daylight time, a ski-equipped Piper PA-18-150 airplane, N7059, sustained substantial damage when the right main landing gear collapsed while landing on a frozen, snow-covered lake, about 28 miles west of Paxon, Alaska. The airplane was operated by the State of Alaska Department of Public Safety, Alaska State Troopers, Anchorage, Alaska, as a visual flight rules (VFR) public use flight under the provisions of Title 14 Code of Federal Regulations Part 91, when the accident occurred. The solo pilot was not injured, and company flight following procedures were in effect, for the flight that departed Anchorage about 1400. In a written statement to the National Transportation Safety Board investigator-in-charge dated April 30, the pilot reported that while on a routine law enforcement flight he landed on a remote, frozen and snow-covered lake. He said that as the skis contacted the flat and smooth snow-covered ice, the airplane pitched forward violently, then to the right, and then back to the left. The pilot’s postaccident examination revealed that the right main landing gear had collapsed, and subsequently folded under the airplane. The airplane sustained substantial damage to the wings and the fuselage. The airplane was equipped with a Burl’s Aircraft LLC, Alpha Omega Suspension System (AOSS), which was installed in accordance with supplemental type certificate (STC) SA02214AK. A postaccident examination of the right main landing gear revealed a fractured suspension strut that had fractured laterally across a bored and threaded suspension tube. The tube had a slight bend with tearing along the fracture line on the inside of the bend. The pilot reported that other than the fractured main landing suspension strut, there were no preaccident mechanical anomalies with the airplane.