ANC16LA072

The commercial pilot reported that, after an uneventful touchdown on the dry, gravel-surface runway, he felt a medium vibration as he applied the brakes. As the airplane continued the landing roll, both main landing gear (MLG) legs sheared off. The propeller impacted the runway, and the airplane came to rest on the bottom of the fuselage, which sustained substantial damage. A postaccident examination of the airplane revealed that both MLG legs failed due to fatigue cracking. The fatigue cracks were extended by intermittent overstress until the legs separated. The overstress extensions likely occurred during the accident landing or immediately before it because there was no corrosion of the crack surfaces.

On September 26, 2016, about 1420 Alaska daylight time, a tailwheel-equipped Helio Courier H-250 airplane, N6324V, sustained substantial damage during the landing roll at the Lake Hood Seaplane Base, Anchorage, Alaska. The certificated commercial pilot sustained no injury. The airplane was registered to, and operated by, a private individual as a visual flight rules (VFR) flight under the provisions of 14 Code of Federal Regulations (CFR) Part 91. Visual meteorological conditions prevailed at the time of the accident, and a VFR flight plan had been filed. The flight originated from a private airstrip near Igiugig, Alaska, about 1200. The pilot reported in a written statement on September 29 that after an uneventful touchdown on the dry and gravel surface of runway 32, a medium vibration was felt as the pilot applied the brakes. As the airplane continued the landing roll, both main landing gear legs sheared off. The propeller impacted the runway, and the airplane came to rest on the bottom of the fuselage without further incident. The airplane sustained substantial damage to the fuselage. A postaccident inspection revealed that the main landing gear legs on both sides sheared off below a support bracket located at the intersection of the tubular steel structure of the fuselage attachment points. The main landing gear legs (left side, part number 250-040-451-0 and right side, part number 250-040-451-1) are constructed of 1/8-inch steel, which is molded and welded to form a 2-inch-square steel tube. The main landing gear legs are covered by a contoured fairing assembly. The pilot reported that the main landing gear legs were manufactured in 1966. He reported that the main landing gear legs had about 3,500 hours total time, about 15,000 cycles, and 15 hours since the last inspection. In a written statement on October 14, the pilot reported that he could not determine when the main landing gear was inspected as they were changed in 2013 for ski-wheel use and he was unsure if a dye penetrant process was performed or not. He reported that the main landing gear legs installed on the accident airplane came off another airplane that had about 3,500 hours total time. The main landing gear legs were secured and transported to the NTSB Materials Laboratory in Washington, D.C., for further examination. In the recommendation section of the National Transportation Safety Board (NTSB) Accident/Incident Reporting Form 6120.1, the pilot reported that the main landing gear legs may previously have experienced hard landings, along with previous cracks, and that these cracks are difficult to detect during inspection. He further reported that it is advisable to have the main landing gear checked by specialists at annual inspections, especially if the main landing gear is being converted from ski-wheels to wheels, or vice versa. METEOROLOGICAL INFORMATION The closest weather reporting facility was the Lake Hood Seaplane Base. At 1353, an Aviation Routine Weather Report(METAR) was reporting in part: wind from 310 degrees at 5 knots; visibility 10 statute miles; sky condition, overcast 5,500 feet; temperature 46 degrees F; dew point 36 degrees F; altimeter 29.89 inHg. TESTS AND RESEARCH An examination by the NTSB Materials Laboratory revealed that both main landing gear legs were fractured through the square tube section between 29 and 30 inches above the axle centerlines. Several fatigue regions were present in the vertically oriented, aft walls of both main landing gear legs particularly near the upper and lower corners. All fatigue regions initiated at multiple sites on the outer (aft) surfaces of the tube structure and propagated inward (forward) through the wall of the main landing gear legs. The NTSB Materials Laboratory examination report is in the public docket for this accident. ADDITIONAL INFORMATION Previous Main Landing Gear Leg Failure ANC05LA097 identified a fracture failure of the right side main landing gear leg (part number 250-040-451-1, with about 7,006 hours in service) with a Helio H-250 during the landing roll. Landing Gear Inspection and Maintenance The Federal Aviation Administration has published Advisory Circular 43.13-1B Acceptable Methods, Techniques, and Practices – Aircraft Inspection and Repair (2001). This document discusses inspection and maintenance of landing gear and states in part: The entire structure of the landing gear should be closely examined for cracks, nicks, cuts, corrosion damage, or any other condition that can cause stress concentrations and eventual failure. Small nicks or cuts can be filed and burnished to a smooth contour, eliminating the point of stress concentration. If a crack is found in a landing gear member, the part must be replaced.