N7507ASchweizer 269C · 1987

On July 23, 2013, at 1520 central daylight time, a Schweizer 269C helicopter, N7507A, impacted terrain during a forced landing near Gary, South Dakota. The pilot and one passenger were not injured, and one passenger sustained minor injuries. The helicopter sustained substantial damage to the fuselage and main rotor system. The helicopter was registered to Novne Aviation LLC, and operated by the pilot under the provisions of 14 Code of Federal Regulations Part 91 as a personal flight. Visual meteorological conditions prevailed for the flight, which was not operated on a flight plan. The local flight originated from Canby, Minnesota, at 1500.According to the pilot, he was descending from approximately 1,000 feet above ground level (agl) and setting up for a landing at an off airport location. At 300 to 500 feet agl, the pilot felt a major vibration with the helicopter. He immediately initiated an autorotation and landed the helicopter. During the forced landing, the helicopter rolled over on its right side and sustained substantial damage.

On September 18, 2013, under the supervision of a Federal Aviation Administration inspector, the Lycoming HIO-360-D1A engine, serial number L-11954-51A, and helicopter were examined. Visual examination of the engine revealed a hole in the crankcase near the number 3 cylinder. Prior to engine disassembly, the inspector attempted to rotate the engine crankshaft by hand; however, the crankshaft was unable to rotate. The engine was then disassembled. Disassembly of the engine revealed the number 3 connecting rod was fractured into multiple sections. The connecting rod and associated hardware were retained for further examination. No evidence of fretting was noted on the base of the cylinders or engine crankcase mating surfaces. The number 3 crankshaft journal displayed dark discoloration. No evidence of oil starvation was noted on the crankshaft bearings. Extensive damage, consistent with the failure of the number 3 connecting rod, was noted throughout the engine.

A review of the airframe and engine logbooks revealed that a major overhaul on the engine was completed on April 20, 1998. The most recent annual inspection was completed on the engine on October 29, 2012, at 881.9 hours since major overhaul. During the annual inspection, all four engine cylinders were removed and overhauled. On May 14, 2013, at 893 hours since major overhaul, the engine oil was changed with no anomalies noted. At the time of the accident, the engine had accumulated 899 hours since major overhaul.

The National Transportation Safety Board's Materials Laboratory examined the engine's number 3 connecting rod and associated hardware. According to the examination, the connecting rod and cap had fractured in multiple locations, and the head of one of the connecting rod bolts had fractured. A fracture surface located on one of the connecting rod yokes exhibited crack arrest and ratchet marks, consistent with progressive crack growth and failure. More than any of the other fractures, this fracture exhibited the largest percentage of progressive fracture relative to overstress features. A thin middle portion of the fracture surface exhibited a morphology consistent with overstress. This overstress area was bounded on the opposite side by progressive cracking that had initiated and grown from the inner bearing surface and bolt hole inner bore of the connecting rod yoke.

The inside bore of the connecting rod bolt hole where the main progressive cracking had initiated exhibited relatively deep grooves. Several of the crack initiation sites corresponded with these grooves on the inside bore.

A fracture surface located in the connecting rod cap exhibited several thumbnail-shaped features indicative of progressive crack growth on the inner bearing surface. These cracks were not located along the same plane and were staggered in location, while being oriented parallel to each other.

The inside surface of the rod cap exhibited multiple cracks less than 0.25 inches in length, perpendicular to the rotational direction of the rod. These cracks were consistent with fatigue that typically occurs in heavy wear during contact between mating surfaces. Fatigue cracking was found emanating from both the connecting rod bolt hole inner bore, as well as from the inside bearing surface. The existence of ratchet marks near the start of the fatigue crack was consistent with multiple crack initiation sites. A thin section of the fracture surface between the fatigue cracks exhibited dimple rupture, indicative of failure from tensile overstress. This fracture pattern was consistent with the final remaining cross section failing from overstress after the fatigue cracks had progressed deep enough. Inspection of the connecting rod found the chemical composition to be consistent with an American Iron and Steel Institute (AISI) 4XXX or 8XXX series alloy steel.

To determine the nature of the cracks on the inner bearing surface of the connecting rod cap, the largest one was backcut and intentionally overstressed to expose the crack faces. Three small thumbnail-shaped cracks were revealed; these cracks exhibited features consistent with fatigue.