Media, PA, USA
N75070
Schweizer 269C
The student pilot was flying the helicopter in cruise flight, at 800 feet, when a loss of tail rotor thrust occurred, with a "sharp metallic-sounding squeal." The student pilot then performed an autorotation which resulted in a hard landing. Examination of the tail rotor drive shaft, forward fitting teeth, revealed smearing and rubbing in the middle of the tooth surfaces. A wear/rubbing failure mechanism had been previously seen on other helicopters, that resulted from insufficient lubrication. Although the forward fitting had a significant amount of grease on it, it could not be confirmed whether grease had always been present. No significant manufacturing defects were found. With over 3,000 helicopters in service, only two similar events had been attributed to insufficient lubrication, and both occurred on same helicopter model within the previous year. Shaft loading, which was "relatively light," only required minimal lubrication to a design that had remained essentially unaltered since the 1960s. The shaft, which required an inspection every 300 hours, had last been inspected 180 hours before the accident.
On February 28, 2004, at 1400 eastern standard time, a Schweizer 269C, N75070, was substantially damaged during a forced landing in Media, Pennsylvania. The certificated student pilot was not injured. Visual meteorological conditions prevailed, and no flight plan had been filed for the flight, which departed South Jersey Regional Airport (VAY), Mount Holley, New Jersey. The solo instructional flight was conducted under 14 CFR Part 91. According to a statement provided by the student pilot to the Pennsylvania State Police, he was westbound, at 800 feet, when "the tail rotor stopped." The student pilot then performed an autorotation to a yard. A witness, who was on his porch at the time, reported that he observed the helicopter "having difficulty flying." He heard the helicopter make a "sharp metallic-sounding squeal," and the tail rotor "quit working." The helicopter then made a "sharp, 45-degree angle toward the ground." Another witness was inside his home when he heard "a loud noise followed by a hard thump on the ground." When he looked outside, he saw the helicopter was "damaged heavily." According to the operator of the helicopter, the helicopter's frame was broken and the tail boom had collapsed. Initial reports from the scene also revealed that the skids were collapsed and the main rotor blades had contacted the ground. According to a Federal Aviation Administration (FAA) inspector and photographs he provided, tail rotor drive shaft continuity was compromised in the vicinity of a forward coupling. A section of tail rotor drive shaft and the drive spline, which was coated with a "significant" amount of grease, were forwarded to the Safety Board for examination. According to the examining engineer's factual report, the drive spline outer diameter teeth and tail rotor shaft fitting inner spline teeth were damaged. The tail rotor shaft was fractured approximately 20.5 inches from the forward end. Optical examination of the tail rotor drive shaft revealed that it was fractured circumferentially, 360 degrees-around, on slant planes, consistent with overstress. The wall on one side was relatively intact, while the opposing side was buckled inward, with damage consistent with bending loading. The fracture location was in line with the tail boom forward bulkhead. The spline teeth appeared relatively undamaged except for a 0.28-inch-wide wear region in the middle (axially) of the teeth (the mating spline teeth on the drive spline were approximately 0.25 inch wide). The wear surface was on a slightly slanted plane, beginning near the root of the driven side of each spline tooth, and increased in height toward the non-driven side of the tooth. The wear surface had a stepped appearance on the driven side, and on the non-driven side, had planar, flat features consistent with smear/rubbing. Minor rubbing was visible both forward and aft of the wear region, with the forward side being more severe, extending up to 0.15 inch from the wear region. Scanning electron microscope (SEM) examination of the stepped portion of the wear surface revealed that it contained a mixture of alternating linear impact marks and areas of fine metallic debris with smear/wear. Backscatter examination of the metallic debris revealed it to be of a similar composition as the base metal. No evidence of hard particle contamination was observed. Energy dispersive spectroscopy (EDS) analysis of the wear surface revealed the presence of iron, chromium, manganese, nickel, and silicon, consistent with the requirements of 9310 steel (AMS 6265), with the addition of aluminum. At the root surface of one tooth, elevated levels of phosphorus and manganese were also observed, consistent with a manganese phosphate conversion coating. A sample of grease was removed from the spline area, dissolved in acetone, and filtered. The resulting debris was mostly composed of very small metallic particulates. Some pieces appeared to be from the edge of a tooth crown. No large sections of spline teeth were found during the examination of all components/debris. A cross-sectional mount was made through some of the internal spline teeth in both undamaged and damaged areas, then etched. The mount revealed the visual presence of a carburized layer and the slant wear plane in the spline tooth. Higher magnification views of the teeth and representative microstructures in the case and core regions revealed smeared/folded over material on the non-driven side of the teeth. Micro-hardness measurements taken from the side of one of the intact areas of the teeth revealed the following hardness: Distance from Surface (in), Knoop, HRC: 0.002 658 56 0.007 624 55 0.013 530 49 0.733 (core) 438 43 The carburized case depth was approximately 0.012 inch, which conformed to the Schweizer specification SHP2-2 drawing requirement of 0.008-0.015 inch. An examination of the drive spline revealed that the exterior splines on the aft end were extensively damaged and worn/fractured down to approximately mid-height. A low magnification, etched cross-sectional metallographic mount through the teeth, revealed a flat fracture/wear plane. The tooth height was approximately 0.050 inch compared to the approximate nominal height of 0.092 inch. The metallographic mount showed the presence of a visual nitrided layer. Folded metal was visible on the non-drive side of the spline teeth. Examples of the case and core microstructures at high magnification were consistent with the requirements of Nitralloy 135 (MIL-S-6709). Micro-hardness measurements taken from the side of one of the teeth showed the following hardness. Per Schweizer specification SHP2-3, the nitrided case depth was approximately 0.008 inch, conforming to the drawing requirement of 0.005-0.008 inch. The specification also stated that an equivalent hardness of HRC 65 should be measured at a depth of 0.002 inch. Three measurements taken at a depth of 0.002 inch were HV 881 (HRC 66) , HV 741 (HRC 62), and HV 837 (HRC 65). Schweizer personnel noted that that wear/rubbing failures had been previously seen, and resulted from insufficient lubrication. However, with over 3,000 helicopters in service, only two prior events had been attributed to the insufficient lubrication, and both occurred on 269C models in the previous year. Shaft loading had been determined to be relatively light, and only required minimal lubrication for a design that had remained essentially unaltered since the 1960s. The shaft, which required inspection every 300 hours, had last been inspected 180 hours before the accident.
A lack of previous lubrication to the tail rotor drive shaft forward fitting teeth, which resulted in their gradual wear, the eventual slippage of the shaft, and the subsequent loss of tailrotor thrust and forced landing.
Source: NTSB Aviation Accident Database
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