Bell 407 N501PH

While in cruise flight at 130 knots, the pilot of the commercial air taxi helicopter "felt and heard vibrations." The pilot described the vibrations as lasting 2-3 seconds in duration and approximately 2-3 seconds apart. He then initiated a descending right turn toward a saturated marshy field. At 20-30 feet above the ground the helicopter started an uncommanded right turn, so the 5,000-hour pilot lowered the collective, rolled off the throttle, and deployed the skid mounted emergency floats. The helicopter rotated approximately 270 degrees and landed "hard." The pilot recalled the power setting was at 87 percent when the event began. Initial on-site examination of the helicopter by the operator revealed a hole on the top of the tail boom driveshaft cover adjacent to the #5 hangar bearing position (above the horizontal stabilizer). The 6-inch long by 3-inch wide hole was large enough to clearly view a separation of the tail rotor driveshaft at the #5 bearing position. Examination of the helicopter revealed that the #5 driveshaft segment was fractured at its forward end adjacent to the journal for the #5 hangar bearing. The initial fracture appeared to occur in the journal for the #5 bearing adjacent to the coupling adapter (commonly called the "Thomas Coupling"). The outer ends of coupling lugs and the Thomas coupling lugs exhibited wear where they had contacted the inner portion of the tail boom drive shaft cover. The forward half of the fracture appeared to have been worn after the initial fracture occurred. The splined end of the #5 driveshaft segment remained in the coupling adapter at its forward end. Some material was missing from the forward half of the fracture on the splined end of the shaft. The fracture face exhibited a "broom straw" appearance consistent with the fracture occurring at or near the melting point for the material. The driveshaft material was made from 2024 aluminum alloy. The retainer ring for the #5 hangar bearing was found plastically deformed and out of its housing, and the cage in the bearing was fractured with the 13 balls (all balls were present as per the engineering drawing). The bearing exhibited overheating signatures. The balls and inner ring exhibited thermal discoloration. The inner ring appeared to have aluminum galled to the surface, and micro-examination of a section though the inner ring revealed the heating had progressed from the ball path to the inner diameter of the ring. The bearing shields were deformed and the elastomeric seal was missing from the shields. An intact ring of the elastomeric lip apparently torn from one of the shields was present. The edge of the elastomeric seal that had been adjacent to the bearing inner ring showed evidence of thermal distress on the edge that was against the inner ring. The bearing cage halves displayed fractures at the spot welds, and segments of the cage halves were fractured. Fatigue fractures were found at two cage welds. Grease samples at hangar bearing #5 were taken from the bearing's forward face, aft face, and grease holes in the outer ring. These samples, when tested, were found to be chemically similar to "Royco 13" (Mil-G-25013) grease that was specified for use in the bearing (p/n 407-340-339-101). After the accident, the manufacturer issued an Alert Service Bulletin No. 407-04-63 that outlines procedures for the replacement of bearings using "Royco 13" grease with new bearings that uses Mobile 28 grease. Readily identifiable colored seals differentiate between the bearings with regard to the type of grease. Bearings with Royco 13 grease have a light blue seal, and bearings with Mobile 28 grease have an orange seal. Additionally, a new part number for bearings with Mobile 28 grease was assigned, p/n 407-340-339-107. According to the manufacturer, Mobile 28 grease is more tolerant to heating.

Quoted verbatim from the NTSB record.