Incident Overview
While in cruise flight at 130 knots, a commercial air taxi helicopter pilot felt and heard vibrations lasting 2-3 seconds, with intervals of 2-3 seconds. The pilot initiated a descending right turn toward a saturated marshy field. At an altitude of 20-30 feet above ground, the helicopter began an uncommanded right turn. The pilot, who had accumulated 5,000 flight hours, responded by lowering the collective, rolling off the throttle, and deploying the skid mounted emergency floats. The helicopter rotated approximately 270 degrees before landing hard. The pilot recalled the power setting was at 87 percent when the event began.
Examination Findings
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, located above the horizontal stabilizer. The hole measured 6 inches long by 3 inches wide, large enough to clearly view a separation of the tail rotor driveshaft at the #5 bearing position. Further examination showed 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, with some material 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.
Bearing and Grease Analysis
The retainer ring for the #5 hangar bearing was found plastically deformed and out of its housing. The cage in the bearing was fractured, with 13 balls present as per the engineering drawing. The bearing exhibited overheating signatures. The balls and inner ring showed thermal discoloration. The inner ring had aluminum galled to its surface, and micro-examination of a section through the inner ring revealed 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 from hangar bearing #5 were taken from the bearing's forward face, aft face, and grease holes in the outer ring. When tested, these samples were found to be chemically similar to "Royco 13" (Mil-G-25013) grease, which was specified for use in the bearing (part number 407-340-339-101).
Manufacturer Actions
After the accident, the manufacturer issued an Alert Service Bulletin No. 407-04-63 outlining procedures for the replacement of bearings using "Royco 13" grease with new bearings using 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.