Madera, CA, USA
N601MT
Bell 407
The pilot executed an autorotative emergency landing following the failure of the number 6 tail rotor drive shaft hangar bearing while maneuvering. The check pilot and pilot were maneuvering the helicopter when it shuddered momentarily, followed by a heavier airframe vibration. The helicopter rolled left as it climbed to 500 feet. The check pilot attempted to turn right towards the airport; however, the helicopter rolled to the left again. The check pilot regained control, shut the engine down, and completed an autorotation to a field. The tail rotor drive shaft coupling was supported by the aft hangar bearing, and inspection revealed that it had sheared in torsional overload due to an overheated and seized number 6 hangar bearing. Post accident examination revealed that the hangar bearing inner race and the balls had large heat affected zones from overheating during operation (the metal in the races ball path had been molten and then had resolidified). The grease was also black and dry, with imbedded metal particles. A series of service bulletins, alert service bulletins, and airworthiness directives (including an emergency AD issued by the FAA) had been issued beginning in October 2001 through 2004 concerning the overheating and failure of the hangar bearings. The bulletins and airworthiness directives dealt with, in part, the modification of the helicopter to include cooling ducts and changes to existing bearings to prevent overheating and eventual bearing failure. The ADs mandated inspections of the hangar bearings every 25 hours of flight; bearing replacement before further flight was called out if metal particles or "black" grease was observed at any bearing.
On February 19, 2004, at 1520 Pacific standard time, a Bell 407, N601MT, made an autorotative emergency landing following a failure of the number 6 tail rotor drive shaft hangar bearing, while maneuvering 1 mile northwest of Madera Municipal Airport, Madera, California. Med Trans Corporation was operating the flight under the provisions of 14 CFR Part 91. The commercial check pilot and the airline transport pilot were not injured; the helicopter sustained minor damage. The flight departed the Madera Children's Hospital about 1400 for the local area initial training flight. Visual meteorological conditions prevailed, and no flight plan had been filed. The Federal Aviation Administration (FAA) inspector reported that the check pilot and pilot were attempting an approach maneuver. The helicopter shuddered momentarily, followed by a heavier airframe vibration. The helicopter rolled left as it climbed to 500 feet. The check pilot attempted to turn right towards the airport; however, the helicopter rolled to the left again. The check pilot regained control, shut the engine down, and completed an autorotation to a field. The tail rotor drive shaft coupling is supported by the aft hanger bearing, and post accident inspection revealed that the number 6 hangar bearing had sheared. The hangar bearing and drive shaft were examined at Bell Helicopter Textron on May 6, 2004, in the presence of an investigator from the National Transportation Safety Board, a representative from the FAA Rotorcraft Standards, and a representative from Bell. The bearing was identified as Bell part number 407-340-339-101. The drive shaft hangar bearing had a fatigue fracture in the bearing cage at one ball pocket. All of the balls showed evidence of skidding on the inner race. The bearing's inner ring was discolored and the grease was black and dry. The inner ring had grooves in the ball path and what appeared to be resolidified metal in the bottom of the ball path. According to the examining metallurgist, the inner ring had been hot enough for the metal to "flow," which created a ridge on the shoulders. Hardness tests and metallurgical sections showed that the examined components had heat affected zones from overheating during operation. A sample of grease from a grease gun used by company maintenance personnel to lubricate the bearings was also submitted to the laboratory. An analysis of the sample showed that it was chemically consistent with Royco 13 grease. On October 3, 2001, Bell Helicopter Textron issued an information letter (407-01-66) regarding greasing of tail rotor drive shaft bearings. It had come to the company's attention that the bearings were being overgreased by maintenance personnel. The letter stated that overgreasing of the bearings will result in overheating. On October 17, 2002, Bell Helicopter Textron issued Alert Service Bulletin (ASB) 407-02-54. The ASB prescribed a means to modify the helicopter using ducts to increase the airflow and cooling properties, thus preventing overheating of components. The FAA issued an Emergency Airworthiness Directive, AD 2002-06-52, on March 15, 2002. The AD addressed a tailwind limitation to restrict certain operations, inspection for bearings, replacement of certain bearings, and lubrication of the bearings. The text of the AD specifically notes that during mandated inspections of the bearings every 25 hours, if metal particles are seen in the expelled grease or if the grease has turned black that the bearings must be replaced before further flight. Transport Canada issued AD number CF-2002-18R2 on November 5, 2003. Its subject was the tail rotor drive shaft bearings. It noted that the initial AD (CF-2002-18) mandated specific downwind hover and vertical flight limitations to avoid overheating in the oil cooler blower area (an interim measure pending design changes). It also called for replacement of the bearing at the oil cooler locations and a specific inspection and lubrication schedule for the replacement bearing, and inspection and lubrication schedules for bearings at other hangar bearing locations. Revision 1 of this AD provided relief to the flight limitations with the incorporation of oil cooler blower inlet ducts and bearing airflow shields. Part B of AD CF-2002-18R2 called for the replacement of bearings in accordance with Bell Helicopter Textron Company (BHTC) ASB 407-01-44, Revision A. On February 10, 2004, Bell Helicopter Textron issued ASB 407-04-63, which superceded the following bulletins: ASB 407-01-44, Revision C dated September 23, 2003; ASB 407-01-47, Revision B dated June 24, 2003; and TB 407-03-43 dated September 22, 2003. The ASB mandated replacement of certain tail rotor drive shaft bearings. It also mandated the replacement of part number 406-040-339-101 and -103 bearings with newer -107 bearings On June 4, 2004, the FAA issued AD 2004-10-07. The AD requires, in part, modification of the oil cooler fairing inlet ducts and replacement of the oil cooler blower and specified segmented shaft bearings in accordance with BHT ASB 407-04-63.
the failure of the number 6 hangar bearing due to overheating. A factor in the accident was the operator's likely inadequate compliance with service bulletins and airworthiness directives on this issue, along with an inadequate maintenance inspection program for the bearings.
Source: NTSB Aviation Accident Database
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