Houma, LA, USA
N501PH
Bell 407
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.
HISTORY OF FLIGHT On March 24, 2003, approximately 1635 central standard time, a Bell 407 helicopter, N501PH, registered to and operated by Petroleum Helicopters, Inc., of Lafayette, Louisiana, was substantially damaged during a forced landing following an in-flight loss of tail rotor control in the vicinity of Houma, Louisiana. The commercial pilot and one passenger were not injured. Visual meteorological conditions prevailed and a company visual flight rules plan was filed for the Title 14 Code of Federal Regulations Part 135 on-demand air taxi flight. The flight originated from an offshore platform (EI-132) in the Gulf of Mexico at 1621, and was destined for Houma, Louisiana. In a statement provided to the NTSB Investigator-In-Charge (IIC), the pilot reported that while in cruise flight at an indicated airspeed of 130 knots and an altitude of approximately 1,200 feet AGL, he "felt" and "heard" vibrations. The pilot described the vibrations as lasting 2-3 seconds in duration and approximately 2-3 seconds apart. The pilot elected to make a forced landing and initiated a descending right turn. At 20-30 feet AGL, the helicopter started an uncommanded right turn. The pilot lowered the collective, rolled off the throttle, and deployed the skid mounted emergency floats. The aircraft rotated approximately 270 degrees and landed "hard" on the saturated marsh terrain. The pilot stated that the engine power setting was at 87 percent when the event began. PERSONNEL INFORMATION The pilot was employed by Petroleum Helicopters, Inc., (P.H.I.) and held a current commercial pilot certificate. He had accumulated a total of 4,519 hours of rotorcraft flight time (85 hours of which were in the accident aircraft). The pilot held a valid FAA Class 2 medical certificate, with no limitations or waivers. AIRCRAFT INFORMATION Manufactured in 1999, the Bell 407, serial number 53401, was delivered new to P.H.I. on December 31, 1999. At the time of the accident, the airframe total time was 2,911.5 hours. The helicopter was powered by an Allison/Rolls Royce 250-C47B turboshaft engine, serial number CAE-847071, which was manufactured on August 7, 1996, and had accumulated a total time of 5,983.30 hours since new. Information provided by the manufacturer showed that the helicopter had no prior reported mishaps. The aircraft was maintained on a 4-event FAA Approved Aircraft Inspection Program (AAIP). Entries in the maintenance records revealed that the helicopter's last inspection was completed on February 8, 2003; 101.25 hours prior to the accident. A detailed review of the maintenance records by the IIC revealed the following information regarding the tail boom assembly and sub-assemblies: Tail boom Assembly: p/n 407-030-801-203, s/n BP1410 Total assembly time: 1,940.25 hours Tail Rotor Driveshaft and Hangar Bearing Assembly: p/n 407-040-302-107 This assembly consists of 4 segments, numbered #3-#6. Each segment has a different serial number. Total part time(s): (#3) s/n A-1962, 2,907.25 hours (#4) s/n A-1958, 2,907.25 hours (#5) s/n A-1954, 2,907.25 hours (#6) s/n A-2218, 1,940.25 hours Tail Rotor Gearbox: p/n 406-040-400-119FM, s/n A-941 Total part time: 3,062.55 hours Time since overhaul: 780.35 hours Tail Rotor Hub and Blade Assembly: p/n 407012-101-111, s/n 53397 Total assembly time: 2,358.15 hours Tail Rotor Driveshaft Hangar Bearing Assembly: p/n 407-340-339-101 Consists of 4 hangar bearings, numbered #3-#6. Each hangar bearing has different serial number. Total part time(s): (#3) s/n ZV10953, 1983 hours (#4) s/n ZV10829, 1953 hours (#5) s/n ZV11263, 1953 hours (#6) s/n ZV 11915, 1953 hours Hangar bearing positions are relative to the tail rotor drive system as counted from forward to aft. METEOROLOGICAL INFORMATION New Orleans, Louisiana, located 36 miles southwest (245 degrees) of the accident site, was the nearest weather reporting station to the accident site. At 1655, New Orleans reported winds from 190 degrees at 7 knots, visibility 7 statute miles, a broken ceiling at 20,000 feet, temperature 23 degrees Celsius, dew point 9 degrees Celsius, and an altimeter setting of 30.03 inches of Mercury. WRECKAGE AND IMPACT INFORMATION The helicopter came to rest upright and intact with the skid mounted floats deployed in a marshy, saturated field. 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. At the direction of the NTSB IIC, due to the terrain and environmental conditions, the operator's on-site personnel arranged to preserve the damaged area and components until the helicopter could be recovered and the components examined. The helicopter was transported by road to the operators Morgan City, Louisiana, base on March 25, 2003, for an initial examination. Examination of the helicopter at Morgan City revealed the emergency floats had been deployed and the landing gear crosstubes were yielded outward. The floats were then deflated and the helicopter was moved into a hangar. Thermal distress and heating was visually observed at the #5 tail rotor drive shaft hangar bearing. The tail rotor drive shaft adapter at the distressed hangar was found fractured and rotational scoring was observed on the inside of the top of the tail boom structure adjacent to where the initial hole was found. The scoring appeared to be consistent with contact from the separated drive shaft coupling. On March 26, 2003, the helicopter and components were transported to the operator's maintenance facility located at Lafayette, Louisiana. A check was performed on the airspeed actuated pedal stop system by applying pitot pressure and testing the function of the system. The check revealed the stop engaged at 55 knots indicated airspeed (I.A.S.) and disengaged at 50 knots I.A.S. which meets operational specifications. The manual pedal stop release T-handle was found safety wired in the stowed position. The tail rotor hub and blade assembly had been removed from the gearbox by the operator to facilitate examination. The tail rotor blades displayed slight scuffing near their respective tips and one blade had an area of trailing edge delamination near it's inboard end approximately 1 inch in length. Measurement of the tail rotor flapping stops revealed gaps of .018 inch and .017 inch respectively which were less than the .055 inch normal gap. An area of splatter containing metal chips and grease were observed on the left lower forward surface of the 90 degree gearbox. The tail boom, tail rotor driveshaft segments, and associated bearings were shipped to the Bell Helicopter Field Investigation Laboratory, Hurst, Texas, for detailed examinations of the failed components. TESTS AND RESEARCH On March 31, 2003, under the supervision of the NTSB IIC at the Bell Helicopter Field Investigation Laboratory, Hurst, Texas, the fractured #5 segment of the tail rotor driveshaft and #5 bearing were examined in detail. Findings during the examination were: 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 through 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 bearings (p/n 407-340-339-101). On February 10, 2004, the manufacturer issued 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. ADDITIONAL DATA The wreckage was released to the owner's representative.
The loss of tail rotor drive and anti-torque control resulting from the fracture and separation of the #5 tail rotor drive segment due to the overtemperature and subsequent failure of the #5 hangar bearing.
Source: NTSB Aviation Accident Database
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