Leet, WV, USA
N61193
Schweizer 269C
While investigating a fire, the pilot noticed an "unusual vibration," and a "needle split" on the rpm gauge. The rotor speed began to decay and the engine speed began to increase. The pilot then entered an autorotation. The helicopter, impacted terrain, was substantially damaged, and a passenger was seriously injured. Examination of the drive train revealed that the main drive pinion, which drove both the main rotor assembly and the tail rotor drive shaft, had failed in fatigue that had initiated in the root of the first (most forward) engaged pinion thread. The failed pinion had been in service for 4,055.4 hours. Including the accident pinion, 10 instances of pinion failures were identified during the investigation. Times in service ranged from 261.5 to 4,243 hours. According to the manufacturer, life limits for the pinion ranged from 6,000 to 8,000 hours depending on application. As a result of this investigation, The FAA issued an SAIB alerting owners and operators of pending revisions to the inspection procedures for the pinion. The manufacturer revised its Handbook of Maintenance Instructions, to include the revised inspection procedures, and submitted design revisions to the FAA to make the part more robust.
HISTORY OF FLIGHT On March 19, 2005, at 1636 eastern standard time, a Schweizer 269C, N61193, was substantially damaged during an impact with trees following an autorotation near Leet, West Virginia. The certificated commercial pilot was not injured, and the sole passenger was seriously injured. Visual meteorological conditions prevailed, and no flight plan was filed for the personal flight from the Mallory Airport (WV12), South Charleston, West Virginia, to the Logan County Airport (6L4), Logan, West Virginia, which was conducted under 14 CFR Part 91. According to the pilot's written statement, while investigating a fire at an altitude of 2,300 feet msl, he noticed an "unusual vibration," and a "needle split" on the rpm gauge. When adding collective during a right 90-degree turn, the rotor speed began to decay and the engine speed began to increase. The pilot then lowered the collective to conserve rotor rpm and entered an autorotation at an airspeed of 55 knots. The main rotor blades contacted trees, and the helicopter contacted the ground in a nose down attitude and rolled onto its right side. The accident occurred during the hours of daylight. The wreckage was located at 38 degrees, 03.07 minutes north latitude, 081 degrees, 59.13 minutes west longitude, at an elevation of 1,135 feet. PERSONNEL INFORMATION A review of Federal Aviation Administration (FAA) records revealed that the pilot held a commercial pilot certificate and flight instructor certificate with ratings for rotorcraft-helicopter and instrument helicopter. His most recent FAA second-class medical certificate was issued on March 25, 2004. He had accrued 750 total hours of rotorcraft flight experience, and had flown 50 hours in rotorcraft in the previous 90 days. AIRCRAFT INFORMATION The helicopter was manufactured in 1992. On February 15, 2001, it was involved in an accident, which damaged the pitch change links, and the 90-degree gearbox. After accumulating 791.1 total hours of operation it was refurbished by Schweizer Aircraft in 2002 and returned to service that same year. The helicopter's most recent annual inspection was completed on November 1, 2004, and at that time, it had accumulated 1,865 total hours of operation. METEOROLOGICAL INFORMATION The reported weather at Yeager Airport (CRW), Charleston, West Virginia, located approximately 38 miles from the accident site, at 1654, included wind from 170 degrees at 16 knots, gusting to 20 knots; visibility 10 miles, ceiling broken at 8,000 feet, temperature 57 degrees Fahrenheit, dew point 28 degrees Fahrenheit, and an altimeter setting of 29.91 inches of mercury. WRECKAGE AND IMPACT INFORMATION After striking several trees, the helicopter came to rest on a road. No debris path was evident, and all the major components of the helicopter were accounted for at the accident site. The main wreckage displayed varying degrees of impact damage. The main swash plate assembly and main rotor blades both exhibited impact damage. The right landing gear skid tube had broken off and the tail boom had separated. Additionally, it was noted that the canopy was damaged and the pilot's compartment had been compromised. Flight control continuity was confirmed and no evidence of preimpact damage or malfunction of the engine was observed. Examination of the drive train, revealed that main drive pinion, part number 269A5103-51, which drove both the main rotor assembly and the tail rotor drive shaft, had fractured through its aft threads and separated from the aft pinion nut. Detailed examination of the component revealed discoloration, and beach marks on the fractured surface. The examination of the drive train was suspended and the main drive pinion was removed. The rearmost 6 inches of the pinion, the aft pinion nut (with a portion of the pinion thread), and the splined drive sleeve were shipped to the Safety Board's materials laboratory for further examination. TESTS AND RESEARCH On April 26, 2005, the Safety Board was notified of another pinion event. An interview was conducted with an airframe and powerplant mechanic regarding that incident which involved a Schweizer 269C-1, N61486 on March 17, 2005. The mechanic stated that he had been contacted by a pilot who was concerned about some "red dust" that was found on top of the "bell casing" of the helicopter. The mechanic advised him not to fly the helicopter, as he believed that something was overheating. When the mechanic examined the helicopter, he discovered that the main drive pinion was broken and had separated at the aft threads. A review of the helicopter's maintenance records showed that the pinion had been in service for 3,115.4 hours. The Safety Board retained this pinion and the aft pinion nut (with a portion of the pinion thread in it) for comparison with the accident helicopter's pinion. Main Drive Pinion Examinations Examinations of the accident pinion and the comparison pinion were continued at the Safety Board's materials laboratory. During the examinations it was discovered that both the accident pinion which had been in service for 4,055.4 hours, and the comparison pinion which had been in service for 3,115.4 hours had failed in similar locations. Similar wear patterns, fretting and corrosion were present on the normally contacting flanks of the pinion threads and aft nuts. The pinion splines that engaged with the upper drive pulley also exhibited wear on the driven flanks. The fracture faces on the threaded portions of the pinions were examined by scanning electron microscope (SEM). The SEM examination revealed that distinct crack arrest lines and striations existed, consistent with fatigue that had initiated in the thread root. Review of the Handbook of Maintenance Instructions (HMI) According to the HMI periodic inspections, table B-2, it was required that the aft pinion nut be subjected to a torque check every 100-hours and the splines on both the pinion and it's mating parts be subjected to an inspection every 300-hours. Re-torque checks were also carried out as part of major component work involving the pinion, such as an upper pulley teardown, or part of a gear pattern check after a transmission overhaul. The HMI appendix B, table B-4, component overhaul schedule, specified that the transmission for the Schweizer 269C should be overhauled every 3,000 hours and the pinion could be replaced at that time, if it failed to meet minimum overhaul criteria. The HMI appendix B, table B-5, component mandatory replacement schedule specified that the transmission input pinion for the Schweizer 269C should be replaced every 6,000 hours. Additional Research/Documentation Additional research by the Safety Board determined that an airworthiness directive had previously been issued by the FAA for fractures discovered in the 269A5103-9 pinions, due to fatigue cracking that had originated in the longitudinal keyway. Subsequent pinions were then designed without the keyway. Warranty information and service difficulty reports, both in the United States and Australia, showed that the other pinion models (269A5103BSC and 269A5103-51) had also failed. These pinions exhibited fretting corrosion, fretting damage on the upper pulley hub and bearings, and in one instance what the reporter described as "nut has a history of loosening." Schweizer Aircraft had on more than one occasion issued guidance for torque checks on the different models of the 269/300 series of helicopters. The Civil Aviation Authority of Australia had also issued an airworthiness directive based on Schweizer Aircraft advisory information, concerning cracking of the pinion assembly. Multiple laboratory reports by an independent materials testing laboratory were also discovered, predating the accident, which attributed previous failures of the pinion to fatigue. Additionally, on September 19, 2005, Schielab b.v. of Holland produced a report, for the Dutch Transport Safety Board describing a fatigue failure in a similar location as the cracking observed by the Safety Board on the accident and comparison pinions. ADDITIONAL INFORMATION The Safety Board completed its Materials Laboratory Factual Report on September 28, 2005. Fatigue was noted on both pinions, which originated in the root of the first thread at the aft end of the pinion. In all, 10 failed pinions including the accident and comparison pinion were identified by the Safety Board. Times in service ranged from 261.5 to 4,243 hours. The average failure time was 2,168.7 hours. In a letter to the Safety Board dated February 6, 2006, Schweizer Aircraft "confirmed" that 10 pinion fractures at the aft nut threads had been identified. According to Schweizer Aircraft, life limits for the main transmission drive pinion ranged from 6,000 hours to 8,000 hours, depending on application. Corrective Actions As a result of this investigation, the FAA issued SAIB NE-06-03R1, which alerted Schweizer 269 owners and operators of a planned revision to the HMI. Schweizer Aircraft subsequently revised their HMI, to include guidance in the event that the pinion nut rotated on the shaft during torque application. A visual inspection was also added to check for fretting damage of the upper pulley hub splines and spline drive, and for galling, fretting, corrosion, and cracks in the bottom of the thread grooves and thread relief. Additionally, Schweizer Aircraft submitted design revisions to the FAA's New York Aircraft Certification Office, which included making the threaded end of the pinion solid instead of hollow, a re-profile of the threads, and the use of a higher strength nut. These changes applied to all models of Schweizer helicopters for commonality and provided better resistance to adhesive wear and galling. At the time of this writing, the FAA was evaluating the proposed changes. WRECKAGE RELEASE The wreckage was released to the owner on April 12, 2005, with the exception of the main drive pinion, which was released on April 11, 2006.
A fatigue fracture of the main transmission drive pinion. Also causal was the manufacturer's failure to remove the affected pinions from service during the period that transpired between the initial failures and the accident flight.
Source: NTSB Aviation Accident Database
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