LAKEWOOD, CO, USA
N937S
Bell 206L-3
During initial descent at the completion of a cross-country flight, the pilot heard a loud bang from the rear of the helicopter and manipulation of the tail rotor control became ineffective. The pilot performed an autorotation to a landing in a field, which sloped up about 6 degrees and was rough and uneven. During landing with about 5 knots forward speed, the lower wire cutter dug into the ground and the skids collapsed twisting the cross tubes aft. The main rotor blades were not damaged and the tail booms buckled downward at the forward attach point. The pilot performed a normal shut down and he and his passenger exited the helicopter with no injuries. Examination of the helicopter following the event revealed that both tail rotor blades had separated approximately 9 inches outboard of the hub. Laboratory tests revealed that one tail rotor blade failed from fatigue. The fatigue origin occurred at the protective strip trailing edge inboard of the outboard tip of the blade doubler. Inspection at the origin revealed the fatigue was a result of chemical machining/etching that took place during the manufacturing process.
HISTORY OF FLIGHT On April 18, 2000, at 1021 mountain daylight time, a Bell 206L-3 helicopter, N937S, sustained substantial damage during an emergency autorotative landing near Red Rocks Mountain Park, Lakewood, Colorado. The airline transport certificated pilot and his passenger were not injured. Visual meteorological conditions prevailed for this business flight operating under Title 14 CFR Part 91 and no flight plan was filed. The flight originated from Kremmling, Colorado, at 0950. According to the pilot, he was in a descent at about 9,500 feet above mean sea level (msl) en route to Centennial Airport, Englewood, Colorado, (approximately 110 degrees magnetic heading at 20 miles) when he heard a "loud bang" from the rear of the helicopter. The pilot said he attempted to move the tail rotor control pedals but got no response so he performed an autorotation to a landing in a field. During the descent, the helicopter spun several times. During the landing, with about 5 knots forward speed, the lower wire cutter dug into the uneven terrain, the skids collapsed when the cross tubes rolled aft, and the tail boom flexed downward which buckled the boom at the forward attach point. The main rotor blades were not damaged. The pilot performed a normal shutdown and he and his passenger exited the helicopter. PERSONNEL INFORMATION The pilot held an airline transport certificate (helicopter) with a type rating in the Bell 206 and Bell 212. He held a commercial pilot certificate in single engine and multiengine land airplanes. He did not have an instrument rating. Federal Aviation Administration (FAA) records provided information that the pilot held a second-class medical certificate with the limitation that he wear corrective lenses. His medical certificate was issued on August 17, 1999. AIRCRAFT INFORMATION The helicopter, serial number 51258, was manufactured in 1988. Total time on the airframe was 4,832.4 hours. It was owned and operated by Mountain States Telephone and Telegraph Company. The tail rotor gearbox was part number 206-040-402-003, serial number A-1526. The tail rotor hub assembly was part number 206-011-810-139, serial number A-6062. The tail rotor blades were part number 206-016-201-127, serial numbers CS 5960 and CS 5963. The tail rotor blades were installed as new blades on January 16, 1994, and had 1,958.1 hours accumulated time. According to information provided by the FAA, Rotorcraft Directorate, Composite Structures, LLC, Monrovia, California, manufactured the tail rotor blades. The service life on the blades was 2,400 hours. Applicable Airworthiness Directives addressing tail rotor blades with the above part numbers were reviewed. The blade serial numbers did not come within the purview of the directives. WRECKAGE AND IMPACT INFORMATION Terrain features at the accident site consisted of an open field sloping approximately 6 degrees up from southwest to northeast. The surface was rough and uneven with a field grass covering. The helicopter came to rest on a heading of 060 degrees, in an upright position, sitting on the underside of the fuselage. Both skids separated and the cross tubes were twisted aft approximately 100 degrees. The underside of the fuselage was crushed in the cross tube attach area and the left side of the windscreen was cracked. The tail boom was deflected down and exhibited wrinkling at the attach point to the fuselage. The aft body of the fuselage was wrinkled forward of the tail boom attach point. The tail rotor 90-degree gearbox was separated from its mounting points and remained in the mounting area. The tail rotor drive shaft was separated at the spline-coupling forward of the gearbox and the tail rotor pitch change rod was fractured. Both tail rotor blades were separated from the helicopter approximately 9 inches outboard from the hub and the fairing around the tail rotor gear box assembly was deformed with white paint transfer and impact witness marks from tail rotor blades. During a ground search back along the perceived track of the helicopter, the outer portions of both tail rotor blades were recovered. Both were within a mile of the accident site. TESTS AND RESEARCH The aft portion of the tail boom, fairing, 90 degree gearbox, tail rotor pitch change rod, tail rotor hub, and both tail rotor blades were sent to the National Transportation Safety Board Office in Arlington, Texas. On April 26, 2000, an investigator from that office supervised an examination of the components at the facilities of Bell Helicopter Textron, Inc., Fort Worth, Texas. The examination provided evidence that one tail rotor blade (serial number CS 5960) fractured as a result of fatigue. The fatigue origin occurred at the protective strip trailing edge inboard of the outboard tip of the blade doubler that was on the side nearest the tail boom. Inspection at the origin area revealed a doughnut shaped groove at the trailing edge of the protective strip. The groove was filled with adhesive from the tail rotor blade manufacturing process. No corrosion was present. Microscopic examination of a metallurgical section at the origin area revealed no evidence of mechanical deformation or electrical arcing at the groove. According to the metallurgist, this indicated the groove was likely a result of chemical machining and/or etching processes used during manufacture prior to bonding the blade assembly. Bell Engineering provided information that chemical machining was used during manufacture of the protective strip to create the tapered thickness required by the engineering drawing. The protective strip was then chemically etched prior to bonding into the blade assembly. Examination of the opposite tail rotor blade (serial number CS 5963), the tail rotor pitch change rod, gearbox, and gearbox mounting components revealed they all failed due to overload. Details of the examination are attached as part of the docket. ADDITIONAL INFORMATION Parties to the investigation were Mountain States Telephone and Bell Helicopter. The wreckage was released to the custody of Century Helicopters, Fort Collins, Colorado. On April 18, 2000. The tail rotor hub, pitch change rod, fairing, blades, and gearbox were retained for further examination. They were returned to Mountain States Telephone, Grand Junction, Colorado, on July 8, 2000. No other parts were retained.
The complete loss of the tail rotor blades due to a fatigue failure of one blade. Factors were sloping and rough uneven terrain for the ensuing forced landing.
Source: NTSB Aviation Accident Database
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