Seneca Falls, NY, USA
N2138Y
Bell 206 L-1
The Bell 206 L-1 helicopter was on a positioning flight. While in cruise flight, the pilot added power for a slow climb, and experienced multiple "bangs" and yawing. He reduced power, determined that he had lost tail rotor thrust, and set up for a precautionary landing in a wooded area. Upon touchdown, the helicopter rolled over. A bolt was found missing from the Thomas coupling located between the aft end of the aft short shaft, and the foward segment of the tail rotor drive shaft. In addition, the bracket for the forward hanger bearing was fractured. Metallurgical examination of the flange from the Thomas coupling with the missing bolt, revealed thread imprint marks on the inside of the bolt hole bore, and lipped material at the edge of the bolt hole bore. The hanger bearing mount had failed in overstress with no evidence of progressive cracking. The helicopter had been operated for 15.2 hours since the Thomas coupling had been disassembled and put back together. The mechanic reported that he had reused the bolts and self-locking nuts, and tightened them to the proper torque. The missing nut and bolt were not recovered. A check of the FAA database for SDRs revealed no other occurrences in the preceding 5 years.
On March 23, 2001, about 1520 Eastern Standard Time, a Bell 206 L-1 helicopter, N2138Y, operated by EMS Air Service of New York, Inc., was substantially damaged during a precautionary landing near Seneca Falls, New York. The certificated airline transport pilot and passenger received serious injuries. Visual meteorological conditions prevailed for the positioning flight. The flight was operated on a company visual flight rules (VFR) flight plan, and conducted under 14 CFR Part 91. According to a written statement by the pilot: "...we were approximately 5-6 miles North-Northwest of the Seneca Falls Ambulance barn at about 900-1,000 feet AGL...I adjusted the power from 70% tq [torque] to about 75% tq to start a slow climb to 1,500 feet AGL. Simultaneously, the aircraft produced a series of loud bangs and yawed several times. I lowered the collective and retarded the throttle to idle, while turning to the left toward an open field. The noise subsided. During the turning descent, I determined I had lost tail rotor control. The field I had automatically turned toward was a plowed, rolling wet field, with trees at the far end. I was concerned about an emergency landing into a plowed wet field, and increased the throttle to fly, with the intent of circling around to an open pasture across the road from the plowed field. I climbed to about 4-500 feet AGL and about 80 KIAS, where I started a right turn back to the pasture." "I lost airspeed and altitude in the turn, and lowered the nose in an attempt to regain the airspeed. After the turn, I realized I would not be able to make the pasture I wanted. As I approached the treetops, I reduced airspeed to stop the forward momentum. When the aircraft started to rotate right, I retarded the throttle to autorotate into the trees. The aircraft rolled left on first contact. I secured the engine by turning the main fuel, battery, and generator switches off. I exited the aircraft to assist [the passenger]...." In a follow-up telephone interview, the pilot reported that loss of tail rotor control was accompanied by a loud banging noise from the rear of the helicopter. He could not identify the location of the noise and due to his concerns of either an engine failure or catastrophic airframe failure, he elected to continue with the precautionary landing as soon as possible. According to an inspector from the Federal Aviation Administration (FAA), there was a hole in the cowling above the aft end of the aft short shaft, and the forward segment of the tail rotor drive shaft. Removal of the cowling revealed the mount for the forward hanger bearing assembly was fractured. In addition, the forward end of the aft short shaft had pulled out of the oil cooler assembly. One bolt was missing from the Thomas coupling, that was located between the aft short shaft and the first segment of the tail rotor drive shaft. The missing bolt and nut were not recovered. The fractured bearing mount, tail rotor drive shaft, and Thomas coupling with the missing bolt were forwarded to the Safety Board Materials Laboratory for examination. According to the report from the Safety Board materials specialist: "...The Thomas coupling at the forward end of the aft short shaft was intact, and did not contain any visible cracks. The individual discs of the coupling were gapped apart between bolt locations, indicative of deformation associated with high angle displacement...[On the Thomas coupling at the aft end of the aft short shaft] the individual discs of the coupling were gapped apart between bolt locations, and the outer discs contained visible wrinkles. Several cracks were noted in areas with wrinkle deformations...The aft end of the aft short shaft contained an oil or grease spray pattern, indicative of offset rotation of the aft short shaft around the bolt, opposite from the missing bolt...Portions of the normal seating positions for the head of the missing bolt, and the washer for the missing nut were relatively free of dirt and grease...The bore surface of the [bolt] hole for the missing bolt in the flanged adapter at the forward end of the first segment contained thread imprint marks...and lipped up material was noted around the perimeter of the hole on both the forward and aft sides...The bolt holes through the coupling discs at the missing bolt position were damaged as if from contact with the shank or threads of the bolt....." The report further stated: "...The legs of the [hanger bearing] bracket were deformed in the aft direction, and the upper edges of the legs contained substantial mechanical damage...The top portion of the hanger was missing, and the bottom portion was fractured...Visual examination of the hanger fractures revealed features typical of overstress fractures. No evidence of fatigue or other type of progressive cracking was noted...." "...[Tail rotor drive shaft] fracture matching showed that only small portions of the two fracture areas mated with each other, indicating that a portion of the shaft was missing from the fracture area...Both pieces of the tail rotor drive shaft segment, contained substantial compression buckling on a spiral plane adjacent to the fracture area. Fracture features and associated deformation on the shaft segment, were typical of fracture as a result of application of excessive torsional loads. The direction of torsion was as if the forward portion of the first segment rotated clockwise (aft looking forward), relative to the aft portion of the segment...." According to the Bell Helicopter maintenance manual for the Bell 206 L-1, the connection between the aft short shaft and the forward segment of the tail rotor drive shaft was made with a Thomas coupling, held together with four bolts using self-locking nuts. Reuse of self-locking nuts was permitted as long as the minimum breakaway torque was met. The maintenance manual also said that the bolts used in the Thomas coupling are torqued to between 50 and 70 inch pounds. The maintenance manual specified the minimum breakaway torque for the self locking nut on the shaft of the bolt to be 3.5 inch pounds. Breakaway torque was the minimum torque necessary to rotate the self locking nut on the threaded shaft prior to application of tightening torque. Removal torque was the torque necessary to rotate the self locking nut on the threaded shaft to loosen it from its present position. The removal torque and breakaway torque on the remaining three bolts of the Thomas coupling were checked. The bolt opposite of the missing bolt was identified as number 2. The other two bolts were identified as number 3 and number 4. The removal torque on number 2 was about 5 inch pounds, and the breakaway torque was about 5 inch pounds. The removal torque on number 3 was about 50 inch pounds and the breakaway torque was 10 inch pounds. The removal torque on number 4 was 40 inch pounds and the breakaway torque was less than 5 inch pounds. According to the director of maintenance (DM) who conducted the 100-hour inspection, part of the inspection procedure included disconnecting the aft short shaft from the forward segment of the tail rotor drive shaft, and the Thomas coupling. When the parts were reassembled, he torqued the nuts on the bolts of the Thomas coupling to the value specified in the maintenance manual (50 to 70 inch pounds). He said that he normally would torque them to the high side value. When asked, he reported that he had reused the bolts and self-locking nuts. When asked how he checked the minimum breakaway torque, the DM replied that if he was able to thread the nut all the way onto the bolt by hand, then he would not use it. He did not have a torque wrench in the range of the breakaway torque specified in the maintenance manual. A check with the director of maintenance for another operator, which had over 70 Bell model 206s, found that that operator used an identical procedure to determine if the self-locking nuts could be reused. That operator also did not possess torque wrenches in the range needed to check for the breakaway torque. In addition, the director of maintenance of that company reported that during reassembly of Thomas couplings, company procedures required that the couplings should be checked three different times prior to a helicopter being returned to service. Torque strips were also added to the nuts and bolts used to hold the Thomas coupling together on all Bell 206 series helicopters, even when it was not called for in the maintenance manual. A review of the maintenance manual for the Bell 206 L-1, found there was no requirement to add a torque stripe to the nut, which would enable a mechanic to note any rotation of the nut on the bolt. Similar conditions were noted for the Bell 206 L-3. However for the Bell 206 A/B series and Bell 206 L-4, the addition of a torque stripe was required. A check of the service difficulty report (SDR) database for the preceding 5 years found no listings for loss of bolts, or loss of torque on the self-locking nuts used in the Thomas couplings. In addition, the preceding flight for the helicopter was a 14 CFR Part 135.293(b) pilot evaluation checkride. According to a written statement from the FAA inspector who conducted the checkride: "...during the flight, no unusual vibration, specifically a higher frequency type vibration associated with the tailrotor system was observed. Upon conclusion of the routine evaluation flight a post flight inspection revealed no obvious abnormalities...." According to the operator, the helicopter had accumulated a total time of 14,017.4 hours at the time of the accident. The helicopter had been operated 15.2 hours since the last disassembly of the Thomas coupling, and an additional 0.4 hours since the completion of the 14 CFR Part 135 checkride.
the loss of a bolt in a Thomas coupling on the tail rotor drive shaft, for undetermined reasons, during climb, while operating over unsuitable terrain.
Source: NTSB Aviation Accident Database
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