Chandler, AZ, USA
N602RW
ROTORWAY A600
The flight instructor and a helicopter-rated pilot were conducting the flight for the purpose of transitioning the pilot to the piston-engine experimental helicopter. About 30 minutes into the flight, while flying straight and level, the pilots felt a jolt followed by loud noise and vibration. The flight instructor reduced power and began looking for a place to land. Erratic engine tachometer indications and additional power reduction then prompted the flight instructor to enter an autorotation for a forced landing. The helicopter had some forward velocity at touchdown on the firm, smooth ground. The skids dug in and the helicopter rolled over. Both pilots were uninjured. The helicopter used a system of pulleys and sprockets to enable the engine to drive the main rotor at the desired rotational speed. The accident helicopter incorporated a design change for the main drive pulley attachment to the engine flywheel. The original design used three 3/8-inch diameter socket-head bolts to attach the pulley to the flywheel and one other drive component; the revised design used four 1/4-inch diameter cross-slot (Phillips) screws to attach the pulley only to the flywheel. Postaccident examination of the components revealed that the four attach screws had failed due to fatigue. Laboratory examination of other screws from the same manufacturing lot indicated that the screws were in compliance with their design specifications. The failed screws had about 20 hours in service. The torque that could be applied to the cross-slot screws was limited by the slippage of the screwdriver in the screw head, which in turn limited the preload on the screws and the induced friction in the pulley-to-flywheel joint. Those conditions contributed to the cyclic loading of the fasteners, which then resulted in their fatigue failure. Subsequent to the accident, the manufacturer reverted to its original flywheel-pulley attach method and replaced the newer design pulley assemblies with the original configuration.
HISTORY OF FLIGHT On March 15, 2011, about 1400 mountain standard time, an experimental Rotorway A-600 Talon helicopter, N602RW, was substantially damaged during an emergency landing near Chandler, Arizona. The certificated flight instructor (CFI) and the helicopter-rated pilot were not injured. The instructional flight was conducted for the purpose of transitioning the pilot to the Rotorway helicopter, and was operated under the provisions of Title 14 Code of Federal Regulations Part 91. Visual meteorological conditions prevailed, and no flight plan was filed for the flight. According to CFI, the helicopter had departed Stellar Airpark (P19), Chandler, for the purpose of practicing some maneuvers. About 30 minutes into the flight, while flying "straight and level" at an altitude of 1,800 feet above mean sea level, he felt a "jolt," followed by "a loud banging noise" and "strong vibration." The CFI reduced the power, and began looking for a place to set down. He noticed that the engine tachometer indications were erratic, so he reduced power further and initiated an autorotation. Just prior to touchdown he added collective to reduce the descent rate and soften the touchdown. After a brief ground run, the skids dug into the soil, the helicopter pitched nose down, and the main rotor blades contacted the ground. The helicopter then rolled onto its left side. Both pilots exited the helicopter on their own, and the CFI then reached into the cockpit to shut off the fuel valve and electrical switches. There were no ground-based witnesses to the accident. The CFI telephoned the manufacturer to report the event, and company personnel recovered the wreckage to their facility later that day. Company personnel began disassembling the helicopter in an effort to determine a possible cause of the accident, and notified the NTSB of the event the following day. Several days after the accident, a Federal Aviation Administration (FAA) inspector examined the helicopter at the company facility. PERSONNEL INFORMATION Flight Instructor The flight instructor was employed by the manufacturer. He held a flight instructor and commercial pilot certificate with helicopter rating. He reported that he had approximately 370 total hours of flight experience, all of which was in helicopters. His most recent flight review was completed in November 2010, and his most recent FAA second-class medical certificate was issued in December 2010. Pilot Under-Instruction The pilot under instruction was an Austrian national, who was in the US for transition training to the accident helicopter type. He held a private pilot certificate with multiple ratings, including airplane, glider, and helicopter. He reported that he had approximately 2,267 total hours of flight experience. His most recent flight review was completed in January 2011, and his most recent first-class medical certificate was issued in June 2010. AIRCRAFT INFORMATION The helicopter was manufactured by Rotorway from primarily Rotorway furnished components. The helicopter was equipped with a Rotorway RI600N 150 hp piston engine. The airworthiness certificate was issued in November 2010; that certificate denoted that the helicopter was an experimental category aircraft with a stated purpose of "Market Survey / Crew Training." The two place helicopter was equipped with a two-blade main rotor and a two-blade tail rotor. The engine was mounted vertically (output shaft oriented up) behind the cockpit/cabin. Normal engine operating speed was about 4,250 rpm, and a speed reduction system was utilized to reduce engine output rpm to a value suitable for the main rotor. The combination drive system/speed reduction unit, with a reduction ratio of approximately 8:1, utilized two pulleys and two sprockets to enable the engine to drive the main rotor. A flywheel attached directly to the engine output shaft, an engine/drive pulley was attached directly to the flywheel, and belts were used to transfer the motion between the engine/drive pulley and the main rotor system. The engine was originally installed new in another helicopter in June 2009. About 20 hours later, the engine was returned to the manufacturer's facility for modifications, "zero-timed," and reinstalled in that helicopter. After another 80 hours, the engine was again removed and sent to the factory for additional modifications, and again zero-timed. After another 17 hours, that same process was repeated, and then repeated again after another 78 hours. That zero-timed engine was then installed in the accident helicopter in February 2011. At that time, the airframe had a total time (TT) of about 86 hours, the engine had a TT of about 195 hours, and a TT since major overhaul (SMOH) of 0 hours. At the time of the accident, the engine had a TT of about 215 hours, and a TTSMOH of about 20 hours. The helicopter's most recent inspection was completed on February 24, 2011. METEOROLOGICAL INFORMATION The 1350 automated weather observation at an airport located about 5 miles east of the accident site included winds from 180 degrees at 4 knots; visibility 35 miles, a broken cloud layer at 20,000 feet; temperature 27 degrees C; dew point -1 degree C; and an altimeter setting of 29.95 inches of mercury. WRECKAGE AND IMPACT INFORMATION The accident site was located about 6 miles south of the origination airport, P19. Manufacturer's personnel recovered the helicopter to their facility before they notified the NTSB or FAA; therefore no NTSB or FAA on-scene investigation was able to be conducted. Review of photographs taken by manufacturer's personnel indicated that the accident site terrain was level, firm desert soil with light scrub vegetation. Intermittent landing skid ground scars extended for about 30 feet, and terminated about 10 feet from the helicopter. The scars indicated that the helicopter touched down first with the right skid, bounced to the left skid, and then both skids contacted and dug in. The helicopter came to rest facing back towards the ground scars. The cockpit/cabin was intact, with the exception of the transparencies. Both main rotor blades were crumpled, and the tail rotor was intact. The tail boom was almost completely severed about 5 feet forward of the tail rotor. Post-accident examination of the helicopter by a FAA inspector revealed that the four screws that attached the engine flywheel to the engine drive pulley had failed, which prevented delivery of engine power to the rotor system. The flywheel, drive pulley, and fractured fasteners, as well as new exemplar fasteners from the same manufacturing lot, were sent to the NTSB Materials Laboratory for examination. Optical examination revealed fretting and wear on the pulley, at the interface with the flywheel. Both faces of the flywheel, and the flywheel attach face of the pulley, contained mechanical damage consistent with rotational contact after failure of the attach screws. All flywheel screws had failed as a result of fatigue propagation, and no metallurgical abnormalities were found with the exemplar screws. Refer to the Materials Laboratory report in the accident docket for detailed information. ADDITIONAL INFORMATION Flywheel-Pulley Attachment Information The original design for the attachment of the engine/drive pulley utilized three 3/8 inch diameter, 1-1/4 inch long, cap bolts to attach (in sequence) the pulley, flywheel, and main drive flange together. Those cap bolts had hexagonally-socketed heads for driving and torqueing purposes. The modified design, which was incorporated on the helicopter, utilized four MS24694-S97 countersunk machine screws which were 1/4 inch diameter, 21/32 inches long, and only attached the drive pulley to the flywheel. Those screws had cross slot (Phillips) heads for driving and torqueing purposes. The torque that can be applied to a cross slot (Phillips) screw is limited by the cam-out of the screwdriver in the screw head. Cam-out occurs when the torque applied to the screwdriver overcomes the friction of the screwdriver-screw interface, and the screwdriver then rides up out of the screw slot. Cam-out is affected by the condition of the screw slot and screwdriver point, and the downforce and torque on the screwdriver. This introduces significant variation in the torque able to be applied to the screw. It also limits the preload on the screws, and the induced friction in the pulley-to-flywheel joint. The manufacturer did not provide the specific reason(s) for the design change. The manufacturer's "Engine Assembly Rebuild Sheet" indicated that the main drive pulley and flywheel were new (TT 0 hours) at the time of the February 2011 engine assembly and installation into the accident helicopter. No records that documented any adjustment, modification, or removal of that assembly since its installation were located. Subsequent to the accident, the manufacturer reverted to its original flywheel-pulley attach method, and replaced installed/delivered new-design pulley assemblies with the original configuration assemblies. Refer to the accident docket for detailed information.
A design modification that changed the fasteners and components used to attach the main rotor drive pulley to the engine, which resulted in fatigue failure of those fasteners and a complete loss of power to the main rotor.
Source: NTSB Aviation Accident Database
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