St. Jacob, IL, USA
N559DD
Robinson R22
The helicopter was substantially damaged when the fan shaft failed causing a loss of power to the main and tail rotors. The instructor pilot reported that while in cruise flight the clutch light flickered several times. He stated that he pulled the clutch circuit breaker, "however the noise continued to worsen and [he] suspected the drive belts were loosening." He reset the clutch circuit breaker and the clutch light illuminated steadily. The instructor pilot reported, "Immediately afterward the engine and rotor RPM needles split indicating a drive train failure, and a loud metal to metal grinding noise began." The flight crew (instructor and dual student) immediately initiated an autorotation. At the termination of the autorotation, the helicopter yawed to the left and did not respond to right pedal inputs. A post-accident examination revealed a complete failure of the fan shaft. The portion of the engine cooling fan assembly aft of the fracture point had departed the aircraft. This component was subsequently found approximately 1.2 miles south of the forced landing site. In addition, the tail rotor blade pitch push-pull tube was completely severed at the forward rod end. Extensive scraping and gouging was noted on the rod end fitting. The fan shaft, fan wheel and bearing were sent to the NTSB Materials Laboratory for examination. The examination noted the fan shaft was fractured in a "V"shape from a transition in shaft diameter adjacent to the bearing at the aft end, forward to the attachment bolt holes. Fracture surface features were "indicative of fatigue propagation." According to the dual student, on the delivery flight one of the drive v-belts broke and an emergency landing under partial power was made. The helicopter was repaired by Robinson technicians who replaced the drive belts and clutch actuator. A visual inspection of the fan shaft and lower bearing was completed at that time and no discrepancies were found. The fan shaft and bearing were not disassembled. Therefore, the mating surfaces of the fan shaft and bearing race were not inspected.
HISTORY OF FLIGHT On November 20, 2002, at 1430 central standard time, a Robinson R22 helicopter, N559DD, piloted by a commercial pilot, was substantially damaged during a forced landing following a loss of main rotor power and an autorotation near St. Jacob, Illinois. This was accompanied by an "intense" vibration as reported by the pilots following the accident. The local instructional flight departed the St. Louis Downtown Airport (CPS), Cahokia, Illinois, at 1400 with a dual student and a certified flight instructor on-board. The dual student was a helicopter-rated commercial pilot and flight instructor receiving training for a instrument helicopter instructor rating. The flight was being conducted under 14 CFR Part 91 and was not on a flight plan. Visual meteorological conditions prevailed at the time of the accident. The dual student and flight instructor reported no injuries. In his written statement, the flight instructor noted: "We were at 2500 feet MSL and approximately 85 knots indicated airspeed, heading north when the failure occurred." The flight was being radar vectored by St. Louis approach control under VFR for a practice instrument approach to St. Louis Regional Airport (ALN), Alton, Illinois. The flight was approximately 2 miles north of the St. Louis Metro-East Airport/Shafer Field (3K6), when a loss of main rotor power occurred. The instructor reported that the clutch light flickered several times and an unusual noise was heard. He stated that he pulled the clutch circuit breaker, "however the noise continued to worsen and [he] suspected the drive belts were loosening." He reported that he reset the clutch circuit breaker and the clutch light illuminated steadily. He noted that "immediately afterward the engine and rotor RPM needles split indicating a drive train failure, and a loud metal to metal grinding noise began." The instructor stated that he took control of the aircraft and began an autorotation. "Upon termination of the autorotation as airspeed was reduced to zero the aircraft yawed to the left into the wind and did not respond to tail rotor pedal inputs. We touched down on the right skid with the nose of the aircraft pointed approximately 30 degrees to the left of our direction of travel. The aircraft rocked forward and right and then rocked back." The dual student, who was flying the aircraft when the loss of rotor power occurred, reported that "the tail rotor pedals began to shake under my feet and then went still." Concerning the instructor's autorotation and landing, he stated: The instructor "flared at the bottom and the ship turned to the left and landed fairly hard on the right front and came to rest in a level condition." PERSONNEL INFORMATION The flight instructor held a commercial pilot certificate with airplane single and multi-engine land, rotorcraft helicopter, instrument airplane and instrument helicopter ratings. He held a flight instructor certificate with rotorcraft helicopter and instrument helicopter ratings. He held a Second Class medical certificate issued on January 10, 2002. He reported total logged flight time as 4,140 hours, with 890 in the same make and model as the accident aircraft. The dual student held a commercial pilot certificate with rotorcraft helicopter, airplane single-engine land and instrument airplane ratings. The airplane and instrument ratings were limited to private pilot privileges. He held a flight instructor certificate, issued January 2002, with a rotorcraft helicopter rating. He held a First Class medical certificate, issued on January 18, 2002, with a limitation for corrective lenses. He reported total logged flight time as 3,200 hours and 1,650 hours in the same make and model as the accident aircraft. AIRCRAFT INFORMATION The helicopter was a 2002 model year, Robinson R22, serial number 3299. The aircraft was issued an airworthiness certificate on February 15, 2002. It was registered to Helicopter Operation, Inc., St. Louis, Missouri, and was used primarily for flight instruction. The helicopter had accumulated a total flight time of 72.6 hours since new with 6.9 hours at the time the airworthiness certificate was issued, according to the airframe logbook. The engine installed was a Lycoming O-360-J2A, S/N L-38406-36A, rated at 180 horsepower. According to Robinson documentation, it is derated to 145 horsepower. It was installed in a normally aspirated, air-cooled, carbureted configuration. Air cooling is supplied by a fan wheel mounted to the engine crankshaft. The fan wheel assembly is enclosed by a fiberglass scroll. An additional airframe logbook entry, dated February 22, 2002, at 17.4 hours, stated: "Removed original A051-1 actuator and A190 V-belts and replaced them with new parts A051-1 S/N 4190 Rev AM Lot 232 Rev X. The fan was rebalanced per the RHC maintenance manual and the aircraft was returned to service." According to the dual student, this entry was related to an incident which occurred during the delivery flight. He reported that one of the drive V-belts broke and an emergency landing under partial power was made. He stated that the helicopter was repaired by Robinson technicians who replaced the V-belts and the clutch actuator. WRECKAGE AND IMPACT INFORMATION The helicopter impacted into a plowed field approximately two miles north of 3K6. A global positioning system (gps) receiver indicated the position of the forced landing site as 38 degrees 46.504 minutes north latitude and 89 degrees 48.558 minutes west longitude. A post-accident examination of the helicopter revealed that the engine cooling fan assembly had departed the aircraft. This component was subsequently found approximately 1.2 miles south of the forced landing site. A gps receiver indicated the location of the fan wheel assembly as 38 degrees 45.297 minutes north latitude and 89 degrees 48.702 minutes west longitude. Complete failure of the fan shaft, running from within the shaft bearing race to the perimeter mounting holes, was observed. The belt tension actuator had failed at the upper attachment fitting. The actuator remained attached to the fan wheel assembly that departed the aircraft. The drive belts were found looped over the sheave. Several cuts or tears were noted the belts. Otherwise they appeared to be intact. The tail rotor control intermediate bellcrank assembly (P/N A331-1) showed extensive scrapes and gouges on the inboard arm. The tail rotor blade pitch push-pull tube (P/N A121-17) was completely severed at the forward rod end. Extensive scraping and gouging was noted on the rod end fitting. Approximately 50% of the structural tubing common to the forward end of the tail boom, immediately aft of the intermediate bellcrank, was worn through. RESEARCH AND TESTING The fan shaft, fan wheel and bearing were sent to the NTSB Materials Laboratory for examination. The examination noted the fan shaft was fractured in a "V" shape from a transition in shaft diameter adjacent to the bearing at the aft end, forward to the attachment bolt holes. The report noted that examination of the fan shaft revealed that the fracture "initiated at a location corresponding to the ... aft break-edge of the bearing inner race. Here circumferential scoring marks were observed. ... All portions of the fracture propagated normal to the exterior surface of the shaft, were flat, and contained multiple gently curving crack arrest marks, all features indicative of fatigue propagation." The clutch actuator was tested under supervision of NTSB personnel at Robinson Helicopter Company. The motor switches and drive gear functioned normally. The tension springs could not be tested due to damage. A review of the R22 maintenance manual indicated that a detailed inspection of the lower actuator bearing is required when the cooling fan and scroll are removed, such as when replacing a drive belt. This includes a visual inspection of the bearing and review of the fan shaft/bearing interface for evidence of movement or fretting. According to Robinson Helicopter, this visual inspection was conducted during repairs following the belt failure incident and no discrepancies were found. A complete visual inspection of the fan shaft and bearing race is not possible without disassembly of the fan shaft from the bearing. The fan shaft is installed into the lower actuator bearing with a .0008 - .0018 inch interference fit and is normally handled as an assembly. ADDITIONAL INFORMATION Parties to the investigation were the Federal Aviation Administration and the Robinson Helicopter Company.
Fatigue fracture of the clutch assembly fan shaft and the resulting complete loss of power to the rotor drive system. A contributing factor was the loss of tail rotor authority due to the secondary failure of the control tube assembly.
Source: NTSB Aviation Accident Database
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