Farmington, CA, USA
N43921
TEXAS HELICOPTER CORP OH-13E/M74
The commercial pilot was conducting aerial application flights in the single main rotor helicopter over a flat, level agricultural field. He had already conducted several uneventful flights that day in the helicopter on other portions of the same field. Just as the pilot completed a course reversal turn, the helicopter crashed in the field. The turn was the last thing the pilot remembered about the flight, and there were no eyewitnesses. Although the field was soft, and the helicopter's airspeed and altitude were low, the helicopter was extensively fragmented. One main rotor blade was found about 500 feet from the rest of the wreckage. That wood main rotor blade was essentially intact, except for the absence of its metal core and multiple linear, spanwise fastener tearouts at the root. The core was fracture-separated in overstress from its blade grip and was bent in a 270-degree arc. All these signatures were consistent with the wood blade, minus the core, separating from the rotor hub while in flight. The loss of the blade, and the subsequent imbalance to the rotor disc, induced vibratory loads that exceeded the structural strength of the helicopter, and resulted in extensive fragmentation of the helicopter before ground impact. No damage patterns consistent with a ground rotor strike were observed. The incompleteness of the maintenance records prevented the determination of the service history of the failed blade as well as its compliance with required inspections. The reason for the failure of the blade wood could not be determined.
HISTORY OF FLIGHTOn February 22, 2012, about 1125 Pacific standard time, a Texas Helicopter Corporation OH13E/M74, N43921, was substantially damaged while conducting an aerial application run on an agricultural field in Farmington, California. The commercial pilot received serious injuries. The flight was operated by Ag Air under the provisions of 14 Code of Federal Regulations Part 137. Visual meteorological conditions prevailed, and no flight plan was filed for the flight. The pilot reported that his typical spray altitude was about 7 feet above ground level, and he increased that to about 20 feet for turns. The flight was conducted at a low airspeed. According to the pilot and the assistant who loaded the chemical payload, the helicopter had been working the local fields since about 0830 that morning, and had applied about 13 loads. The helicopter lifted off to begin another spray run after reloading by the assistant, who then turned his attention elsewhere. Shortly thereafter, he heard the sounds of the helicopter crashing, and turned to look, but he did not witness the accident. He ran to the accident site, assisted the pilot, and summoned for help. The helicopter was highly fragmented, with a debris field that measured a few hundred feet square. The pilot reported that all that he recalls of the accident is that he completed a left course reversal turn. PERSONNEL INFORMATIONFederal Aviation Administration (FAA) information indicated that the pilot held a commercial pilot certificate with a rotorcraft-helicopter rating, and a private pilot certificate with airplane single-engine land and instrument ratings. He reported a total flight experience of 20,000 hours, 19,000 of which were in helicopters, and 5,000 of which were in the accident helicopter make and model. His most recent FAA second-class medical certificate was issued in August 2011. He was the previous owner of the current operator. AIRCRAFT INFORMATIONFAA information indicated that the original airframe model (a Bell model 47) was converted by Texas Helicopters to the M74 in 1977, and the helicopter was registered to the accident operator (Ag-Air) in 1989. The basic configuration consisted of a single main rotor, with two wood main rotor blades (MRBs) with metal inserts (mass balance or "core"), powered by a piston engine. The cockpit was partially enclosed by a large transparent plastic 'bubble.' The tail rotor assembly was supported by an open frame steel tube truss, and the landing gear consisted of metal skids. Normal MRB operating range is 322 to 370 rpm, and each MRB weighs about 92 pounds. Review of NTSB records indicated that the helicopter was involved in two NTSB-investigated accidents, in 1990 and 1991. The second accident was fatal to that pilot, and the helicopter damage level was cited by the NTSB as "destroyed." Ag Air was the operator for both of those accidents. No records of maintenance action associated with the repair of the helicopter after either accident were located. In June 1992, a Lycoming VO 435 series piston engine was installed in the helicopter; that was the engine installed at the time of this subject accident. While an FAA Form 337 for the installation of the engine was located, the FAA registration indicated that the helicopter was still equipped as delivered by Texas Helicopters, with a Lycoming VO-540 series piston engine. METEOROLOGICAL INFORMATIONThe 1155 automated weather observation for Stockton Municipal Airport (SCK), Stockton, California, located about 9 miles west-southwest of the accident site, included variable direction winds at 3 knots, visibility 9 miles, clear skies, temperature 15 degrees C, dew point 8 degrees C, and an altimeter setting of 30.25 inches of mercury. AIRPORT INFORMATIONFAA information indicated that the original airframe model (a Bell model 47) was converted by Texas Helicopters to the M74 in 1977, and the helicopter was registered to the accident operator (Ag-Air) in 1989. The basic configuration consisted of a single main rotor, with two wood main rotor blades (MRBs) with metal inserts (mass balance or "core"), powered by a piston engine. The cockpit was partially enclosed by a large transparent plastic 'bubble.' The tail rotor assembly was supported by an open frame steel tube truss, and the landing gear consisted of metal skids. Normal MRB operating range is 322 to 370 rpm, and each MRB weighs about 92 pounds. Review of NTSB records indicated that the helicopter was involved in two NTSB-investigated accidents, in 1990 and 1991. The second accident was fatal to that pilot, and the helicopter damage level was cited by the NTSB as "destroyed." Ag Air was the operator for both of those accidents. No records of maintenance action associated with the repair of the helicopter after either accident were located. In June 1992, a Lycoming VO 435 series piston engine was installed in the helicopter; that was the engine installed at the time of this subject accident. While an FAA Form 337 for the installation of the engine was located, the FAA registration indicated that the helicopter was still equipped as delivered by Texas Helicopters, with a Lycoming VO-540 series piston engine. WRECKAGE AND IMPACT INFORMATIONThe NTSB investigator-in-charge did not respond to the accident site. FAA inspectors and personnel from the operator did respond on-scene, and provided on-scene photographs and other information. The field being sprayed was level, with small, soft furrows and fresh crops about 6 inches high. The helicopter was highly fragmented, but the ground scars were minimal in dimension and quantity. The wreckage was roughly grouped in five main areas: 1) Landing skids, spray gear, and tail boom 2) Pilot's helmet and miscellaneous items 3) Engine, transmission and rotor head assembly (largest ground scar) 4) Lower fuselage/cockpit floor (with pilot) and instrument pedestal 5) One MRB The second, third, and fourth groups were located about 100 to 200 feet southwest of the first group, which was the initial ground contact point. One MRB, arbitrarily designated as 'Blade A' for the investigation, was found about 500 feet southeast of the first wreckage group. Examination of the recovered airframe and engine was conducted at the recovery facility about 2 months after the accident. The cockpit area was fracture-separated from the rest of the helicopter. Exclusive of breaks attributed to breakup of the helicopter, continuity for the flight controls (collective, cyclic, and tail rotor) was established. The tail rotor and tail rotor gearbox remained attached to the aft tail boom, which was fracture-separated from the helicopter. Both tail rotor blades were partially fractured The engine was relatively intact, and the main rotor mast and hub remained attached to it. Since the pilot did not report any engine problems, and all damage was consistent with engine power at the time of the breakup of the helicopter, only a limited examination of the engine was conducted. No engine damage that was not associated with the helicopter breakup or ground impact was observed. MRB A was essentially intact, except that its full–span steel core was absent, and the blade root section had multiple linear, spanwise fastener pullouts. The core was fracture-separated from its grip, and that fracture was determined to be due to overload only. The core was bent into an arc of approximately 270 degrees. The dataplate from MRB A was absent from the blade, and was never located. The reason for its absence could not be determined. The other MRB was highly fragmented, and separated from its core. The full length of that core remained attached to its respective blade grip, and that core was bent and twisted. ADDITIONAL INFORMATIONMaintenance and Maintenance Records According to the General Manager of Texas Helicopters, metal main rotor blades are subject to a 5,000-hour life limit, but wood blades, such as those on the accident helicopter, have no established life limits, and are replaced or repaired on-condition. Only wood blades may be rebuilt and zero-timed, and this must be accomplished in accordance with the Texas Helicopters Maintenance & Overhaul Instructions document. In addition to preflight, 100-hour, and annual inspections of the MRBs, the instructions for continued airworthiness require MRB inspections at 50, 600 and 1,200 hours. Only partial maintenance records for the helicopter were located. Based on the maintenance records available, the MRBs had a minimum time in service of about 1,140 hours on the accident helicopter, and a maximum time of about 1,737 hours. The records indicated that the MRBs had been removed and reinstalled on the helicopter several times. It could not be determined whether the MRBs had been installed on any other helicopters when they were not installed on the accident helicopter. No records of the 50, 600, or 1,200-hour mandatory MRB inspections were located.
The in-flight separation of the wood portion of one main rotor blade while maneuvering at low level, the reason for which could not be determined because postaccident examination did not reveal any anomalies that would have precluded normal operation.
Source: NTSB Aviation Accident Database
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