History of Flight
On February 22, 2012, about 1125 Pacific standard time, a Texas Helicopter Corporation OH13E/M74, registration 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 14 Code of Federal Regulations Part 137. Visual meteorological conditions prevailed, and no flight plan was filed.
The pilot reported his typical spray altitude was about 7 feet above ground level, increasing to about 20 feet for turns, and the flight was conducted at 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. After reloading by the assistant, the helicopter lifted off to begin another spray run; the assistant then turned his attention elsewhere. Shortly thereafter, he heard the sounds of the helicopter crashing and turned to look, but did not witness the accident. He ran to the accident site, assisted the pilot, and summoned help. The helicopter was highly fragmented, with a debris field measuring a few hundred feet square.
The pilot reported that all he recalls of the accident is completing a left course reversal turn.
Pilot Information
Federal Aviation Administration (FAA) information indicated 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 total flight experience of 20,000 hours, 19,000 in helicopters, and 5,000 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 Information
FAA information indicated the original airframe (a Bell model 47) was converted by Texas Helicopters to the M74 in 1977, and the helicopter was registered to 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 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 accidents. No records of maintenance action associated with repair after either accident were located.
In June 1992, a Lycoming VO 435 series piston engine was installed; that was the engine installed at the time of this accident. While an FAA Form 337 for the installation was located, the FAA registration indicated the helicopter was still equipped as delivered by Texas Helicopters with a Lycoming VO-540 series piston engine.
Meteorological Information
The 1155 automated weather observation for Stockton Municipal Airport (SCK), Stockton, California, about 9 miles west-southwest of the accident site, included variable direction winds at 3 knots, visibility 9 miles, clear skies, temperature 15°C, dew point 8°C, and an altimeter setting of 30.25 inches of mercury.
Wreckage and Impact Information
The NTSB investigator-in-charge did not respond to the accident site. FAA inspectors and personnel from the operator responded on-scene and provided photographs and information. The field being sprayed was level, with small, soft furrows and fresh crops about 6 inches high. The helicopter was highly fragmented, but ground scars were minimal.
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 Blade A, was found about 500 feet southeast of the first wreckage group.
Examination of 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, continuity for 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. Both tail rotor blades were partially fractured.
The engine was relatively intact, and the main rotor mast and hub remained attached. Since the pilot did not report any engine problems, and all damage was consistent with engine power at the time of breakup, only a limited examination was conducted. No engine damage not associated with breakup or ground impact was observed.
MRB A was essentially intact except 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 overload only. The core was bent into an arc of approximately 270 degrees. The dataplate from MRB A was absent and never located; the reason 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 was bent and twisted.
Additional Information
According to the General Manager of Texas Helicopters, metal MRBs have a 5,000-hour life limit, but wood blades (as on the accident helicopter) have no established life limits; they are replaced or repaired on-condition. Only wood blades may be rebuilt and zero-timed, in accordance with the Texas Helicopters Maintenance & Overhaul Instructions document. In addition to preflight, 100-hour, and annual inspections, the instructions require MRB inspections at 50, 600, and 1,200 hours.
Only partial maintenance records were located. Based on available records, the MRBs had a minimum time in service of about 1,140 hours and a maximum of about 1,737 hours. Records indicated the MRBs had been removed and reinstalled several times. It could not be determined whether the MRBs were installed on other helicopters when not on the accident helicopter.
No records of the 50, 600, or 1,200-hour mandatory MRB inspections were located.