History of Flight
On February 2, 2010, about 1400 Pacific standard time, an Arrow Falcon Exporters OH-58A helicopter, registration N9286U, executed a hard landing in an orchard shortly after takeoff near Escalon, California. The helicopter was being operated by Cavanagh Flying Service under 14 Code of Federal Regulations Part 91 as a positioning flight. The commercial pilot sustained serious injuries, and the helicopter incurred substantial damage. The flight departed from an orchard near Escalon. Visual meteorological conditions prevailed, and no flight plan was filed.
Approximately 30 minutes before the accident, the pilot left two voicemail messages for his mechanic, reporting that the helicopter was vibrating heavily, lacked sufficient power, and that the dc generator caution light was illuminated. He described experiencing "big maintenance problems" and stated his workload was heavy, but he intended to fly the helicopter home.
According to a family member, while fueling the helicopter prior to the accident flight, the pilot noted vibrations. After takeoff, he reported over the radio that the generator light was on, and shortly thereafter stated, "I'm going down."
In a written statement, the pilot reported that at an altitude of about 200 feet above ground level, the engine lost power. He performed a forced landing, during which the helicopter landed hard, causing substantial damage to the entire fuselage structure.
Aircraft Information
The helicopter, serial number 69-16198, was equipped with a Rolls-Royce/Allison T63-A-720 gas turbine engine, serial number 405588. In February 1997, it received a restricted class airworthiness certificate for agricultural and pest control operations.
Logbook records showed that at the last annual inspection on December 7, 2009, the airframe had accumulated 7,549.3 total flight hours, with a corresponding Hobbs meter time of 3,602.7 hours. The engine logbook entry for that inspection indicated "Eng TT 1,776.3." An entry on December 27, 2009, at a Hobbs time of 3,610.4, documented replacement of the fuel control unit. The most recent logbook entry was a 25-hour inspection on January 31, 2010, at a Hobbs time of 3,633.6. The Hobbs meter at the accident site read 3,636.6 hours.
Tests and Research
External Engine Examination
The engine was removed in the presence of the NTSB investigator-in-charge and representatives from Rolls-Royce and Bell Helicopters. The engine compartment was largely intact with minimal impact damage. Inward indentations were observed on the outer combustion case near the fuel igniter, and the fuel nozzle/igniter boss appeared canted about 10 degrees. Manual rotation of the N1 and N2 rotors showed continuity with no foreign object damage noted at the compressor inlet.
Examination of the engine fuel control unit (FCU) revealed that the pneumatic control line (part number 6870035), which connects the power turbine governor to the FCU, had fractured at the FCU connector. The line was displaced about 1/4 inch laterally from its fitting.
Pneumatic Control and Oil Sump Accessory Lines
The pneumatic control line had fractured 0.42 inches from the tube end, adjacent to the end of the ferrule. A rub mark approximately 0.125 inches in diameter was found on the line next to an oil sump accessory housing line (part number 6853464AL), which displayed a corresponding rub mark. Visual comparison with an exemplar tube indicated the pneumatic line was bent outwards in two planes.
Metallurgical examination by Rolls-Royce revealed circumferential score marks on the outer surface near the fracture. Scanning electron microscope analysis showed crack signatures progressing radially in the scored area and circumferentially around the tube. Rolls-Royce determined the fracture features were indicative of high cycle fatigue. Cross-sectioning of the flared tube end revealed two additional cracks originating on the outer wall that had not propagated through to the inner surface. The pneumatic line met the specified stainless steel type, wall thickness, and diameter dimensions.
Engine Test
After replacing the damaged pneumatic and oil scavenge lines and the impact-damaged outer combustion case with slave units, the engine was installed in a Rolls-Royce test cell and run for about one hour throughout its power range. No anomalies were noted. During replacement, fretting damage was observed on the fuel nozzle/igniter adjacent to the combustion liner, and a 2-inch section of the double lip flange at the base of the combustion liner was missing.
DC Starter-Generator
The starter-generator (Lear Siegler, Inc., model 23032-022, manufactured June 6, 1980) was removed and examined. The main bearing shield had separated from the bearing, exposing the inner race at the splined shaft. The shield was free on the shaft and appeared distorted; both the race and shield exhibited a blue tint. Hand rotation produced a grinding sound, and about 1 millimeter of lateral play was noted in the splined shaft at the main bearing.
Additional Information
Rolls-Royce Customer Service Letter A-1166, dated November 15, 1990, and revised February 5, 2007, reiterated the importance of proper alignment, clamping, and torquing of engine tubing during installation. It stated that failures in external tube lines could be caused by bent tubes inducing misalignment at the flare, resulting in cracked flares or fretting of the tube at the end of the ferrule, as well as tube-to-fitting misalignment from poorly aligned fittings.
The Rolls-Royce Operation and Maintenance Manual (Section 73-20-02) for the accident engine included a warning that failure to properly install, align, and tighten fuel, oil, and air fittings and tubes could result in an engine failure. According to the Rolls-Royce representative, a leak in the pneumatic control line could reduce engine power, and a complete failure could cause power loss to sub-idle conditions.