Accident Overview
On July 2, 2016, at about 0230 Pacific daylight time, a Bell OH-58A single-engine agricultural helicopter, registration N6251G, experienced a partial loss of engine power while in the vicinity of Brawley Municipal Airport (BWC), Brawley, California. The commercial pilot was not injured. The helicopter sustained substantial damage during a precautionary landing to a cropped field. The aircraft was registered to the Bank of Utah and operated by Farm Aviation, Inc., of Brawley, California, as a Title 14 Code of Federal Regulations Part 137 aerial application flight. Night visual meteorological conditions prevailed at the time of the accident. A flight plan was not filed for the local flight, which departed shortly before the accident.
Pilot's Account
The pilot reported that after departing BWC with a full chemical load, he maneuvered about one-quarter mile from the airport and made a 90° turn to align with the field to be sprayed. Shortly after lining up, the engine began to surge, and the torque meter needle was "bouncing rapidly." The pilot initiated a precautionary landing. The helicopter landed hard in an open field and came to rest upright.
Initial Examination
An on-site examination conducted by a Federal Aviation Administration (FAA) representative revealed that the tailboom and tail rotor driveshaft had sustained impact damage consistent with a main rotor strike as a result of the hard landing. The examination of the airframe and engine found no anomalies. Helicopter maintenance documentation was not available during the investigation. According to the pilot, the airframe had a total operating time of 13,688 hours at the time of the accident, and the engine had 5,618 hours. The last 100-hour inspection was performed on June 15, 2016, about 74 hours prior to the accident flight.
Engine Examination
The engine was removed and shipped to a facility for further examination. On July 28, 2016, at Aero Maritime in Phoenix, Arizona, an engine examination was conducted in the presence of an FAA inspector and a Rolls-Royce investigator. The engine was set up in a test cell. During the test run, the engine's bleed valve did not close at any speed, and the engine produced less than 250 horsepower. Chip detectors revealed metal debris: the lower detector had metal debris present, and the upper detector had light amounts. After replacing the bleed valve with an overhauled unit, another test run resulted in an increased performance rating of about 380 horsepower (approximately 7% below minimum allowable power for a serviceable engine) before reaching the maximum specified operating temperature. The engine was deemed unserviceable due to low power production.
No component log card or historical information for the bleed valve was found. According to the operator, the compressor, turbine, and bleed valve assemblies were overhauled in July 2014 and assembled as an engine. Rolls-Royce specified an overhaul period of 1,500 hours for the bleed valve assembly. The engine had 766 hours since overhaul at the time of the accident. Disassembly of the bleed valve revealed that the diaphragm had several holes and cracks. The poppet valve in the bleed valve assembly had radial play consistent with worn bushings. According to Rolls-Royce, the non-functioning bleed valve exhibited wear inconsistent with comparable service time.