On June 23, 2008, about 0700 Pacific daylight time, an Arrow Falcon Exporters, Inc., OH-58C helicopter, N912HP, was substantially damaged during an emergency landing following a reported loss off engine power while hovering west of Fresno, California. The helicopter was registered to Del Rio Aviation, Inc., Paso Robles, California, and operated by Double Tree Helicopters, Turlock, California, under the provisions of 14 Code of Federal Regulations Part 137. The commercial pilot, sole occupant of the helicopter, was not injured. Visual meteorological conditions prevailed, and no flight plan had been filed for the local aerial application flight that was originating at the time of the accident.
The pilot reported that he had filled the spray tanks with water and had lifted off from the back of a truck platform when he heard three loud noises followed by a loss of engine power. The pilot initiated a hovering autorotation to the landing platform, however, was only able to maneuver the forward portion of the helicopter over the platform. Subsequently, the helicopter slid off the platform and impacted the ground tail boom first, resulting in structural damage to the tail boom and fuselage. On June 23, 2008, the Allison T63-A720 engine was examined by a representative from Rolls Royce under the supervision of a Federal Aviation Administration (FAA) inspector. The examination revealed that all fuel, lube and pneumatic lines, "B" nuts and their associated fittings were secure. Several black "carbon-like" particles lay loose in the base of the exhaust collector. Inspection of the inlet plenum chamber did not reveal any evidence of foreign material or missing hardware.
Inspection of the compressor inlet revealed dirt deposits on the inlet guide vanes as well as a small ferrous type material adjacent to the inlet guide vane at the six o'clock position. A single first stage compressor blade contained a small nick to the leading edge of the blade near the tip. The engine was then removed from the airframe and the compressor was partially disassembled.
The compressor module was removed from the accessory gearbox and the compressor case halves were removed for further inspection. Inspection revealed that 15 third stage compressor vanes were fractured near the root and were separated. The majority of the separated vanes were located in the lower compressor case halve. The remaining attached vanes were bent over in the direction of engine rotation. One second stage compressor vane located in the lower case halve was fractured near the root and was separated. Multiple third stage and fourth stage compressor blades were damaged at the leading edge. Compressor case lining material was missing from both compressor case halves.
During removal and disassembly of the compressor, oil was evident at the number one bearing supply and scavenge fittings. The compressor bleed valve was dirty but moved freely from full open to full close. No radial play of the poppet valve was noted. The outer combustion case and combustion liner were removed. The fuel nozzle inlet screen was clean. Visual inspection of the first stage nozzle shield and first stage nozzle vane exhibited metal spray fused to the surface of the pressure side.
The compressor was sent to the facilities of Rolls Royce for further metallurgical examination under the supervision of an FAA inspector.
The examination of the compressor section revealed that the compressor case lining material was partially missing and eroded from both compressor case halves with the highest level of erosion on the top half, downstream of the first stage vane band. One second stage vane airfoil was separated. The area of separation was found consistent with high cycle fatigue (HCF). An additional second stage vane airfoil exhibited a crack, consistent with HCF near the root, but did not separate. A semi-quantitative x-ray energy dispersive analysis (XEDA) revealed deposits at the suction side vane root were consistent with oxidized braze alloy, indicating the braze alloy was exposed during engine operation. It was also found that the chemistry, microstructure and hardness of the second stage vanes were consistent with properly processed AMS 5604 (17-4 PH steel).