Background
On June 23, 2008, about 0700 Pacific daylight time, an Arrow Falcon Exporters, Inc. OH-58C helicopter, registration N912HP, was substantially damaged during an emergency landing following a reported loss of engine power. The helicopter was registered to Del Rio Aviation, Inc., of Paso Robles, California, and operated by Double Tree Helicopters of Turlock, California, under 14 Code of Federal Regulations Part 137 for aerial application. Visual meteorological conditions prevailed, and no flight plan was filed for the local flight that was originating at the time of the accident.
Accident Sequence
The commercial pilot, the sole occupant, reported that after filling the spray tanks with water, he lifted off from the back of a truck platform. He then heard three loud noises followed by a loss of engine power. The pilot initiated a hovering autorotation to return to the landing platform but 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. The pilot was not injured.
Post-Accident Examination
On the same day, the Allison T63-A720 engine was examined by a representative from Rolls Royce under Federal Aviation Administration supervision. The examination revealed that all fuel, lube, and pneumatic lines and fittings were secure. Black carbon-like particles were loose in the exhaust collector base. The inlet plenum chamber showed no evidence of foreign material or missing hardware. The compressor inlet had dirt deposits on the inlet guide vanes and a small ferrous type material adjacent to a guide vane at the six o'clock position. A single first stage compressor blade contained a small nick on its leading edge near the tip. The engine was removed from the airframe and the compressor partially disassembled.
Further inspection of the compressor module revealed that 15 third stage compressor vanes were fractured near the root and separated, with the majority located in the lower compressor case half. The remaining attached vanes were bent over in the direction of engine rotation. One second stage compressor vane in the lower case half was fractured near the root and separated. Multiple third 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, 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. 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 Rolls Royce for further metallurgical examination under FAA supervision. The examination revealed that the compressor case lining material was partially missing and eroded from both 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, and the area of separation was 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. Semi-quantitative x-ray energy dispersive analysis (XEDA) revealed deposits at the suction side vane root consistent with oxidized braze alloy, indicating the braze alloy was exposed during engine operation. The chemistry, microstructure, and hardness of the second stage vanes were consistent with properly processed AMS 5604 (17-4 PH steel).