MD Helicopters 369 N506PH

The pilot was performing an approach to land when the engine power surged, and the engine turbine outlet temperature (TOT) rose after the pilot lowered the collective. The pilot performed an autorotation to uneven terrain where the helicopter landed and rolled over onto its left side. The engine continued to run after impact, and the pilot shut it down. Examination of the wreckage found no anomalies with the airframe. The engine bleed valve was found in the incorrect, closed position. The engine was relocated to a service center where it ran normally using the accident bleed valve and after the bleed valve was opened manually before the test run. The valve was observed to operate at a slower rate than normal during the engine run. The engine was then equipped with an exemplar bleed valve modified to operate manually. When the engine was brought to full power, the bleed valve held closed, and the engine power reduced, the engine surged, and the TOT increased like it did on the accident flight. A teardown examination of the bleed valve revealed that the bushing in which the bleed valve stem resides was manufactured undersized. The bushing measured .307 inch inside diameter, and official documents state the bushing should measure .310 inch inside diameter. This reduced the clearance between the valve and the bushing; however, the bleed valve assembly had operated on the engine for about 94 hours prior to the accident and was not identified in any maintenance writeups prior to the accident. The examination of the bleed valve also revealed an accumulation of corrosion on multiple internal sub-component surfaces, which included the valve stem and the undersized bushing. The corrosion tested positive for the engine wash compound used by the operator, and the helicopter had undergone an engine wash the day before the accident. The operator indicated the engine manufacturer’s guidance for the wash was followed, which included blocking the bleed valve closed to prevent wash intrusion, washing the engine with a diluted wash solution, rinsing the engine, and drying the engine by conducting a 10-minute engine run. The engine manufacturer stated that engine wash intrusion into the internal components of the bleed valve would be very small if those steps were followed. Due to the extended storage time in non-climate-controlled environments, the investigation could not determine the exact amount of corrosion on the bleed valve at the time of the accident; however, the presence of the wash compound in the corrosion found on the bleed valve indicates that some wash solution penetrated into the bleed valve during a wash and remained following the rinse and drying of the engine. The investigation could not determine if the wash procedures were strictly followed or why the wash solution entered the bleed valve and remained. Although the bushing installed on the bleed valve stem was manufactured incorrectly, the bleed valve likely operated properly until corrosion developed inside the bushing following the engine wash. However, the bleed valve was likely more susceptible to this type of failure due to the incorrect bushing dimensions. Due to these factors, the bleed valve failed to function properly during the accident flight, which resulted in a partial loss of engine power.

Quoted verbatim from the NTSB record.