ERA13IA313 · Bell Helicopter Textron Canada 206 L-4

The commercial pilot reported that, during an overwater sightseeing flight in the single-engine helicopter and while at 1,500 ft, he heard a "bang," followed by the "engine out" warning. He then saw that the N2 (power turbine) indication was dropping. The pilot decided to perform an autorotation, and just before lowering the collective and rolling the throttle to flight idle, he saw the "engine chip" light illuminate. During the landing flare, the pilot deployed the skid-mounted floats, bled off all forward airspeed, and completed a successful autorotation.

The turbine section of the turboshaft engine had recently been overhauled, and the engine had operated for about 2 hours before the incident. The No. 2 bearing in the compressor section and its corresponding races were found damaged and appeared dry with evidence of high-temperature exposure. The forward side of the bearing cage exhibited significantly more damage than the aft side; the forward side was deformed, and the bearing balls on that side appeared rough and had large areas of material loss. Metallurgical examination revealed thermal distress to the raceway surfaces consistent with the bearing operating with reduced oil flow.

To confirm oil flow, the engine gearbox was attached to an oil supply, and the engine oil pump was rotated by a hand drill. Three of the four jets from the oil supply tube produced streams of oil; however, the fourth jet, which normally supplied oil to the aft face of the No. 2 bearing, did not. Visual examination of the oil supply tube revealed that a dark, thick substance was adhered to the face and chamfer. Analysis of the substance determined that, although it was the correct-specification turbine oil, it was thermally degraded. Initial flow tests revealed that the oil supply tube that lubricated the No. 2 bearing was operating below the total flow requirement. The tube was cleaned ultrasonically multiple times, and, with each subsequent cleaning, the amount of debris collected decreased. After the cleanings, the No. 2 bearing orifice met the flow requirement.

Nearly the entire surface of the pressure oil screen, except for the area covered by an O-ring, was also covered in a thick, dark substance. A sample of the substance was also determined to be consistent with correct-specification but thermally degraded turbine oil. Additional components were also covered with coked and degraded turbine oil.

In addition, the engine oil pressure regulator valve was found backed out, and the poppet guide was atypically close to the outer lip of the housing. The atypical position indicated that adjustments, which were not documented, were likely made in response to high oil pressure indications. However, any adjustment to the oil pressure regulator valve would have been contrary to the engine manufacturer's maintenance manual, which cautioned not to do so for high oil pressure, which would have been "cause to suspect other oil system problems have developed."

The origin of the coking and buildup of degraded turbine oil within the supply system could not be determined; however, the extent to which the oil pressure regulator valve was found adjusted indicated that it had likely occurred over time, which in turn then masked a growing oil blockage problem within the oil lubrication supply paths.

Verbatim NTSB analysis and probable cause from the NTSB dataset