Photo 1.
Sulfidation of number 1 GP turbine
During the examination of the engine, it was installed in an approved engine test facility where a full test was attempted; however, the engine vibrated excessively and the test was aborted. Some smoke was evident during the spool down of the engine. The engine was then disassembled for inspection. Of the 38 blades on the number 1 gas producer (GP) turbine, 29 were damaged significantly. One was broken off close to its root, and most of the others were missing pieces. Examination of the remaining fracture surface of the blade that broke close to its root showed indications of a progressive failure (fatigue crack). The other fracture surfaces showed failure by overload. More detailed examination of the GP turbine blades showed extensive sulfidation corrosion1 (see Photo 1). The blade surfaces were flaking and there were fatigue cracks in many of the blades. The turbine first-stage nozzle labyrinth seal was damaged and the outer fibreglass packing was partially missing. There was soot at the joint between the turbine nozzle and number 1 GP turbine and thick, black, wet soot around the labyrinth seal and on the turbine coupling.
On 10 February 2000, Rolls-Royce Allison issued a Commercial Service Letter, Hot Corrosion-Sulfidation, describing hot corrosion (sulfidation) and suggesting that operators look for it. However, no periodic maintenance inspection was directed by Transport Canada (TC) and none was implemented by the operator with regard to sulfidation corrosion.
Engine compressor washing is recommended in corrosive and dirty environments. It is also prescribed when engine performance has depreciated, as noted from power assurance checks. However, there has been concern that some tap water can in itself be corrosive. Operators rarely do compressor washes when their helicopters are operating away from a salt environment. They will wash compressors when they can see dirt building up at the air intake, or when an engine is performing below specifications. It is not clear if compressor washing effectively cleans the GP turbine wheel.