The pilot of the Cessna 208B reported that, after a normal departure from Badu Island, he established the aircraft in cruise at about 5,000 ft for the flight to Saibai Island. The engine had performed normally up to that time. Shortly after becoming established in the cruise, the pilot heard a muffled bang from the engine compartment. On checking the engine instruments, he saw that the gas generator RPM (Ng) had stabilised at about 52% and the turbine temperature was at about 700 degrees C. (The maximum allowable temperature was 740 degrees C.) The pilot initially thought that these symptoms may have indicated an engine fire, so he shut down the engine and feathered the propeller. However, appropriate checks indicated that there was no fire. During these observations, the pilot had turned the aircraft towards Badu Island.
The pilot then attempted two engine starts, neither of which was successful. On each occasion, Ng stabilised at 17%, fuel flow was 110 lb/h, and engine light-off occurred at 850 degrees C. Eventually, Ng stagnated at 42-43% and the propeller unfeathered, but there was no indication of engine torque.
The pilot transmitted a distress call after the first start attempt. After the second attempt, he decided to concentrate on flying the aircraft and realised that his best option was to try to land on a narrow beach on the northern side of Badu Island. He was able to achieve this without any damage to the aircraft.
The aircraft was powered by a Pratt and Whitney Canada PT6A turboprop engine. Examination of the engine revealed that a compressor turbine (CT) blade had failed at about mid-span. The liberated section of blade had struck two adjacent blades, causing them to break. Metallurgical examination of the remaining section of the failed CT blade indicated that the fracture line passed through the centre of a deep sulphidation pocket. Impact damage was observed on all the remaining CT blades. Detailed examination of these blades revealed a total of seven blades that exhibited some degree of cratering due to sulphidation. Inspection of the compressor section of the engine also revealed significant corrosion consistent with operation in a salt-laden atmosphere.
"Sulphidation" refers to the reaction of sulphur containing compounds with metallic components that have been exposed to a hot gaseous environment. Components of gas-turbine engines located in the hot gas path, such as blades and vanes, are exposed to sulphidation during normal operation. Corrosive sulphates are formed during the combustion process from sulphur in the fuel and sodium and potassium salts present in the fuel and air, in particular the air in marine environments. If the accumulations of sulphur-containing salts are not removed from the surfaces of the turbine blades and vanes, the protective oxide coating will be attacked and the underlying alloy rapidly corroded.
The PT6A engine maintenance manual required desalination water-washes to be applied to both the compressor and the turbine after the last flight each day. The operator was conducting compressor washes prior to the first flight of each day, using the compressor wash ring installed on the engine. A special wash tube assembly tool for installation into the gas generator igniter boss, to enable wash solution to be introduced directly to the first-stage turbine blades, was available from the engine manufacturer. The operator was not using this tool. As a result, effective washing of the turbine blades was not achieved, allowing salt deposits to build.
The engine was fitted with an Engine Condition Trend Monitoring (ECTM) system which recorded a number of key engine parameters such as fuel flow, inlet turbine temperature, torque, and Ng. Use of this system allowed operators to delete a fixed-time hot section inspection (HSI) requirement in favour of basing the HSI interval on the results of engine condition trend monitoring. Use of the ECTM system was acceptable to the Civil Aviation Safety Authority (CASA), provided the procedures were in accordance with CASA Airworthiness Advisory Circular 6-29 `PWC PT6A Series Engines HSI Policy' 5/98. Investigation revealed that the operator was not complying fully with all the requirements of the circular.
