What happened
On August 6, 2005, a PILATUS PC6/B_1H2, registration F-GZDO, was conducting a scheduled parachute drop operation at the Ocaña Aerodrome in Toledo. The aircraft was on its third flight of the day, carrying a pilot and four passengers. While climbing through approximately 14,000 ft, the crew heard a loud noise followed by a total loss of engine power. Following the engine failure, the passengers successfully exited the aircraft via parachute. The pilot then managed to glide the aircraft back to the departure aerodrome, performing an emergency landing without further incident. There were no injuries to the crew or passengers, though the aircraft sustained major damage to the engine.
The investigation
The investigation focused on the engine's condition and the operational environment. The pilot reported that during the climb, the Inter-Turbine Temperature (ITT) spiked above 1,000 °C, significantly higher than the expected 680 °C. An inspection of the Pratt & Whitney Canada PT6A-20B engine revealed substantial damage to the power turbine blades and the stator, along with damage to the compressor blades. No foreign object ingestion was found.
The investigation examined the nature of parachute operations, noting that the engine was subjected to frequent, high-stress cycles. Because flights occurred in short intervals, the engine was often restarted before it had cooled to standard temperatures, leading to "warm start-ups." Additionally, high ambient temperatures (29-30 °C) and potential calibration issues with the ITT thermocouple harness were considered factors in the engine's thermal state.
Findings
- The engine was subjected to repetitive thermal stress due to the nature of parachute operations, which involve frequent takeoffs, landings, and engine shutdowns.
- The practice of performing "warm start-ups" prevented the engine from reaching prescribed cooling values between flights.
- Repeated exposure to ITT values exceeding prescribed limits likely caused the fatigue and subsequent failure of the power turbine blades.
- The maintenance manual lacked specific procedures for evaluating the cumulative impact of repeated, short-duration high-temperature events.