What happened
On 15 July 1999, a Saab-Scania SF34 0A, registration G-GNTE, was conducting a public transport flight from Aberdeen Airport when the crew experienced a sudden impact on the right side of the aircraft. Shortly after takeoff, the crew noted a sensation similar to a bird strike, followed by the illumination of the No 2 engine turbine over-temperature light. Although initial engine instruments appeared normal, the commander reduced power to the No 2 engine. Within approximately 20 seconds, the engine suffered a complete power loss. The crew declared a Mayday and performed a visual return to Aberdeen, landing safely. There were no injuries to the 14 passengers or 3 crew members.
The investigation
An examination of the No 2 engine revealed an uncontained failure within the Gas Generator Turbine (GGT). A rupture in the engine casing allowed fragments to exit the engine, puncturing the engine cowling and an aircraft de-icing pipe. One piece of debris even struck the leading edge de-icing boot on the right tailplane.
During a detailed strip examination of the engine, investigators identified a radial fracture in the Aft Cooling Plate (S2) of the second stage turbine. This component had reached 11,454 cycles, approaching its 12,000-cycle retirement limit. Further metallurgical analysis in the USA revealed that the fracture originated from axial grooves found inside the air cooling holes of the plate. These grooves were traced back to an anomaly in the electrical discharge machining (EDM) process used during manufacturing.
Findings
- The engine failure was caused by a fatigue crack in the second stage turbine cooling plate.
- The fatigue crack initiated from microscopic axial grooves located within the air cooling holes.
- These grooves were inadvertently created during the electrical discharge machining (EDM) process at the manufacturer's facility.
- The manufacturing defect reduced the fatigue resistance of the component, allowing a crack to propagate through the plate during service life.