What happened
The CT7-9B departed runway 13L and reached an altitude of approximately 400 feet AGL when the flight crew heard a loud bang. Following this sound, the upper cowling of the right engine separated from the aircraft. The right engine subsequently shut itself down, and the right propeller automatically feathered. The crew declared an emergency and completed a single-engine landing without further incident; all occupants were not injured.
The investigation
An examination of the right CT7-9B turboprop engine determined that the Stage 2 turbine disk was missing. Approximately one third of the disk was located within the right landing light compartment. Investigators found that one end of a disk fragment had progressed through a cooling air hole. Evidence indicated a fatigue fracture had initiated from multiple origins on the inboard side of the cooling air hole.
Research into the component revealed a layer of deformed microstructure and an associated zone of increased disk hardness near the area where the fatigue crack originated. Investigations showed that test disks had been drilled using dull and worn drill bits, which caused plastic deformation and increased the hardness of the inner diameter hole surfaces. The manufacturer reported that this specific type of deformed microstructure correlates to a zone of reduced fatigue properties.