A single engine, inter-island commuter plane experienced an in-flight engine shutdown necessitating the pilots to make a challenging and successful landing on a mountain side highway. The daring landing went viral on sociaI media. Subsequent inspection of the turbine engine revealed one of its compressor turbine blades failed during the flight. The engine had been serviced recently and ESi was engaged to investigate the failure on behalf of the airline operator.
Our Approach
ESi review of maintenance records revealed the CT disk assembly had undergone maintenance about 40 hours prior to the failure, including inspections for existing cracks, and none were observed. As part of this periodic maintenance, glass bead blast cleaning was performed on the assembly.
The assembly consists of the CT disk which has serrated firtree female slots on its outer diameter that engage 57 turbine blades with matching male firtrees at their base. The firtree shape allows for easy blade replacement and benefits the distribution of blade centrifugal stress and heat transfer during disk rotation and hot gas exposure in service. Importantly the geometric design of the firtree slots and mating blade firtrees are carefully controlled with clearances that permit blade damping movement to avoid critical, detrimental harmonic oscillations in service.
ESi performed a lab-based inspection, including optical microscopic and scanning electron microscopic examinations of the failed blade, other turbine blades and firtree slots on the CT disk. The failed blade was confirmed to have experienced high-cycle fatigue cracking that progressed rapidly to its failure. Several other blade firtree attachments and CT disk slots were observed to have residues containing microscopic round spheres. Comparison with exemplar glass bead blast material confirmed the subject observed spheres had similar size and elemental composition.
Services Utilized
Expertise Utilized
The Outcome
The presence of bead blast media was attributed to the maintenance shop’s failure to adequately mask the firtree areas of the disk assembly prior to glass bead blast cleaning. This discrepant material in the precisely machined firtree connections obstructed the critical clearances, impeded proper blade damping, and ultimately led to resonance excitation, resulting in rapid high-cycle fatigue turbine blade failure.
