21st of October 2013, Improper Maintenance, Blog #724
- V2Aviation
- 10 hours ago
- 3 min read
With a crew of two and eight passengers, a Cessna 208B Grand Caravan was operating a flight from Kahului Airport (OGG/PHOG) to Waimea-Kohala Airport (MUE/PHMU, both in Hawaii, on this day in aviation history.
After an uneventful take-off, when the aircraft passed approximately 8000 feet (MSL - Mean Sea Level) over open water, the crew heard a loud bang and a grinding sound, which was followed by a loss of engine power. Sparks were observed coming out of the engine's right exhaust. At this time, they were ~13 miles south of Kahului, their departure airport. Sparks were seen coming out of the engine exhaust. ATC was informed, and they were cleared to land at runway 02, at Kahului.
The relevant emergency checklist was completed, at which time it was clear they would not make it back to Kahului. The crew decided to perform an emergency landing on the Piilani Highway (state route 31) and informed ATC of their intentions. During the landing rollout on the highway, the airplane struck two highway traffic signs, which resulted in substantial damage to the right wing. After coming to a stop, the pilot advised ATC that they were safely on the ground and evacuating the aircraft. There were no injuries. After recovery of the aircraft to a secure location, the engine was removed and sent to the manufacturer for examination. Maintenance service records established that the engine had accumulated 9,343.2 hours since new (TSN) and 19,316 cycles since new (CSN). The engine had accumulated 5,249.1 hours and 10,997 cycles since the last overhaul. The engine had accumulated 138 hours since the last hot section inspection.
The postaccident examination of the airplane's engine revealed that all of the compressor turbine (CT) blades had separated near the blade platforms, and the remaining stubs were gouged and battered. The fracture surface of one of the blades displayed smooth features characteristic of fatigue fracture; all other blade fracture surfaces displayed coarse features characteristic of overload fracture. The fatigue-fractured blade's microstructure in the fracture area was normal, and the blade material and hardness met engineering drawing requirements. The CT hub displayed a frosted appearance over the entire front and aft surfaces, and glass beads and bead fragments were observed between the fir-tree joints of the blade platforms and the CT disc, consistent with the disc assembly having been cleaned by aggressive glass media blasting in the assembled condition. The engine manufacturer specifies that all media blast cleaning be performed with the CT disc and blades disassembled. The glass bead contamination of the firtree joints caused the CT blades to be unevenly restrained, and it altered the blades' designed vibration frequency, making them susceptible to the aerodynamic vibrations from the combustor gas flow. Therefore, the fatigue fracture of the blade was most likely due to the glass bead contamination. The CT disc assembly had been removed from the engine for repair about two months before the accident, and this was likely when the glass bead media cleaning occurred and the beads became embedded in the exposed fir tree joints. When the CT disc assembly is installed in the engine, the fir tree joints are protected from exposure; therefore, the glass bead contamination could not have occurred when the engine was installed on the airplane.
The NTSB concluded that the probable cause of the accident was; Improper maintenance resulted in contamination of the engine's compressor- turbine disc and blade assembly by glass bead remnants, which resulted in a blade failure.
The NTSB report, which served as the source for this blog, can be accessed by clicking on the .pdf file below;
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