Mokulele Flight Service Flight 1770

According to the flight crew, as the airplane passed through about 8,000 ft mean sea level during climb over open ocean water, they 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. The flight crew initiated a turn back towards the departure airport, which was about 13 miles north of their position. The crew accomplished the emergency checklists and elected to perform an emergency landing on a highway. During the landing roll, the airplane struck two highway traffic signs, which resulted in substantial damage to the right wing.

A 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 course 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 fir-tree 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.

Verbatim NTSB analysis and probable cause from the NTSB dataset