Overview
On October 21, 2013, about 1845 Hawaii standard time, a Cessna 208B, registration N861MA, sustained substantial damage during an emergency landing on the Piilani highway (state route 31), about 13 miles south of Kahului Airport (OGG), Kahului, Hawaii. The aircraft was operated by Mokulele Airlines as flight 1770 under Title 14 Code of Federal Regulations Part 135. The flight crew consisted of an airline transport pilot and a commercial pilot, with eight passengers aboard. No injuries were reported.
Flight Details
The scheduled domestic flight originated from OGG at about 1830, with a planned destination of Waimea-Kohala Airport (MUE), Kamuela, Hawaii. Visual meteorological conditions prevailed, and an instrument flight rules flight plan was filed. During climb out, as the airplane passed through about 8,000 feet mean sea level, the crew heard a loud bang and grinding noise, followed by a loss of engine power. Sparks were observed from the right side engine exhaust. The crew did not attempt an engine restart. They turned back toward the island of Maui and performed emergency checklists, electing to land on the highway. During the landing rollout, the airplane struck two highway traffic signs, causing substantial damage to the right wing.
Injuries
None of the 10 occupants (two pilots and eight passengers) were injured.
Aircraft Information
The accident airplane was a 2000 Cessna model 208B, serial number 208B0825, with a maximum takeoff weight of 8,750 pounds. It was powered by a Pratt and Whitney PT6A-114A engine, serial number PCE-PC-1213, rated at 675 horsepower. The engine had accumulated 9,343.2 hours since new and 19,316 cycles since new. The last overhaul was at 5,249.1 hours and 10,997 cycles prior to the accident. A hot section inspection was performed on August 8, 2013, by Southwest Airmotive, which included replacement of the 3rd stage compressor disc, compressor impeller, and power turbine disc.
Engine Examination
Post-accident examination at Pratt & Whitney facilities revealed that the compressor turbine (CT) blades were fractured, with about 20 to 60 percent of their spans remaining. A fatigue crack was identified on blade number 57. The engine was disassembled, and glass-like beads were found between the "fir-tree" joint of the blade platforms and the CT disc. The beads, approximately 195 to 160 µm in size, were composed predominantly of silica and consistent with glass media blasting. The manufacturer's overhaul manual specifies that all media blast cleaning must be performed with the disc and blades disassembled and cleaned before assembly. The contamination was introduced during the disc assembly cleaning process.
Probable Cause
The failure of a compressor turbine blade due to a fatigue crack, which resulted from glass bead contamination in the compressor turbine disc assembly introduced during cleaning.