LAX97LA247

The pilot was hovering with an external load, when he heard a loud noise and noticed a power loss. He turned and executed a precautionary landing with a hard touchdown. The main rotor blades flexed downward and impacted the tail boom. An inspection revealed that over half of the blades in the No. 2 power section were fractured. A metallurgical examination revealed that two consecutive blades showed evidence of solutioning and re-solidification; however, the fracture surfaces did not show similar evidence of heat distress. Discoloration was also found on the power turbine wheel at a point corresponding to the location of the two blades. The operator stated that the aircraft had flown about 138 hours since all the turbine blades were replaced. None of the pilots who had flown the aircraft since then reported experiencing any hung or hot starts.

On July 18, 1997, at 1415 hours mountain standard time, a Sikorsky S-58T, N47B, experienced a loss of engine power and made a hard landing 15 miles southwest of Page, Arizona. The aircraft sustained substantial damage; however, the pilot, the sole occupant, was not injured. The aircraft was operated by Air One Helicopters of San Jose, California, and was conducting external load operations under 14 CFR Part 133 when the accident occurred. The flight originated from the Page Municipal Airport at 0530. Visual meteorological conditions prevailed at the time and no flight plan was on file.

The pilot was over a concrete pour site when the engine power loss occurred. The pilot and ground crew heard a loud explosion at the moment of the power loss. He turned away from the site and executed an immediate precautionary landing. As the aircraft touched down, the main rotor blades flexed downward and struck the tail boom. The pilot described the weather at the time as hot and windy.

A postaccident inspection of the engine was conducted at the manufacturer's facility in Longueuil, Quebec, Canada. The inspection, which was supervised by a senior investigator from the Transportation Safety Board of Canada, revealed that turbine blades No's. 15 through 38 in the No. 2 power section had fractured at various heights between the tip and root. The power turbine shroud displayed gouges, circumferential machining, and outward radial deformation. The power turbine guide vanes' airfoils displayed evidence of heat erosion at the 2 o'clock and 8 o'clock positions. The power turbine guide vane downstream side, the No. 3 bearing and support structure displayed mechanical damage.

A metallurgical examination of the blades conducted by the manufacturer showed that, in their opinion, the surface of the No's. 27 and 28 blades showed evidence of exposure to overheating conditions, including solutioning and re-solidification of the blade material. The fracture surfaces of those blades, however, did not show similar evidence heat distress. The manufacturer stated that it was also their opinion that this indicated the overheating exposure to the two blades had occurred some time after their installation, but before the blades' ultimate structural failure. The manufacturer verbally stated that an arc of discoloration was found on the power turbine wheel which corresponded to the location of the two blades on the wheel.

The aircraft's engine installation requires that the start sequence be conducted with the power turbine held in a static position due to the airframe main rotor configuration. The manufacturer also stated that uneven gas path temperature distributions can result in overheating to isolated or adjacent blades, and further stated that hung or hot starts have the potential to create these conditions.

A review of the history of the accident engine revealed that in July 1996, the No. 2 power section was removed from service due to excessive vibration. An examination conducted by the manufacturer found that the vibration had been caused by the fracture of one power turbine blade through tensile overload promoted by localized overheating. The engine was repaired by the manufacturer on November 19, 1996. The repair included the replacement of 39 of the 41 power turbine blades, the power turbine shaft, the exhaust duct, and the power turbine housing with reconditioned components. The remaining two power turbine blades, the Nos. 3 and 4 bearings, and the power turbine shroud were replaced with new components. The last No. 2 power section overhaul was conducted on December 1, 1987, at a repair facility other than the manufacturer's.

The operator stated that the aircraft had flown about 138 hours since the repair to the turbine section. He stated that none of the pilots who had flown the aircraft during that time reported any hung or hot starts.

After the completion of the examination of the No. 2 power section at the manufacturer's facility, the fractured turbine blades were lost before the engine was returned to its owner. Subsequent attempts by the manufacturer to locate the blades have been unsuccessful.