CEN12TA550

The pilot reported that, shortly after leveling off the helicopter at 1,500 ft mean sea level, he heard a "loud exploding noise" coming from the rear of the helicopter followed by a "long sustained whining noise" coming from the engine and a constant warning tone in the background. The pilot then saw the gas producer (N1) and power turbine (N2) gauge needles falling rapidly. He identified the situation as an engine failure, so he performed an autorotation. The helicopter subsequently touched down on a street, slid, rotated counterclockwise about 1 1/2 turns, then came to a stop. The main rotor blades contacted a utility pole adjacent to the street, and the tail fin, stabilizer, and tail rotor broke off. A postaccident examination of the engine revealed that a 4-inch segment of the 4th-stage turbine shroud was fractured in the plane of the 4th-stage turbine wheel. The 4th-stage turbine was missing one blade, and the blade had fractured off near the inner platform. A three-blade span of the related segment of the rim was also missing. An examination of the missing blades' fracture surfaces showed evidence consistent with high-cycle fatigue likely caused by an engine speed/airframe interaction of unknown origin, which resulted in the blade failure and subsequent loss of engine power.

On August 16, 2012, at 0150 eastern daylight time, an MD Helicopters, Inc. 369E, N551CP, impacted terrain during a forced landing near Columbus, Ohio. The certificated commercial pilot and an observer, who was also a commercial pilot, were not injured. The helicopter sustained substantial damage. The helicopter was registered to the City of Columbus and operated by the Columbus Police Department under the provisions of 14 Code of Federal Regulations Part 91 as an aerial observation flight. Night visual meteorological conditions prevailed for the local flight that had originated from the Columbus Police Heliport (OH52), Columbus, Ohio.The pilot reported that he leveled off at 1,500 ft mean sea level (msl) and had completed a turn to the west when he heard a "loud exploding noise" from the rear of the helicopter, followed by a long sustained "whining" noise coming from the engine and a constant warning tone in the background. The pilot saw that the gas producer (N1) and power turbine (N2) gauge needles were a falling "rapidly." The pilot performed an autorotation and touched down on a street where the helicopter slid and rotated counter-clockwise about 1 1/2 turns. The helicopter's tail fin, stabilizer, and tail rotor were broken off, and the main rotor blades were substantially damaged from contact with a utility pole adjacent to the street. The helicopter was powered by Rolls Royce M250-C20B, serial number CAE-837045, turboshaft engine. The engine accumulated a total time of 4,667 hours and to cycles of 2,987. The engine was overhauled on May 25, 2011 and accumulated a total time since overhaul (TSO) of 1,023 hours and cycles since overhaul (CSO) of 529. During the last overhaul, new turbine wheel stages 1-4 were installed. The engine was removed on June 13, 2012 for a compressor bearing problem at TSO 832.1 hours and CSO 529. The helicopter's engine was examined by a National Transportation Safety Board powerplants specialist. The N1 drive train turned free and continuous between the compressor and the starter generator pad. The N2 drive train was locked. The left-hand compressor discharge tube was undamaged. The right-hand compressor discharge tube was pierced in one location and dented in one location. The hole and dent locations and direction were in the plane of the 4th stage turbine rotor. The hole was located at the 6 o'clock to 9 o'clock location with reference to the tube and was about 2-1/2 inches long with the width varying from ½ inch to 1 inch. The edge material condition of the hole at 9 o'clock was 'petaled' inwards; consistent with a high-speed particle entering the tube and the edge material condition at the 6 o'clock location was 'petaled' outwards, consistent with a high-speed particle material exiting the tube. The dent location was between the 12 o'clock and 9 o'clock location with reference to the tube and was approximately 2 inches in axial length and 3 inches wide. There were two rectangular shaped impressions within thedent surface, each approximately 3/8 inch square. The 3rd stage turbine nozzle was intact and undamaged. The 3rd stage turbine wheel was intact. The curvic coupling was undamaged. The outer shroud was intact; however, a 90-degree segment of the outer rim was heavily scored, consistent with contact against the shroud. The 4th stage turbine nozzle was generally intact; however a 4-inch segment of the 4th stage turbine shroud was fractured in the plane of the 4th stage turbine wheel, consistent in location with the fracture of the exhaust collector housing. The 4th stage shroud showed heavy rotational scoring. All the nozzle vanes were present and undamaged. The 3rd stage turbine shroud showed heavy rotational scoring. The 4th stage turbine was missing one blade. The missing blade was fractured near the inner platform. An examination of the fracture surface showed high cycle fatigue. A 3-blade span of the related segment of the rim was also missing. The two adjacent blades were fractured near the tips. One adjacent blade was missing the entire rim segment and was twisted, while the other adjacent blade was partially attached to the remaining rim. All other blades were present and appeared to be undamaged. The curvic coupling was intact and undamaged. The N2 bearing set consisted of the No. 3, No. 4, No. 5 and No. 6 bearings. The No. 5 bearing snap ring was found dislodged from its groove. The power turbine inner shaft was intact; however, the No. 6 bearing inner race was heavily deformed at the roller bearing contact surface.