Accident Details
On August 16, 2012, at 0150 eastern daylight time, an MD Helicopters, Inc. 369E (registration N551CP) impacted terrain during a forced landing near Columbus, Ohio. The helicopter was registered to the City of Columbus and operated by the Columbus Police Department under 14 Code of Federal Regulations Part 91 as an aerial observation flight. Night visual meteorological conditions prevailed for the local flight, which had originated from the Columbus Police Heliport (OH52).
The certificated commercial pilot and an observer, also a commercial pilot, were not injured. The helicopter sustained substantial damage. The pilot reported leveling off at 1,500 ft mean sea level (msl) and completing a turn to the west when he heard a "loud exploding noise" from the rear, followed by a long sustained "whining" noise from the engine and a constant warning tone. The pilot observed the gas producer (N1) and power turbine (N2) gauge needles falling rapidly. He performed an autorotation and touched down on a street, where the helicopter slid and rotated counter-clockwise about 1 1/2 turns. The 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.
Engine Examination
The helicopter was powered by a Rolls Royce M250-C20B turboshaft engine (serial number CAE-837045). The engine had accumulated 4,667 total hours and 2,987 cycles. It was overhauled on May 25, 2011, accumulating 1,023 hours time since overhaul (TSO) and 529 cycles since overhaul (CSO). 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.
A National Transportation Safety Board powerplants specialist examined the engine. The N1 drive train turned freely 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 tube was pierced and dented in one location each. The hole and dent locations were in the plane of the 4th stage turbine rotor. The hole measured about 2-1/2 inches by 1/2 to 1 inch, with inward petaling at 9 o'clock and outward petaling at 6 o'clock, consistent with a high-speed particle entering and exiting. The dent was approximately 2 inches by 3 inches with two rectangular impressions.
The 3rd stage turbine nozzle and wheel were intact; the outer shroud had a 90-degree segment of the outer rim heavily scored. The 4th stage turbine nozzle was generally intact, but a 4-inch segment of the shroud was fractured in the plane of the 4th stage wheel, consistent with the exhaust collector housing fracture. The 4th stage shroud showed heavy rotational scoring. The 4th stage turbine was missing one blade, fractured near the inner platform; examination showed high cycle fatigue. A 3-blade span of the related rim segment was also missing. Two adjacent blades were fractured near the tips, one twisted, one partially attached. All other blades were present and undamaged. The curvic coupling was intact.
The N2 bearing set included No. 3, No. 4, No. 5, and No. 6 bearings. The No. 5 bearing snap ring was dislodged from its groove. The power turbine inner shaft was intact, but the No. 6 bearing inner race was heavily deformed at the roller bearing contact surface.