WPR11LA220

The pilot reported that the helicopter was in cruise flight at 12,500 feet mean sea level over mountainous terrain when power stopped being transmitted to the main rotor system. He performed an autorotation to an open area. During the landing, the tail rotor sustained damage to both blades as they went through about 1 foot of snow. After landing, the engine was still running but was not spinning the main rotor blades. Postaccident examination of the airframe revealed no anomalies that would have precluded normal operation. The drive train was fully functional. The engine was run in a test cell, which confirmed that it met specifications for all parameters including performance. The reason for the loss of engine power could not be determined.

HISTORY OF FLIGHT On May 13, 2011, at 0917 Pacific daylight time, an MD Helicopter (MDHI) 369D, N124AL, made an off airport forced landing near Columbia, California. Airlift Helicopters of Alaska, Inc., was operating the helicopter under the provisions of 14 Code of Federal Regulations (CFR) Part 91. The airline transport pilot and one passenger were not injured; the helicopter sustained substantial damage to the tail rotor section. The cross-country ferry flight departed Fresno, California, at an undetermined time with a planned destination of Reno, Nevada. Visual meteorological conditions prevailed, and no flight plan had been filed. The pilot stated that the helicopter was in cruise flight at 12,500 feet mean sea level (msl) over mountainous terrain. The engine remained running, but power stopped being transmitted to the main rotor system. He performed an autorotation to an open area. During the landing, the tail rotor sustained damage to both blades as they went through about 1 foot of snow. He reported that after landing, the engine was still running, but not spinning the main rotor blades. Post Accident Examination Investigators from the airframe and engine manufacturers (MDHI and Rolls-Royce, respectively) examined the helicopter under the supervision of a Federal Aviation Administration (FAA) inspector at the operator’s facility in Reno on May 26, 2011. Their complete reports are part of the public docket. Pertinent excerpts follow. No anomalies were identified with the airframe or engine that would have precluded normal operation. Airframe Lateral, longitudinal cyclic and collective main rotor control linkage continuity was established throughout the full range of movement from the cockpit controls to the upper flight controls and to the rotor hub. The anti-torque flight control pedal brackets fractured on both sides at a bracket on the rudder pedal support frame. From that point, there was linkage continuity to a bellcrank where the bellcrank end and control rod end bearing had broken off. Manipulation of the bellcrank caused the applicable changes to the tail rotor blade pitch settings. No anomalies were identified with the fuel system. Battery power was supplied to the cockpit and instrument panel. All appropriate warning lights illuminated, and on test all segments functioned. There were no chip lights illuminated. The engine reignition system, N2 rpm control (beep switch) to the power governor, and the lateral and longitudinal trim actuators were functional. The main transmission rotated freely by hand. Drive to the tail rotor output pinion and driveshaft was present. Transmission oil was observed in the sight gauge, and both chip detectors were clean. The over-running clutch was functional. Both forward and aft KAFLEX drive shaft couplings were intact, and exhibited no visible damage. Engine The engine was an Allison Model 250-C20B gas turbine engine, serial number CAE 831345. Initial inspection revealed that the compressorrotated freely, and was continuous to the starter generator gearshaft. The N2 power turbine rotor turned freely by hand, and was continuous to the power turbine output. No engine anomalies were noted. The engine was shipped for follow-up examination to the Rolls-Royce facility in Indianapolis, Indiana. The examination took place on July 18, 2011, under the supervision of an FAA inspector from the Indianapolis Flight Standards District Office. The engine was placed into a test stand; it was run through an as-received test run schedule, and met all tested parameters. It started, accelerated to ground idle normally, and then advanced to takeoff power successfully. The compressor bleed valve closed within specifications, and the acceleration from flight autorotation to takeoff test was performed successfully. The anti-ice system, governor droop test, and engine vibrations were within specifications. Five selected performance calibrations were completed, and the calculated performance was 506 shaft horsepower (SHP) versus the 420 per the model specifications.