Accident Overview
On May 21, 2014, at approximately 1011 eastern daylight time, a Bell 206L-4 helicopter, registration N55SL, landed hard following a total loss of engine power during initial climb at Beverly Municipal Airport (BVY) in Beverly, Massachusetts. The commercial pilot and one passenger were not injured, and the helicopter sustained substantial damage. The flight was operated by Helicopters, Inc. under Title 14 Code of Federal Regulations Part 91 as an aerial observation flight. Day visual meteorological conditions prevailed, and no flight plan was filed. The local flight was originating at the time of the accident.
Pilot Report
According to the pilot, he was assigned to fly a photographer with a local news station to the Watertown, Massachusetts area to respond to a truck accident. The helicopter was parked on a trailer, and the preflight and engine start portions were normal. After checking local wind conditions, the flight was cleared for departure by air traffic control tower personnel. Shortly after takeoff, the engine "suddenly and abruptly" lost all power. The pilot attempted an autorotation to a taxiway but chose to land in the grass adjacent to the taxiway when it became apparent he would not reach the taxiway. During the flare and landing, the main rotor blades contacted the tail boom, severing it just aft of the horizontal stabilizer. The helicopter came to rest on its landing gear skids, and the pilot and passenger exited the aircraft.
Examination Findings
An inspector from the Federal Aviation Administration examined the wreckage. The engine examination revealed that the combustion section outer case (OCC) was ruptured in one of two elbow, or "armpit," areas. The OCC was removed and forwarded to the NTSB Materials Laboratory for further examination. The OCC contained a rupture aft of the left armpit, manifested as case material protruding outward, with mating fracture surfaces located along a horizontal butt weld line of the case. This fractured weld line was situated where a wire mesh reinforcement patch was brazed to the case. The fracture extended into the braze joint around the left connecting flange and the butt weld of the center connecting flange. Much of the fracture surface along the flange joints outside the case butt weld exhibited a 45-degree slant, consistent with overstress fracture. The fracture surface along the case weld exhibited a relatively flat orientation.
Metallurgical Analysis
The weld fracture surface exhibited crack arrest marks and ratchet marks consistent with progressive fracture. In the progressive regions, the fracture surface appeared dark with visible crack arrest marks, more evident toward the periphery. Portions outside the butt weld exhibited a brighter luster but still showed crack arrest features. The thickness of the case along the progressive regions averaged 0.027 inches. Examination of the opposite braze patch revealed small cracks on the outside surface, consistent with smaller versions of the defects observed on the fractured braze patch. The initiation area showed that fatigue cracks started on the inside face of the case weld at multiple sites. No material defects such as pores or inclusions were found at these initiation sites. At the end of the fatigue crack in the weld, features representing a transition to other failure modes contained fatigue striations microscopically. Fatigue striations were present outside the main weld line. Outside the fatigue regions, dimple rupture, indicative of overstress failure, was observed.
Maintenance History
A review of maintenance logbook entries indicated that the most recent 150-hour inspection was performed on the engine on April 12, 2014. About 41.5 hours had accrued since that inspection. The total time on the engine at the time of inspection was 11,142 hours. The inspection included a detailed visual inspection followed by a Leak-Teck or fluorescent penetrant inspection. A general condition inspection of the OCC was also required as part of the post-flight inspection. No anomalies were noted.