NTSB Factual Report

On May 21, 2014, about 1011 eastern daylight time, a Bell 206L-4, N55SL, landed hard following a total loss of engine power during initial climb at Beverly Municipal Airport (BVY), Beverly, Massachusetts. The commercial pilot and one passenger were not injured, and the helicopter was substantially damaged. The helicopter was operated by Helicopters, Inc. under the provisions of Title 14 Code of Federal Regulations Part 91. Day, visual meteorological conditions prevailed for the aerial observation flight, and no flight plan was filed. The local flight was originating at the time of the accident.

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 of the flight were normal. After checking the local wind conditions, the flight was cleared to depart 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 the he would not make 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 the landing gear skids and the pilot and passenger exited the aircraft.

An inspector with the Federal Aviation Administration responded to the accident site and examined the wreckage. The engine examination revealed that the combustion section outer case (OCC) was ruptured in one of two elbow, or "armpit" areas.

A review of maintenance logbook entries revealed that the most recent 150-hour inspection was performed on the engine on April 12, 2014. About 41.5 hours had accrued on the engine since this inspection. The total time on the engine at the time of this 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.

The OCC was removed from the engine and forwarded to the NTSB Materials Laboratory for examination. The OCC contained a rupture aft of the left armpit. The rupture was manifested as case material protruding outward, with the mating fracture surfaces located along a horizontal butt weld line of the case. This fractured weld line was located 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.

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, which were more visibly evident towards the periphery. The portions of the fracture surface outside the butt weld exhibited a brighter luster, but still exhibited crack arrest features. Using a measurement standard for comparison, the thickness of the case was measured along the progressive regions of the fracture surface. The thickness of the case averaged 0.027 inches.

An examination of the opposite braze patch revealed small cracks present on the outside surface. These were consistent with smaller versions of outer surface defects observed on the fractured braze patch.

Examination of the initiation area revealed the fatigue cracks started on the inside face of the case weld at multiple crack initiation 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 that appeared to represent transition to other failure modes were also found to contain fatigue striations microscopically. Fatigue striations were present outside the main weld line. Outside the fatigue regions, dimple rupture, which was indicative of overstress failure, was observed.

NTSB Probable Cause

Maintenance personnel’s inadequate inspection of the engine’s outer combustion case, which resulted in the failure of the case due to fatigue cracks that initiated on the inside surface of the case and the subsequent total loss of engine power.