NTSB Factual Report

On August 12, 2016, about 1225 mountain daylight time, a Robinson R44 II helicopter, N247DB, experienced a loss of engine power during a spray run. The pilot subsequently initiated an off airport forced landing near Gunnison, Utah. The commercial pilot was not injured. The helicopter's tailboom was substantially damaged during the hard landing. The helicopter was registered to and operated by a private individual as a Title 14 Code of Federal Regulations Part 137 agricultural application flight. Visual meteorological conditions prevailed at the time of the accident, and no flight plan had been filed. The local flight originated from an off-airport site near Gunnison about 0900.

The pilot reported that he had accomplished one spray load and returned to the loading spot for another load. After fill-up, he returned to the field. During the second pass over the field, the pilot heard a "pop", as well as, a "thump" that resonated through the helicopter about the same time that he was turning off the spray system. The pilot glanced out the right-side door and saw chemical spewing in the air, and along the pilot side of the helicopter. He immediately went to kill the pump by using the kill push button located on the cyclic, by the spray on/off switch. As he was doing so, the engine and rotor rpm began to drop. The pilot reported that it sounded like the engine was "sputtering for air" or being "choked out." The pilot lowered the collective and rolled on throttle to re-establish the rpm to try and revive the engine. As he was performing this procedure, the engine lost total power. The helicopter was about 50-75 feet above ground level, and there was not time to try an engine restart. The helicopter subsequently landed hard and remained upright. During the hard landing, the main rotor blade(s) severed the tailboom.

The pilot reported that a new Simplex Spray System had been installed on the helicopter about 16 hours prior to the accident. The spray system pump is mounted on the pilot (right) side skid. The engine air filter compartment is located above and behind the spray pump.

A review of the accident site photos did not reveal any evidence of chemical residue on the side of the fuselage between the pump and the engine intake manifold.

A postaccident examination of the spray system revealed that a plastic pipe nipple on the spray system fractured. The close nipple and the engine air intake were sent to the National Transportation Safety Board Material Laboratory, Washington D.C. for examination. The materials laboratory specialist reported that the close nipple broke in a two-step sequence. An initial progressive crack was noted through the wall (length: about 1/8 inch of the circumference). The region along the aft side of the close nipple fracture surface revealed fractographic features consistent with a slow crack growth, a progressive fracture mode in polymers. The balance of the fracture exhibited fractographic features consistent with overstress fracture and a region of final fracture. The lower close nipple piece removed from the pump outlet was examined and revealed the slow crack growth region, with the initiation region and a representative area of crazing identified. The fractography generally revealed mechanical damage in the fracture surface immediately adjacent to the thread root. The threat root exhibited features consistent with mold under-fill. Also evident was step features consistent with multiple crack initiation sites typically found in craze cracking in rigid thermoplastics. The inside surface of the close nipple piece removed from the pump outlet, and in the roots of the aft facing thread root revealed secondary cracks. The initiating regions revealed features consistent with mold under-fill.

Examination of the internal and external surfaces of the engine intake filter revealed no cake deposits.

The entire Material Laboratory Factual Report is included in the public docket for this case.

The engine examination revealed that the crankshaft rotated freely, and the valves moved approximately the same amount of lift in firing order. Thumb compression was obtained in all cylinders. The spark plug center electrodes were circular and clean with no mechanical deformation. The spark plug electrodes were gray, with normal operating signatures consistent to the Champion Aviation Check-A-Plug AV-27 Chart. The fuel nozzles were free of contaminants. The magnetos were removed from the engine and installed on another engine. No anomalies were detected with the magnetos during the engine run. The air intake scat hose displayed a light coating of dark colored dust that was greasy to the touch. The number two-cylinder intake tube was disconnected, and no residue was detected.

The engine was subsequently test run in a test cell. The engine ran with no anomalies noted.

NTSB Probable Cause

A total loss of engine power for reasons that could not be determined based on the available evidence.