Accident Overview
On August 12, 2016, about 1225 mountain daylight time, a Robinson R44 II helicopter, registration N247DB, experienced a loss of engine power during a spray run. The pilot initiated an off-airport forced landing near Gunnison, Utah. The commercial pilot was not injured. The helicopter sustained substantial damage to its tailboom during the hard landing. The helicopter, registered to and operated by a private individual, was conducting a Title 14 Code of Federal Regulations Part 137 agricultural application flight. Visual meteorological conditions prevailed, and no flight plan was filed. The local flight originated from an off-airport site near Gunnison about 0900.
Sequence of Events
The pilot reported completing one spray load and returning to the loading spot for another. After filling, he returned to the field. During the second pass, the pilot heard a "pop" and a "thump" that resonated through the helicopter as he turned off the spray system. Looking out the right-side door, he saw chemical spewing into the air and along the pilot side of the helicopter. He immediately attempted to kill the pump using the kill push button on the cyclic, near the spray on/off switch. Simultaneously, engine and rotor rpm began to drop. The pilot described the engine as sounding like it was "sputtering for air" or being "choked out." He lowered the collective and rolled on throttle to re-establish rpm and revive the engine, but total power loss occurred. At about 50-75 feet above ground level, there was insufficient time for an engine restart. The helicopter landed hard and remained upright. The main rotor blades severed the tailboom during the hard landing.
Spray System and Engine Examination
The pilot reported that a new Simplex Spray System had been installed about 16 hours before the accident. The spray system pump is mounted on the pilot side skid, with the engine air filter compartment located above and behind the pump. A postaccident examination of the spray system found a fractured plastic pipe nipple. The close nipple and engine air intake were sent to the NTSB Materials Laboratory. The laboratory specialist reported that the close nipple broke in a two-step sequence: an initial progressive crack through about 1/8 inch of the circumference, with fractographic features consistent with slow crack growth in polymers, followed by overstress fracture. The lower close nipple piece from the pump outlet showed slow crack growth initiation region and crazing. Mechanical damage was noted near the thread root, which exhibited mold under-fill. Secondary cracks were found inside the nipple piece and in the thread roots.
Examination of the engine intake filter showed no cake deposits. The engine crankshaft rotated freely, valves moved in firing order, and thumb compression was obtained in all cylinders. Spark plug electrodes were circular, clean, and gray with normal operating signatures. Fuel nozzles were contaminant-free. Magnetos were tested on another engine with no anomalies. The air intake scat hose had a light coating of dark, greasy dust. The number two-cylinder intake tube was disconnected with no residue. The engine was test run in a test cell and operated without anomalies.
No evidence of chemical residue was found on the fuselage between the pump and engine intake in accident site photos.