WPR15LA138

The commercial pilot reported that, during the agricultural application flight, the engine started to shake violently. The pilot executed a left turn and positioned the helicopter into the wind, but the helicopter started to descend, and the engine then lost power. The pilot conducted an autorotation, and the helicopter subsequently impacted the ground, which resulted in substantial damage to the tailboom. Postaccident examination of the engine revealed no evidence of preimpact mechanical malfunctions or failures that would have prevented normal operation.

On March 28, 2015, about 1800 Pacific daylight time, a Hiller UH-12E, N130HA, sustained substantial damage following a loss of engine power and subsequent forced landing approximately 6 miles east of Firebaugh, California. The pilot, the sole occupant, was not injured. The helicopter was registered to Mid-Cal AG Aviation, Inc., and operated as an agricultural aerial application flight under the provisions of Title 14 Code of Federal Aviation Regulations (CFR) Part 137. Day visual meteorological conditions prevailed, and no flight plan was filed. The local flight originated at the loading site approximately 6 miles east of Firebaugh about 1700. The pilot reported he had conducted a preflight check about 2 hours before takeoff and no anomalies were noted. Prior to departing with the first load of insecticide, the pilot noted 24 gallons of fuel on board as indicated by the fuel gauge. The pilot dispersed 10 loads of insecticide in 45 minutes. He returned to the loading site for the 11th load, and attempted a normal landing from a hover with 8 gallons of fuel remaining. During the landing flare, the engine momentarily "sputtered". The pilot assumed that the "sputter" was the result of the fuel moving to the rear of the tank, and air entering the fuel system during the landing flare. The level of fuel in the tank was verified with a graduated fuel stick, which indicated 5 gallons of fuel remaining. The engine ran for about 5-7 minutes at 2,500 rpm while 18 gallons of fuel were added to the fuel tanks, and more insecticide was loaded. Prior to and during takeoff, the pilot visually scanned the engine instrumentation gauge, and did not notice any abnormalities. After a dispersal of the 11th load, the engine started to shake violently. The pilot executed a left turn, and positioned the helicopter into the wind, but the helicopter started to descend, and the engine lost power. The pilot performed an autorotation and the helicopter subsequently impacted the terrain, which resulted in substantial damage to the tailboom. The helicopter was recovered from the accident site for further examination. The helicopter and engine were inspected on October 21, 2015, at the operator's facility. During the examination, a throttle cable bracket mounted on the intake duct was found separated. Three rivets used to secure the bracket were sheared, and the fourth rivet was not located. A longitudinal snubber, used as one of the engine mounts, was sheared. A subsequent microscope examination with a 5 to 50X stereo-zoom microscope of the rivets revealed that all fracture surface features were consistent with overstress. The complete engine examination report is appended to this accident in the public docket. No evidence of pre-impact mechanical malfunctions that would have prevented normal operations were noted.