NTSB Factual Report

On June 4, 2012, about 0645 Pacific daylight time, a Hiller UH-12B helicopter, N93467, impacted fruit trees in an orchard near Pasco, Washington. The commercial pilot, who was the sole occupant, was not injured, but the helicopter, which was being operated by Larsen Helicopter Operations, sustained substantial damage. The 14 Code of Federal Regulations Part 91 fruit orchard drying operation was being performed in visual meteorological conditions. The pilot departed Tri-Cities Airport, Pasco, Washington, about 0615. No flight plan had been filed.

According to the pilot, who had just completed drying one section of the orchard with the air blast from his main rotor blades, he ascended to about 100 to 150 feet above the trees in order to look for any obstacles around a separate section of the orchard that he was going to dry next. He then started to descend to the low level that he would be working at over the trees. During that descent, he realized that the helicopter's main rotor rpm, which had been in the lower part of the normal operating range at the beginning of the descent, had begun to decrease. He therefore began to roll on the throttle in order to compensate, but the rpm continued to decay. He then decided to lower the collective in an attempt to recover the rotor rpm, but by then he was about 20 feet above the trees. He therefore rolled the throttle to full power, and tried to pump the collective, but the rpm continued to decay. Because he still had some forward airspeed and enough rotor rpm to keep the helicopter in a controlled slow descent, the pilot maintained a level attitude and maneuvered the helicopter to a position between two rows of trees, where it settled to the terrain in the upright position. Just before the helicopter settled to the terrain, both its main rotor and the tail rotor came in contact with the fruit trees.

After an initial examination at the accident site by a Federal Aviation Administration Inspector, the helicopter was taken to the facilities of AvTech Services, in Maple Valley, Washington. There, both the engine and the airframe were examined by an NTSB Aviation Safety Investigator. That examination confirmed spark at all spark plug leads, compression in all cylinders, gear continuity to the accessory section, and normal wear of the spark plug electrodes. In addition, after disassembly, the carburetor bowl was determined to contain a small amount of fuel consistent with aviation low lead, and except for a small sliver of metal in its inlet finger screen, the carburetor showed no evidence of preimpact anomalies or malfunction. The oil filter contained no metal or foreign debris, and the internal gearing appeared to be well lubricated, with no visible sign of thermal distress.

The engine could not be test run due to impact damage to the crankcase, but at the time of the completion of the examination, no evidence of any preimpact malfunction associated with the airframe or engine had been found. And, although the pilot reported that during the engine start process it appeared that there may not have been sufficient voltage coming from the battery, he did not report any unusual engine sounds or engine performance irregularities at any time during the flight.

NTSB Probable Cause

The pilot's failure to maintain the helicopter's rotor rpm while maneuvering at a low altitude over a fruit orchard.