Hiller UH-12B N93467 accident near Pasco, Washington

Probable cause

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

NTSB analysis

The pilot of the helicopter was maneuvering at a low altitude over a fruit orchard to dry the trees. After completing one section of the orchard, he climbed higher in order to check for obstacles near the adjacent orchard that he planned to dry next. While descending into position to work the adjacent orchard, the pilot noticed that the main rotor rpm was beginning to decay. Although he added some power, the rpm decay continued. He therefore made the decision to lower the collective, but realized he was too low to do so. Because he still had some forward airspeed and was able to maintain a controlled descent, the pilot maneuvered to land in an opening between the trees; however, the helicopter impacted trees just before it touched down. A postaccident examination of the engine and airframe revealed no evidence of mechanical malfunctions or failures that would have precluded normal operation.

Text from the NTSB. Open the NTSB docket for WPR12LA244 for the full record.