Robinson R44 N488AS accident near Bella Vista, California

Probable cause

A partial loss of engine power during low-altitude maneuvering for reasons that could not be determined because postaccident examinations of the engine and airframe revealed no evidence of any preaccident mechanical malfunctions or failures that would have precluded normal operation.

NTSB analysis

The commercial pilot reported that, while making a left descending turn in the helicopter about 75 to 100 ft above a pond to set up for landing to a field, he heard three "pops," followed by the low rotor horn sounding and the caution light illuminating. The pilot assumed that the engine had lost power, and he immediately and abruptly lowered the collective, pulled aft on the cyclic, and rolled the throttle to full open. To avoid obstructions, he steepened the right turn. Realizing that the rotor rpm was below 50% and that the helicopter would not reach the field, he subsequently landed the helicopter on the pond; it then sank. The pilot was unsure if the engine was running or not.

Postaccident airframe and engine examinations revealed no evidence of any preaccident mechanical failures or malfunctions that would have precluded normal operation. The reason for the partial loss of engine power could not be determined.

Text from the NTSB. Open the NTSB docket for WPR17LA140 for the full record.