On August 14, 1990, a Bell UH-1B (registration N64789) operated by Intermountain Air Services was involved in an aviation accident near Bieber, CA. Investigators recorded the probable cause as: THE PILOT'S FAILURE TO MAINTAIN AIRCRAFT CONTROL WHEN INTENTIONALLY SLOWING HIS AIRCRAFT TO OFF LOAD WATER ON FIRE. CONTRIBUTING TO THE ACCIDENT WAS THE PILOT'S FAILURE TO ARM AND RELEASE THE EXTERNAL LOAD. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
While approaching ascending terrain to drop water, a firefighting helicopter lost altitude and settled into trees after a left tail turn and low rotor rpm warning.
Accident Details
The helicopter was engaged in a firefighting mission and approached southerly ascending terrain from the east. As the aircraft neared the terrain, it slowed to execute a water drop. During this maneuver, the tail turned to the left and the helicopter lost altitude, subsequently settling into the trees. The ambient temperature was reported to be between 82 and 86 degrees Fahrenheit, with wind from the north at 5 to 8 knots.
Pilot Actions
Upon encountering the loss of altitude, the pilot heard an audio warning of low rotor rpm and observed the corresponding warning light. He then attempted to release the water load.
Investigation Findings
An examination of the engine and its components revealed no evidence of a malfunction or failure. The investigation did not identify a specific mechanical cause for the accident.