Bell UH-1E N333XL accident near Gustine, California

Probable cause

Failure of the 90° gearbox due to the lack of lubricating oil, which resulted in a loss of rotational drive to the tail rotor and a subsequent loss of control of the helicopter.

NTSB analysis

The pilot was conducting aerial spray applications of a chemical to a field. He had checked the oil quantity of the 90° gearbox during the preflight check but did not recall the quantity of oil at the time. The pilot had operated the helicopter for about 45 minutes and, while making a turn, lost control of the helicopter. The helicopter descended and impacted terrain.

Maintenance records revealed no anomalies. Postaccident examination of the 90° gearbox revealed severe damage to the internal components and no evidence of lubricating oil inside the housing. The pinion was severely deformed, which resulted in the loss of rotational drive to the tail rotor assembly and the subsequent loss of control.

Examination of the oil level sight glass revealed that the inside was covered with a material that did not allow for sufficient viewing of the oil level. It is likely that, when the pilot looked at the oil level sight glass during his preflight check, he interpreted what he saw as an adequate amount of oil.

Text from the NTSB. Open the NTSB docket for WPR22LA084 for the full record.