Airbus Helicopters AS350 N1WT incident near Clewiston, Florida

Probable cause

Seizure of the hydraulic pump pulley bearing due to its improper installation by the manufacturer, leading to a failure of the hydraulic pump belt and the resultant partial loss of hydraulic assistance to the pedals.

NTSB analysis

The No. 2 hydraulic system (HYD2) pulley bearing likely failed due to being installed skewed onto its pulley, resulting in abnormal bearing loads which led to its premature failure. The plastic deformation observed on the raceways and rolling elements, as well as contact damage on the bearing cage, were consistent with sliding of the rolling elements against the raceways and cage due to abnormal loading and forced rotation of a seized bearing.

The seizure of the HYD2 pulley bearing led to the deterioration of the drive belt, resulting in its failure and the subsequent loss of hydraulic pressure in HYD2. Loss of hydraulic assistance from HYD2, which included the tail rotor servo control, resulted in higher pedal forces to control the helicopter in the yaw axis. It is likely the HYD2 pulley bearing did not show obvious visual evidence of deterioration during past inspections, including the last scheduled inspection about 13 flight hours prior to the incident, and visual detection of the deteriorating bearing may not have been possible without removal of the bearing from the pulley.

Text from the NTSB. Open the NTSB docket for ENG20LA031 for the full record.