Eurocopter AS350B2 N1231A accident near Pacoima, California

Probable cause

The contamination of a spool bearing in the power turbine governor during manufacture, which led to the bearing’s failure in flight, a subsequent loss of engine power during departure, and the resultant hard landing.

NTSB analysis

During a downwind departure from the airport, the engine began to lose power. The pilot initiated an immediate autorotation back toward the airport while adjusting the airspeed and flight controls to clear power lines along the airport boundary. After clearing the power lines, the helicopter settled hard on the runway centerline and then began to rock, slide, and yaw right. The pilot attempted to regain control of the helicopter, and it eventually came to a stop upright on its skids facing about 160 degrees right of the runway centerline. The aft portion of the tail boom had separated.

Postaccident testing and examination of the engine revealed a degraded power turbine (PT) governor spool bearing. Examinations also revealed the presence of diamond particles, which are used in the manufacturing process for the ball bearings, in the governor spool bearing. It is likely that diamond particle contamination caused the governor spool bearing to fail, which rendered the PT governor incapable of modulating governor servo pressure to the fuel control unit. Although no physical evidence was found indicating that the electronic overspeed system activated, the pilot's statement about two increases in rpm and then a decrease in rpm indicates that the system likely activated. The loss of PT governing ability coupled with the activation of the electronic overspeed system likely resulted in fuel flow oscillations that decreased the fuel flow below the lean blowout limit of the engine and led to the loss of engine power.

Text from the NTSB. Open the NTSB docket for WPR11FA045 for the full record.