Eurocopter AS 350 B2 N217LA accident near Lancaster, California

Probable cause

The pilots’ decision to conduct a practice autorotation at low altitude in high density altitude conditions and their failure to attain and maintain an appropriate descent rate. Contributing to the accident was the high density altitude and the pilots’ failure to comply with Los Angeles Police Department guidance.

NTSB analysis

Due to inclement weather at their normal training airports, the commercial pilot and flight instructor chose to practice maneuvers for an upcoming proficiency flight at an airport in the high desert that had clear weather conditions. While en route to that airport, the pilots discussed the “4H’s - High, Heavy, Hot, and Humid” and their effects on performance. The pilots intended to initiate all maneuvers about 500 feet above ground level (agl). This was contrary to the Los Angeles Police Department instructor's guide, which specifies that such maneuvers be initiated 700 feet agl. After performing several training maneuvers with no incident, the pilots performed two autorotations. During the first autorotation, the approach became unstabilized, and the flight instructor initiated a power recovery. The flight instructor reported that during the second autorotation, the helicopter developed a high sink rate and was pushed away from the intended landing spot by the wind. The flight instructor said that he again initiated a power recovery, but the engine did not seem to be producing the necessary power, and the helicopter landed hard.

Postaccident examination revealed no evidence of mechanical malfunctions or failures that would have precluded normal operation. Downloaded data from a recording device on the helicopter revealed that the accident autorotation was initiated about 300 feet agl and resulted in an average descent rate of 1,747 feet per minute during the 8 seconds before the helicopter struck the ground. The calculated density altitude at the time of the accident was 5,214 feet; this condition would have made the engine less responsive, which would have increased the rate of descent. If the maneuver had been initiated at the company-approved altitude of 700 feet, the pilot might have had sufficient altitude to recover from the maneuver in the existing conditions.

Text from the NTSB. Open the NTSB docket for WPR09GA397 for the full record.