Robinson Helicopter Company R44 II N7523S accident near Grants, New Mexico

Probable cause

The loss of engine power due to an excessively rich fuel mixture that occurred during operations at idle power settings at a high density altitude. Contributing to the accident was the flight instructor’s poor decision-making in choosing to practice an unnecessary autorotation over mountainous terrain.

NTSB analysis

The helicopter, which had been based at sea level, had recently been sold, and the flight instructor and a commercial pilot were flying it across the country to its new base. While in cruise flight over mountainous terrain at high density altitude, the flight instructor initiated a practice autorotation for the purpose of maintaining proficiency. During the autorotation, the engine stopped producing power. The instructor continued the autorotation to a landing; however, during the landing, the main rotor blades severed the tail boom. A postaccident engine run at high density altitude revealed that the engine ran rough at an idle power setting with a full rich fuel mixture. Leaning the fuel mixture improved engine performance. It is likely that the fuel control unit was set by maintenance personnel to provide a rich mixture at sea level, which resulted in poor engine performance at high density altitudes.

Text from the NTSB. Open the NTSB docket for CEN12LA289 for the full record.