Mcdonnell Douglas Helicopter 500N N521HB accident near Los Alamitos, California

Probable cause

The flight instructor's delayed remedial action during the instructional autorotative landing, which resulted in a failure to maintain an adequate rotor rpm to avoid a hard landing.

NTSB analysis

The certified flight instructor (CFI) and the pilot he was instructing were practicing emergency procedures, in particular, power recovery autorotations. At 700 feet above ground level (agl), the CFI rolled the throttle off (idle) and entered the autorotation. At 50 feet agl he rolled the throttle on late, realized the ground was rapidly approaching, and decided to perform a running landing. The pilot reported that he inadvertently applied partial throttle, which did not allow the engine to regain full power. With the throttle positioned at 25 percent, the rotor rpm decayed, and when the helicopter contacted the ground, a rotor blade dipped down and struck the tail boom. The pilot stated that the helicopter and engine had no mechanical failures or malfunctions during the flight.

Text from the NTSB. Open the NTSB docket for WPR10TA025 for the full record.