ERA23LA205 · Schweizer 269C UNDESIGNAT

After the instructional flight was airborne for slightly more than an hour, the helicopter began descending slightly followed by repetitive low main rotor rpm warnings. The student corrected after the first two warnings, but did not correct after the third. The flight instructor took over the controls and rolled the throttle slightly off, then opened it all the way to the stop, recovering the engine rpm. The flight instructor then initiated a precautionary landing and turned toward an open field. During the descent, when operating in the shaded area of the height velocity diagram (between 100 and 200 ft above the ground and 40 knots airspeed ), the low main rotor rpm again sounded. Knowing the helicopter was flying in the shaded area of the height velocity diagram, the flight instructor nosed the helicopter over to increase airspeed and entered an autorotation. About 40 to 50 ft above ground level (agl), he started a slight deceleration though the helicopter was in a near-vertical descent. He leveled the helicopter at a height of about 10 to 20 ft agl while rapidly approaching the ground. He applied collective to cushion the landing but landed hard, damaging all main rotor blades and the tail boom. Following the accident, the engine was started twice and operated to partial power (due to the main rotor blade damage), with no anomalies noted. The reason for the reported loss of engine power could not be determined from the available evidence.

Verbatim NTSB analysis and probable cause from the NTSB dataset