CHI04FA177 · Robinson R-22 Beta

A helicopter piloted by a student pilot and a flight instructor was destroyed when it impacted terrain during a landing approach. A witness reported that the helicopter appeared to be making a normal approach toward the helicopter pad. He reported the helicopter was about 100 feet above the ground and going about 30 knots. When he looked at the helicopter again, he saw it "bobble" twice (yawing motion). Within 2-3 seconds, the helicopter rolled, "fell out of the sky" and impacted the ground. The flight instructor reported he could not remember much about the accident. He reported that the student pilot was at the controls and they were flying a right downwind approach to the helicopter pad. He reported that he did not think there was anything mechanically wrong with the helicopter, and did not remember anything being out of the ordinary. The inspection of the helicopter revealed that here were no ground impact marks other than the ground scars immediately below the main wreckage of the helicopter. The helicopter came to rest on its right side in a nose down attitude. The main rotor blades had severed the tailcone. The tail rotor drive shaft was found in one piece about 150 feet from the main wreckage with a 90-degree bend in it. The post accident inspection of the helicopter's airframe and engine revealed no preexisting anomalies that could be associated with a pre-impact condition. The Robinson Helicopter Company issued Safety Notice SN-24 in September 1986 with a revision in June 1994. The safety notice stated the following warning: LOW RPM ROTOR STALL CAN BE FATAL. The safety notice stated, "Rotor stall … can occur at any airspeed and when it does, the rotor stops producing the lift required to support the helicopter and the aircraft literally falls out of the sky." The safety notice explained, "As with the airplane wing, the blade airfoil will stall at a critical angle, resulting in a sudden loss of lift and a large increase in drag. The increased drag on the blades acts like a huge rotor brake causing the rotor RPM to rapidly decrease, further increasing the rotor stall. As the helicopter begins to fall, the upward rushing air continues to increase the angle-of-attack on the slowly rotating blades, making recovery virtually impossible, even with down collective."

Verbatim NTSB analysis and probable cause from the NTSB dataset