Accident Overview
A helicopter being operated by a student pilot and a flight instructor was destroyed when it impacted terrain during a landing approach. The helicopter came to rest on its right side in a nose-down attitude. The main rotor blades severed the tailcone, and the tail rotor drive shaft was found in one piece about 150 feet from the main wreckage with a 90-degree bend.
Witness Account
A witness reported that the helicopter appeared to be making a normal approach toward the helicopter pad. The witness stated the helicopter was approximately 100 feet above the ground and traveling at about 30 knots. When the witness looked again, the helicopter was seen to "bobble" twice, described as a yawing motion. Within 2 to 3 seconds, the helicopter rolled, "fell out of the sky," and impacted the ground.
Pilot Statements
The flight instructor reported that he could not remember much about the accident. He stated that the student pilot was at the controls and they were flying a right downwind approach to the helicopter pad. The instructor indicated that he did not think there was anything mechanically wrong with the helicopter and did not recall anything being out of the ordinary.
Wreckage and Inspection
Inspection of the wreckage site revealed no ground impact marks other than the ground scars immediately below the main wreckage. A post-accident inspection of the helicopter's airframe and engine found no preexisting anomalies that could be associated with a pre-impact condition.
Safety Notice
The Robinson Helicopter Company issued Safety Notice SN-24 in September 1986, with a revision in June 1994. The notice included the warning: "LOW RPM ROTOR STALL CAN BE FATAL." It stated that 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 notice explained that, as with an 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 acts like a huge rotor brake, causing rotor RPM to rapidly decrease and 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.