Robinson R22 BETA N7779M accident near Del Valle, Texas

Probable cause

The divergence of the main rotor from its normal plane of rotation for reasons that cannot be determined because the pilot’s control inputs leading up to the event are unknown, resulting in mast bumping and main rotor blade separation.

NTSB analysis

The helicopter was flying across a river when witnesses heard a loud "pop" or "bang" and saw something fall from the helicopter into the river. They heard the helicopter crash, saw smoke, and ran to the crash site. One main rotor blade was found approximately 780 feet from the wreckage. The other main rotor blade was found approximately 975 feet from the wreckage in the opposite direction. Both blades had separated from the main rotor hub at their coning hinges. All components of the helicopter were accounted for with the exception of portions of both coning bolts. The investigation was unable to locate the object observed falling from the helicopter or determine what it was. Examination of the rotor head components, including the recovered portions of the coning bolts, revealed fracture features consistent with overload failure and mechanical damage indicative of mast bumping (contact between the main rotor blades and the mast). The main rotor blade pitch links and attaching hardware displayed compression buckling and bending overload features consistent with an out-of-control main rotor that had diverged from its normal plane of rotation. The helicopter manufacturer reported that the damage signatures were consistent with both low-G mast bumping and low rotor rpm rotor stall. Low-G mast bumping is a phenomenon specific to two-bladed teetering rotor systems in which the rotor disc becomes unloaded, typically due to a cyclic pushover. Low rotor rpm rotor stall is a different phenomenon in which the main rotor blades experience an aerodynamic stall due to low rotor rpm caused either by the pilot raising the collective too much or by a loss of engine power followed by the pilot lowering the collective too slowly. Rotor blade stall can lead to mast bumping. The pilot’s control inputs leading up to the event are unknown, and, therefore, the reason for the main rotor divergence could not be determined.

Text from the NTSB. Open the NTSB docket for CEN11FA422 for the full record.