Accident Description
During an agricultural application flight, the helicopter was operating at a slow airspeed approximately 10 feet above the ground. The pilot reported a severe vibration throughout the airframe and decided to land immediately. While turning toward a landing area, the vibration suddenly ceased. The helicopter then fell to the ground on its nose and rolled over onto its top.
Examination Findings
One main rotor blade and half of the rotor hub were located more than 100 yards from the wreckage. The second blade remained attached to the remainder of the hub, found about 150 feet away. The separated pieces of the fractured main rotor hub were recovered and subjected to metallurgical examination.
Metallurgical Analysis
The hub fractured through its center. The fracture originated from fatigue cracks that began at the tension-torsion (TT) pin hole on the leading edge side of one hub arm. The starting point of the fatigue crack was determined to be inside the TT pin hole, where corrosion deposits typical of aluminum alloys were present. The crack would not have been visible during an external inspection.
Inspection Requirements
The fatigue crack and subsequent fracture occurred in areas not covered by the recurring inspection requirements of Airworthiness Directive (AD) 73-20-03.