Accident Sequence
During a routine maneuver, the pilot initiated a right turn, then felt and heard a thump. He lowered the collective and attempted to straighten the turn, but experienced a strong vibration and the nose of the aircraft pitched down. A ground crewman who witnessed the accident reported that while the helicopter was about 35 feet above ground level, a large piece of the tail and a rotor blade separated from the aircraft, followed by the tail rotor hub assembly, the remaining blade, and the 90-degree gearbox. The helicopter then began a 360-degree right spiral, completed two and a half rotations, and struck the ground in a 30-degree nose-down attitude. The witness noted that the engine appeared to still be producing about 60 percent power at impact.
Investigation Findings
The tail and tail rotor gearbox were recovered approximately 125 feet from the main fuselage. Metallurgical examination of the vertical tail fin, tail rotor gearbox, and tail rotor blades revealed an area of chafing that had worn through the vertical fin's spar, initiating a fatigue crack. The fatigue crack originated in an area where the vertical fin driveshaft cover was fastened on the left side of the aircraft near the upper end. Chafing had occurred between the cover and the spar, gradually wearing away the spar material.
Maintenance Requirements
According to Bell, the manufacturer of the UH-1B, the helicopter requires a daily inspection to visually check the area of the upper vertical fin, as specified in the Aviation Unit Technical Manual. This inspection is intended to detect potential fatigue cracking in the area where the crack occurred in this accident.