Overview
A helicopter engaged in aerial pesticide application suffered a mechanical failure during a routine pass. The pilot heard a loud pop, followed by oscillations in the collective control and severe airframe vibrations. The aircraft began to oscillate, prompting the pilot to decelerate to approximately 15 knots and attempt an emergency landing.
Sequence of Events
During the landing attempt, a spray boom attached to the helicopter contacted the ground, causing the helicopter to roll onto its left side. The pilot slowed the aircraft to about 15 knots before the boom struck the terrain.
Examination Findings
Post-accident examination revealed that the tension-torsion bar pin, which connects the main rotor blade drag brace, had failed due to fretting corrosion and fatigue. A second tension-torsion pin from a sister helicopter was also found to exhibit corrosion and a fatigue crack. The failed pins had accumulated 369.5 hours and 489.4 hours of service, respectively, while the retirement life for the pin is specified at 643 hours. The presence of fretting corrosion and fatigue was identified as the underlying cause of the pin failures.