Maintenance Work
Two mechanics arrived on the morning of the accident to balance the main rotor blade, as requested by the pilot. They installed balancing equipment and the helicopter was flown on a test flight. The mechanics then performed the necessary adjustments, which required using an adjustable wrench on top of the rotor head to hold a bolt while the nut below was removed and later torqued back on. After completing the work, the men gathered their tools. The pilot asked the lead mechanic if all tools had been collected, and the mechanic replied affirmatively.
Test Hover and Incident
The three occupants—the pilot and the two mechanics—boarded the helicopter for a test hover to verify the adjustments. As the main rotor speed reached 100 percent, they heard a loud bang. While hovering, the mechanics observed that the balancing equipment readings had worsened compared to the first flight, and the helicopter exhibited increased vibrations. The pilot landed the helicopter to allow the mechanics to make further adjustments.
Discovery of Missing Tool
Upon inspection, the mechanics realized that the adjustable wrench used on top of the main rotor head was missing. A subsequent post-accident examination revealed that one main rotor blade, the tail boom, and the lower vertical stabilizer had been damaged by the unaccounted-for wrench.