Primary finding
Probable cause
The mechanics' failure to account for all the tools following a maintenance procedure and the pilot's failure to perform a preflight check before a maintenance test flight.
Investigator assessment
Analysis narrative
The pilot of the helicopter told the lead mechanic that the main rotor blade needed to be balanced. Two mechanics showed up on the morning of the accident and installed the balancing equipment. A test flight was flown. The two men performed the necessary work, which required a second adjustable wrench on the top of the rotor head to hold the bolt while the nut below was removed and later torqued back on. When the work was finished, the men gathered their tools. The pilot asked the lead mechanic if they had gathered all their tools and the mechanic replied "yes." The three men got on the helicopter for a test hover to check their work. As the main rotor speed reach 100 percent, the three occupants heard a "bang." As the helicopter hovered, the two mechanics thought the balancing equipment readings were worse than the first flight, and the vibrations in the aircraft had increased. The pilot landed the helicopter so that the mechanics could make additional adjustments. The mechanics discovered that they were missing the adjustable wrench that had been used on the top of the main rotor head. Postaccident examination revealed that one main rotor blade, the tail boom, and the lower vertical stabilizer had received damage from the unaccounted for wrench.
Source record
Factual narrative
The pilot of the helicopter told the lead mechanic that the main rotor blade needed to be balanced. Two mechanics showed up on the morning of the accident and installed the balancing equipment. A test flight was flown. The two men performed the necessary work, which required a second adjustable wrench on the top of the rotor head to hold the bolt while the nut below was removed and later torqued back on. When the work was finished, the men gathered their tools. The pilot asked the lead mechanic if they had gathered all their tools and the mechanic replied "yes." The three men got on the helicopter for a test hover to check their work. As the main rotor speed reach 100 percent, the three occupants heard a "bang." As the helicopter hovered, the two mechanics thought the balancing equipment readings were worse than the first flight, and the vibrations in the aircraft had increased. The pilot landed the helicopter so the mechanics could make additional adjustments. The mechanics discovered that they were missing the adjustable wrench that had been used on the top of the main rotor head. Post accident examination revealed that one main rotor blade, the tail boom, and the lower vertical stabilizer had received damage from the unaccounted for wrench.