N90270Hughes 269A · 1966

Prior to the accident flight, the pilot reported experiencing vibrations in the helicopter. The pilot (who was also a mechanic) and another mechanic adjusted the main rotor dampers to address the issue. Following this adjustment, a 10-minute ground run was performed without any observed problems. The pilot then conducted a test flight during, which no issues were detected. Upon landing and while reducing the rotor rpm, the helicopter began to shake violently. The pilot attempted to perform the ground resonance recovery procedure and climbed the helicopter, but the vibration worsened, and he subsequently landed. After contacting the ground, the helicopter shook and spun uncontrollably before coming to a stop. The airframe and main rotor were substantially damaged during the accident sequence.

Federal Aviation Administration inspectors examined the helicopter after the accident and found that the yellow main rotor blade’s damper had significantly higher torque than the red and blue blades, and that none of the dampers were torqued to the specification in the helicopter’s maintenance manual. The manual also described that incorrect torque adjustments of the dampers could result in “…conditions that may result in ground resonance and destruction of the helicopter. During a subsequent discussion with the assisting mechanic, he stated that he, “may have unintentionally over-torqued the blade [damper].” Based on this information, it is likely that the mechanics’ improper maintenance of the helicopter’s main rotor dampers resulted in the ground resonance event experienced at the conclusion of the post maintenance test flight test flight.