All helicopters have an inherent vibration. The type and intensity varies as a function of rotor design and isolation systems. Understanding basic vibration levels and being alert to changes can be an important tool for preventing fatal accidents. Difficulty with tracking and balancing the main rotor system is a condition that should raise concern with pilots and mechanics.
Two accidents involving Robinson R22 helicopters, one in Australia in June, 2003 and the other in Israel in February, 2004, involved increasing vibration levels in the main rotor system. In both aircraft, the vibrations were corrected with track and balance only to reappear a short time later. In fact, the accident in Israel happened during one of the track and balance flights. In both cases, investigations revealed that corrosion from water penetration initiated a fatigue crack in the main rotor blades.
More than a year prior to the first accident, Robinson Helicopter released a Service Letter (SL-53) regarding potential development of main rotor blade fatigue cracks when the helicopter is operated under conditions where the loads on the main rotor exceed the design limits. In part the letter stated, “The first indication of a fatigue crack in progress may be a rotor that will not stay balanced after being adjusted.”
Then in July of 2003 Robinson Helicopter issued a R22 Safety Notice again stating that vibrations that reappear after tracking and balancing the main rotor system should be consider suspect.
Safety Notice SN-39
UNUSUAL VIBRATION CAN INDICATE A MAIN ROTOR BLADE CRACK
A catastrophic rotor blade fatigue failure can be averted if pilots and mechanics are alert to early indications of a fatigue crack. Although a crack may be internal to blade structure and not visible, it will likely cause a significant increase in rotor vibration several flight hours prior to final failure. If a rotor is smooth after balancing but then goes out of balance again within a few flights it should be considered suspect.
Knowing this information is important to help pilots and mechanics prevent future accidents.