The mass of a spinning rotor blade produces a strong force that pulls the blade away from the hub. It is known as centrifugal force and the greater the rpm or the higher the blade’s mass, the stronger the force. For helicopter engineers this presents a challenge because while the blade must be sturdily attached to the hub, it also needs the flexibility to change pitch.
Each manufacturer has designed its own method to address this issue. In the 1960s Bell helicopter designed something called a torsion-tension (TT) strap for use on the 206 series helicopter. The straps are made of a durable, elastic material and resemble a pair of elongated rubber bands with reinforced grommets on each end. One end is attached to the hub and the other to the blade. They are very strong in tension and at the same time can twist (torsion) to allow blade pitch change.
In the mid 1970s three 206B accidents were attributed to a TT strap failure where the component ripped apart under the centrifugal forces of the main rotor. In August of 1976 Bell responded by making the straps 1,200-hour life limited components, instead of on condition. Then in 1980, following the fourth failure of a TT strap in a Bell 212 the company added a 24 month retirement life.
Research by Bell showed that all four failures were the result of operations in a highly corrosive environment. Company engineers have been working on a solution to the TT strap problem by testing new materials and corrosion-resistant coatings. However, many customers have complained that the 24 month requirement is too short, increases their operating costs and is not necessary for operators who do not regularly fly near sand and salt water. Nevertheless, Bell Helicopter remains convinced that it is desirable to err on the side of caution, and that considering the factual history it is prudent to maintain the 24-month calendar life retirement until they can engineer a safe replacement.