AD FAA-2011-0593

We are superseding an existing emergency airworthiness directive (EAD) for the specified Schweizer model helicopters that was previously sent to all known U.S. owners and operators. That EAD currently requires removing each locknut and verifying sufficient drag torque and retorquing, or if the locknut does not have sufficient drag torque, replacing the locknut with an airworthy locknut. This AD retains the existing EAD requirements but also requires within a specified time, modifying the expandable bolts and installing a cotter pin. This AD is prompted by a locknut working loose from a bolt attaching the tailboom support strut at the aft cluster fitting because the locknut installed on the expandable bolt did not have the proper threads. We are issuing this AD to modify each expandable bolt to allow adding a cotter pin to prevent the strut and driveshaft separating from the helicopter and subsequent loss of control of the helicopter.