Airworthiness Directives; Thrush Aircraft, Inc. Model 600 S2D and S2R Series Airplanes

We are adopting a new airworthiness directive (AD) to supersede AD (AD) 2006-07-15, which applies to Thrush Aircraft, Inc. Model 600 S2D and S2R (S-2R) series airplanes (type certificate previously held by Quality Aerospace, Inc. and Ayres Corporation). AD 2006-07-15 currently requires repetitive inspections of the 1/4-inch and 5/16-inch bolt hole areas on the wing front lower spar caps for fatigue cracking; replacement or repair of any wing front lower spar cap where fatigue cracks are found; and reporting of any fatigue cracks found to the FAA. AD 2006-07-15 also puts the affected airplanes into groups for compliance time and applicability purposes. Since we issued AD 2006-07-15, FAA analysis reveals that inspections are not detecting all existing cracks and shows the incidences of undetected cracks will increase as the airplanes age. Consequently, this AD retains the actions of AD 2006-07-15 and imposes a life limit on the wing front lower spar caps that requires replacement of the wing front lower spar caps when the life limit is reached. This AD also changes the requirements and applicability of the groups discussed above and removes the ultrasonic inspection method. We are issuing this AD to prevent wing front lower spar cap failure caused by undetected fatigue cracks. Such failure could result in loss of a wing in flight.