Airworthiness Directives; Air Tractor, Inc. Models AT-501, AT-502, AT-502A, AT-502B, and AT-503A Airplanes

We are adopting a new airworthiness directive (AD) for certain Air Tractor, Inc. Models AT-501, AT-502, AT-502A, AT-502B, and AT-503A airplanes, which supersedes AD 2002- 26-05 and AD 2002-11-05 R1. Since we issued AD 2002-26-05 and AD 2002- 11-05 R1, we determined that additional airplanes should be added to the applicability section and determined the safe life for new production airplanes and replacement spar caps should be extended. We also developed an alternative method of compliance (AMOC) to the requirements of this AD. This AD retains the actions required in AD 2002-26-05 and AD 2002-11-05 R1, adds additional airplanes to the Applicability section, extends the safe life for new production airplanes and replacement spar caps, and incorporates an AMOC. We are issuing this AD to prevent fatigue cracks from occurring in the wing lower spar cap before the established safe life is reached. Fatigue cracks in the wing lower spar cap could result in failure of the spar cap and lead to wing separation.