The FAA is revising an existing airworthiness directive (AD) that applies to all Boeing Model 747 series airplanes. That AD currently requires the implementation of a corrosion prevention and control program. We issued that AD to prevent degradation of the structural capabilities of the affected airplanes. This new AD clarifies the applicability of the existing AD by specifying which Boeing Model 747 airplanes are affected by this AD because we have determined that certain new variants that have not yet been certified will not be subject to the requirements of this AD. This AD results from reports of incidents involving corrosion and cracking in transport category airplanes, which have jeopardized the airworthiness of the affected airplanes. We are issuing this AD to prevent degradation of the structural capabilities of all Boeing Model 747-100, 747-100B, 747- 100B SUD, 747-200B, 747-200C, 747-200F, 747-300, 747-400, 747-400D, 747-400F, 747SR, and 747SP series airplanes.