Summary
This document revises an earlier proposed airworthiness directive (AD), applicable to certain Boeing Model 747 series airplanes, that would have required repetitive inspections for cracking of the skin, bear strap, and sill chord of the lower lobe cargo door cutout, and repair, if necessary. For certain airplanes, the proposed AD also would have provided an optional modification of the lower lobe cargo door cutout, which would have ended the pre-modification repetitive inspections, but would have necessitated new post-modification repetitive inspections after a certain time. This new action, prompted by new crack findings, revises the proposed rule by expanding the optional modification of the lower lobe cargo door cutout and reducing the compliance threshold for the post-modification inspections. The proposed actions are necessary to find and fix cracking of the skin, bear strap, and sill chord of the lower lobe cargo door cutout, which could lead to reduced structural integrity of the lower lobe cargo door cutout, and result in rapid depressurization of the airplane. This action is intended to address the identified unsafe condition.