Summary
We propose to adopt a new airworthiness directive (AD) for certain Boeing Model 737-300, -400, and -500 series airplanes. This proposed AD would require repetitive high frequency eddy current (HFEC) inspections for cracking of the 1.04-inch nominal diameter wire penetration hole in the frame and frame reinforcement, between stringers S-20 and S-21, on both the left and right sides of the airplane, and related investigative/ corrective actions if necessary. This proposed AD results from reports of cracking in the frame, or in the frame and frame reinforcement, common to the 1.04-inch nominal diameter wire penetration hole intended for wire routing. We are proposing this AD to detect and correct cracking in the fuselage frames and frame reinforcements, which could reduce the structural capability of the frames to sustain limit loads, and result in cracking in the fuselage skin and subsequent rapid depressurization of the airplane.