Summary
We propose to adopt a new airworthiness directive (AD) for all The Boeing Company Model 737-400 series airplanes. This proposed AD was prompted by reports of cracks in the upper chord of the overwing stub beams at body station (STA) 578 emanating from the rivet location common to the crease beam inner chord and the overwing stub beam upper chord. This proposed AD would require repetitive inspections for cracking, and related investigative and corrective actions if necessary. Replacement of the overwing stub beam would terminate the repetitive inspections for cracking at the replacement location only, and post-replacement inspections would be required if the replacement was done. We are proposing this AD to detect and correct cracking in the upper chord of the overwing stub beam caused by high flight cycle fatigue stresses from both pressurization and maneuver loads. Cracking of the overwing stub beam could adversely affect the fuselage structural integrity and result in possible decompression of the airplane.