Summary
We propose to supersede Airworthiness Directive (AD) 2005-20- 01, which applies to all The Boeing Company Model 737-100, -200, -200C, -300, -400, and -500 series airplanes. AD 2005-20-01 requires repetitive inspections of the vertical stiffeners at left buttock line (LBL) and right buttock line (RBL) 6.15 for cracks; and replacement of both stiffeners with new, improved stiffeners if any stiffener is found cracked. Since we issued AD 2005-20-01, we have received reports of cracks found in the left and right side keel beam upper chords when replacing vertical stiffeners. In addition, we have determined that the replacement stiffener installation degraded the fault current bonding path and could introduce an ignition source in the fuel tank in the event of an electrical hot short or a lightning strike. This proposed AD would require, depending on airplane configuration, replacing the vertical stiffeners at LBL and RBL 6.15 on the rear spar of the wing center section, installingangle and bonding jumpers, installing brackets, applying sealant, and applying paint. We are proposing this AD to address the unsafe condition on these products.