AD NUMBER: 2019-16-12

PRODUCT: All The Boeing Company Model 737-100, -200, -200C, -300, -400, and -500 series airplanes.

ACTION: Final Rule.

SUMMARY: The FAA is superseding Airworthiness Directive (AD) 2005-20-01, which applied to the aircraft listed above.

AD 2005-20-01 required 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.

This new AD requires, depending on airplane configuration, replacing the vertical stiffeners at LBL and RBL 6.15 on the rear spar of the wing center section, installing angle and bonding jumpers, installing brackets, applying sealant, and applying paint.

This AD was prompted by reports of cracks found in the left and right side keel beam upper chords when replacing vertical stiffeners.

This AD was also prompted by possible degradation of the fault current bonding path that could introduce an ignition source in the fuel tank in the event of a fault current being imparted onto the fuel tank structure.

DATES: This AD is effective October 9, 2019.

COST: The FAA estimates that this AD affects 171 airplanes of U.S. registry. Operators may incur the following costs in order to comply with this AD:

• Stiffener replacement, angle and bonding jumper installation, bracket installation, and sealant and paint application: Up to 257 work-hours × $85 per hour = Up to $21,845 per airplane. Parts: $14,730. Total cost per airplane: Up to $36,575.

FMI: AD