Airworthiness Directives; Boeing Model 747 Airplanes

The FAA is superseding an existing airworthiness directive (AD), which applies to certain Boeing Model 747 airplanes. That AD currently requires inspections to detect cracks in the front spar pressure bulkhead chord, and repair if necessary. This new AD requires repetitive high frequency eddy current (HFEC) inspections of the body station (BS) 1000 bulkhead chord for cracks, repetitive detailed inspections of the bathtub fittings, if installed, for cracks, and corrective action if necessary. Initiation of the new inspections ends the inspections of the existing AD. This AD also revises the applicability of the existing AD to include additional airplanes. This AD is prompted by reports of cracks in the BS 1000 bulkhead chord. We are issuing this AD to detect and correct fatigue cracks in the BS 1000 bulkhead chord, which, if not repaired before they reach critical length, could result in the failure of the adjacent structure and skin and lead to in-flight depressurization of the airplane.