Summary
The FAA is revising an earlier proposed airworthiness directive (AD) for certain Boeing Model 747 airplanes. The original NPRM would have required repetitive tests to detect hot air leaking from the trim air diffuser ducts or sidewall riser duct assemblies (collectively referred to in this proposed AD as "TADDs"), related investigative actions, and corrective actions if necessary. The original NPRM also would have provided an optional terminating action for the repetitive tests. The original NPRM resulted from reports of sealant deteriorating on the outside of the center wing fuel tank and analysis that sealant may deteriorate inside the tank due to excess heat from leaking TADDs. This action revises the original NPRM by referring to improved inspection procedures and extending the repetitive interval for certain related investigative actions. We are proposing this supplemental NPRM to prevent leakage of fuel or fuel vapors into areas where ignition sources may be present, which could result in a fire or explosion.