Summary
This document proposes the supersedure of an existing airworthiness directive (AD), applicable to certain Boeing Model 747-200B, -300, -400, -400D, and -400F series airplanes, that currently requires repetitive inspections to detect cracking of fire extinguisher discharge tubes in certain engine struts, and corrective action, if necessary. For certain airplanes, that AD also provides for an optional modification of the fire extinguisher discharge tubes, which constitutes terminating action for the repetitive inspections. This action would make the previously optional modification of the fire extinguisher discharge tubes mandatory for all affected airplanes and would add one airplane to the applicability. This proposal is prompted by a report that the check tee valve at the top of an engine strut can be damaged such that no extinguishing agent can get to the engine. The actions specified by the proposed AD are intended to prevent blockage of the check tee valve and cracks in the fire extinguisher discharge tubes in the engine struts, which could prevent the fire extinguishing agent from being delivered to the engine or reduce the amount delivered to the engine, which could permit a fire to spread from the engine to the wing of the airplane.