Summary
This document revises an earlier proposed airworthiness directive (AD), applicable to certain Boeing Model 747-400, -400D, and -400F series airplanes; equipped with GE or P&W series engines, that would have required modifications and functional tests of the wiring of the wire integration unit and the air supply control test unit (ASCTU) of the engine bleed air distribution system. This new action revises the proposed rule by adding a new requirement. The actions specified by this new proposed AD are intended to prevent inadvertent commanded shutdown of the engine bleed air distribution systems due to an erroneous ASCTU command. That shutdown could cause depressurization of the airplane and subsequent ice build-up on the engine inlets during descent, which could result in ingestion of ice into the engine(s) and consequent loss of thrust on one or more engines. This action is intended to address the identified unsafe condition.