Airworthiness Directives; Boeing Model 747-100, 747-200B, 747-300, and 747SR Series Airplanes

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We are revising an earlier proposed airworthiness directive (AD) for certain Boeing Model 747-100, 747-200B, 747-300, and 747SR series airplanes. The original NPRM proposed to require installation of a closeout panel and moisture curtains for the main equipment center. The original NPRM also proposed to require changing the drain tubes for the power drive units (PDU) and the pitot static tubes and installing larger moisture shrouds. The original NPRM resulted from a report of water contamination in the electrical and electronic units in the main equipment center. This action revises the original NPRM by adding airplanes to the applicability and removing certain others, and removing certain requirements. We are proposing this supplemental NPRM to prevent the malfunction of one or more electrical and electronic units in the main equipment center, which could adversely affect the airplane's continued safe flight.