Airworthiness Directives; Boeing Model 747SP, 747SR, 747-100, 747-200, and 747-300 Series Airplanes; Equipped With Pratt & Whitney Model JT9D-3, -7, and -7Q Series Engines and Model JT9D-7R4G2 Engines

This amendment supersedes an existing airworthiness directive (AD), applicable to certain Boeing Model 747SP, 747SR, 747-100, 747- 200, and 747-300 series airplanes, that currently requires repetitive operational tests of the reversible gearbox pneumatic drive unit (PDU) or the reversing air motor PDU to ensure that the unit can restrain the thrust reverser sleeve, and correction of any discrepancy found. This amendment requires installation of a terminating modification, repetitive functional tests of that installation to detect discrepancies, and repair if necessary. This amendment also removes airplanes from the applicability and adds certain new requirements. The actions specified by this AD are intended to ensure the integrity of the fail-safe features of the thrust reverser system by preventing possible failure modes in the thrust reverser control system that can result in inadvertent deployment of a thrust reverser during flight. This action is intended to address the identified unsafe condition.