Airworthiness Directives; Boeing Model 737-100, -200, -300, -400, and -500 Series Airplanes

This amendment supersedes an existing airworthiness directive (AD), applicable to certain Boeing Model 737-300 and -400 series airplanes, that currently requires either repetitive leak checks on the forward lavatory service system and repair, as necessary, or draining of the system and placarding the lavatory inoperative. This amendment also requires periodic changing of the seals of certain lavatory drain systems; replacing "donut valves" with other FAA-approved valves; revising certain leak test intervals; and revising the pressurization and fluid level requirements for testing. The actions specified by this AD are intended to prevent damage to engines, airframes, and property on the ground that is associated with the problems of "blue ice" that forms from leaking lavatory drain systems on transport category airplanes and subsequently dislodges from the airplane fuselage.