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

This amendment adopts a new airworthiness directive (AD), applicable to certain Boeing Model 737-100, -200, -300, -400, and -500 series airplanes, that requires installation of a placard that warns the cabin crew not to put the selector valve for the forward lavatory water supply in the ``DRAIN'' position during flight. This amendment also requires installation of an isolation valve in the drain line downstream of the selector valve, which would terminate the requirement for the placard installation. This amendment is prompted by reports of damage to the horizontal stabilizer, and engine flameout caused by ice formed from water drained inadvertently through a mispositioned selector valve. The actions specified by this AD are intended to prevent damage to the engines, airframe, or horizontal stabilizer, and/ or to prevent a hazard to persons or property on the ground, as a result of ice that could dislodge from the airplane.