Summary
The FAA proposes to supersede an existing airworthiness directive (AD) that applies to certain Boeing Model 747 series airplanes. The existing AD requires a one-time inspection of the potable water and drain lines in the cargo compartments for indications of overheating of the heater tape, exposed foam insulation, missing or damaged protective tape, or debris around the potable water fill and drain lines; and corrective action, if necessary. This proposed AD would require that the inspection of the water and drain lines be repetitively performed, using new service information, until new ribbon heaters are installed, which would terminate the repetitive inspections. This AD would also remove certain airplanes from the applicability. This proposed AD results from a report of a fire in the aft cargo compartment started by a potable water line heater tape. We are proposing this AD to prevent overheating of the heater tape on potable water fill and drain lines, which could ignite accumulated debris or contaminants on or near the potable water fill and drain lines, resulting in a fire in the airplane.