Airworthiness Directives; The Boeing Company Airplanes

We are adopting a new airworthiness directive (AD) for certain The Boeing Company Model 737-100, -200, -200C, and -300 series airplanes. This AD was prompted by a report from the airplane manufacturer that airplanes were assembled with air distribution ducts in the environmental control system (ECS) wrapped with Boeing Material Specification (BMS) 8-39 or Aeronautical Materials Specifications (AMS) 3570 polyurethane foam insulation, a material with fire-retardant properties that deteriorate with age. This AD requires reworking certain air distribution ducts in the ECS. We are issuing this AD to prevent ignition of the BMS 8-39 or AMS 3570 polyurethane foam insulation on the duct assemblies of the ECS due to a potential electrical arc, which could start a small fire and lead to a larger fire that may spread throughout the airplane through the ECS.