Summary
This document proposes the adoption of a new airworthiness directive (AD) that is applicable to certain Boeing Model 737, 747, 757, and 767 series airplanes. This proposal would require rework of certain duct assemblies of the environmental control system (ECS) or replacement of the duct assemblies with new or reworked duct assemblies. This action is necessary to prevent potential ignition of fiberglass insulation material installed on the outside of the ECS ducts, which could propagate a small fire and lead to a larger fire. This action is intended to address the identified unsafe condition.