Airworthiness Directives; Boeing Model 757 Series Airplanes

This amendment adopts a new airworthiness directive (AD) that is applicable to all Boeing Model 757 series airplanes. This action requires repetitive inspections to detect damage of the tubes of the fire extinguishing and smoke detection systems, and duct support brackets of the auxiliary power unit (APU); and corrective actions, if necessary. This amendment is prompted by reports of incidents in which the tubes of the fire extinguishing and smoke detection systems chafed against the stiffener rings and support brackets of the pneumatic duct of the APU. The actions specified in this AD are intended to detect and correct such chafing, which could result in a hole in the tube of the fire extinguishing system and consequently, could prevent the proper distribution of the fire extinguishing agent within the aft cargo compartment in the event of a fire. Such chafing also could result in a hole in the smoke detection system, which could result in the delay of detection of a fire in the aft cargo compartment.