Summary
This document proposes the adoption of a new airworthiness directive (AD) that is applicable to certain EMBRAER Model EMB-135BJ and EMB-145XR series airplanes. This proposal would require repetitive inspections for cracking in the firewall of the auxiliary power unit (APU), and repair of the firewall if necessary. This proposal would also provide an optional terminating action for the repetitive inspections. This action is necessary to detect and correct cracking in the APU firewall, which could result in reduced structural integrity of the firewall, and a consequent uncontained APU fire that could spread to the airplane structure. This action is intended to address the identified unsafe condition.