Summary
This document revises an earlier proposed airworthiness directive (AD), applicable to certain Boeing Model 777 series airplanes, that would have required repetitive inspections of the safety spring wear plate doublers attached to the auxiliary power unit (APU) firewall, measurement of wear of the doublers, and follow-on actions, if necessary. That proposed AD also would have provided for optional terminating action for the repetitive inspections. That proposal was prompted by reports indicating that excessive wear was found on the safety spring wear plate doublers on the APU firewall of Boeing Model 777 series airplanes. This new action revises the proposed rule by extending the compliance time for a certain action and referencing a new service bulletin. For certain airplanes, this new action also adds a one-time inspection to detect improper clearance between the safety spring wear plate doubler and the APU firewall, and corrective action, if necessary. The actions specified by this new proposed AD are intended to detect and correct wear of the safety spring wear plate doublers on the APU firewall, which could result in a hole in the APU firewall, and consequent decreased fire protection capability.