Summary
This document proposes the supersedure of an existing airworthiness directive (AD), applicable to AlliedSignal Inc. (formerly AirResearch and Garrett) TSCP700-4B and -5 Series Auxiliary Power Units, that currently requires restretching the first stage low pressure compressor (LPC) tie rods, or replacing affected disks at or before 8,000 cycles since new (CSN). This action would eliminate the option of restretching the tie rods, and would require removing from service affected disks, replacing them with serviceable parts, and establishing a life limit of 8,000 CSN for affected disks. This proposal is prompted by a report of a first stage LPC disk rim separation due to low cycle fatigue on an APU that had its tie rods restretched in accordance with the current AD. The actions specified by the proposed AD are intended to prevent first stage LPC disk rim separation due to low cycle fatigue, which could result in an uncontained APU failure and damage to the aircraft.