Summary
This document proposes the supersedure of two existing airworthiness directives (ADs), applicable to AlliedSignal Inc. ALF502 and LF507 series turbofan engines, that currently require rework or replacement of No. 4 and 5 bearing oil system hardware, initial and repetitive inspections of the oil system, optional installation of an improved oil filter bypass valve, and repetitive inspection of No. 4 and 5 bearing oil inlet tube, to ensure the integrity of the reduction gear system and overspeed protection system. This action would require replacement of the existing power turbine bearing housing assembly with a new, improved power turbine bearing housing assembly, and installation of a reworked or modified fourth turbine rotor disk assembly as a part of a design change to the new No. 4 bearing configuration that eliminates the requirement for repetitive inspections of oil system and No. 4 and 5 bearing oil inlet tube assembly. This proposal is prompted by one report of a contained power turbine rotor shaft separation forward of the Stage 4 low pressure turbine (LPT) rotor on an AlliedSignal Inc. ALF502R-5 engine. The LPT failure was caused by improper inspection of the engine oil system required by AD 97-05-11 R1. The actions specified by the proposed AD are intended to prevent No. 4 and 5 duplex bearing failure, which can result in a Stage 4 LPT rotor failure, an uncontained engine failure, and damage to the aircraft.