Summary
This document proposes the adoption of a new airworthiness directive (AD) that is applicable to certain Pratt & Whitney (PW) PW4000 series turbofan engines not incorporating modifications described in certain PW service bulletins listed in the applicability section. This proposal would require high pressure compressor (HPC) blade tip grinding of the rotor assembly, installation of aluminum oxide coated HPC blade tips in stages 9 through 12, modification of HPC 8th through 14th stage stators, incorporation of 1st stage high pressure turbine (HPT) vanes with increased airflow area which also requires additional HPT hardware modifications, and incorporation of HPC 13th-15th stage zirconium oxide blade tips. This proposal is prompted by reports of HPC surge caused by excessive HPC rear stage rotor-to-case clearance. The actions specified by the proposed AD are intended to prevent HPC surge, which can result in engine power loss at a critical phase of flight such as takeoff or climb.