Event Overview
On December 26, 2018, Korean Air flight 0753, an Airbus A220-300 with registration HL8314, was climbing through 29,000 feet on a flight from Busan, Korea (PUS) to Nagoya, Japan. The flight crew reported hearing a loud bang, followed by vibrations and an engine fire warning. They shut down the left (No. 1) engine, a Pratt & Whitney (P&W) PW1521G-3, and returned to Busan without further incident. There were no reported injuries to passengers or crew. The engine had accumulated 417 hours and 529 cycles since new.
Engine Examination
The engine was removed and shipped to P&W's Columbus Engine Center in Columbus, Georgia for disassembly. Examination revealed damage to the turbines and several holes in the low pressure turbine (LPT) case. Despite the fire warning, no fire damage was found.
A sector of six adjacent LPT stage 3 blades (Nos. 52 to 57) were fractured between about 0.25 and 1 inch above the blade root platform. Blade No. 52 had a flat, planar, elliptical-shaped fracture surface at the rear half, while the forward half of that blade and the other five blades had coarse, grainy fracture surfaces.
Metallurgical Findings
Blade No. 52 and the other LPT stage 3 blades were sent to P&W's Materials and Process Engineering laboratory. The metallurgical examination showed that blade No. 52 separated due to a fatigue crack that originated from a preexisting intergranular crack near the trailing edge. Fatigue progressed to the trailing edge and forward to the mid‑chord area before the remainder fractured in overload. Blades Nos. 53 to 57 had separated in overload.
The grain size and hardness of blade No. 52 conformed to requirements for IN‑100 nickel alloy. Energy dispersive spectroscopy (EDS) away from the fatigue crack origin produced a spectra consistent with IN‑100. However, EDS at the preexisting intergranular crack and the fatigue crack origin showed peaks of zirconium and hafnium.
Manufacturing Details
Within the PW1000 engine series, some LPT stage 3 blades are made of beta titanium and others of IN‑100. Beta titanium blades have had numerous fractures and are being replaced with IN‑100 blades. This Korean Air event is the only reported IN‑100 LPT stage 3 blade fracture.
The LPT stage 3 blade is cast IN‑100 nickel alloy. According to the manufacturer Arconic, the nickel alloy is melted in a zirconia crucible and poured into molds. The mold containing the fractured blade was the first one cast. Arconic's records showed that MAR‑M‑247 alloy had been melted in the crucible just before the IN‑100 alloy for these blades. Hafnium is an alloying element of MAR‑M‑247.
Arconic performed post‑casting inspections including visual, fluorescent penetrant inspection (FPI), and X‑ray; no anomalies were detected. The fatigue crack originated from a subsurface intergranular crack. Because the origin was subsurface, it would not have been detectable by visual or FPI, which require surface‑breaking defects. A subsequent review of X‑ray inspection records focused on the fatigue crack origin area detected a casting anomaly, but the manufacturer stated the anomaly lacked contrast to be detectable by typical production‑level inspection.