HL8314

On December 26, 2018, KoreanAir flight 0753, an Airbus A220-300, registration HL8314, experienced a commanded inflight shutdown of the left (No. 1) engine, a Pratt & Whitney (P&W) PW1521G-3, while climbing through 29,000 feet on a flight from Busan, Korea (PUS) to Nagoya, Japan. The flight crew reported hearing a loud bang that was followed by vibrations and an engine fire warning. The flight crew shut down the engine and the airplane returned to PUS without further incident. The examination of the engine revealed damage to the turbines and several holes in the low pressure turbine (LPT) case. Although there was a fire warning, the examination of the engine did not reveal any fire damage. There were no reported injuries to the passengers and crew that were on board the airplane. At the time of the event, the engine had accumulated only 417 hours and 529 cycles since new.

The engine was removed from the airplane and shipped to P&W’s Columbus Engine Center, Columbus, Georgia for disassembly and examination. The examination of the engine revealed there was a sector of six adjacent LPT stage 3 blades, Nos. 52 to 57, that were fractured between about 0.25- and 1-inch above the blade root platform. There was one LPT stage 3 blade, No. 52, that had a flat, planar, elliptical-shaped fracture surface at the rear half of the blade. In comparison, the fracture surfaces on the forward half of blade No. 52 as on the other five blades in that cluster of blades, Nos. 53 to 57, were coarse and grainy.

LPT stage 3 blade No. 52 along with all of the other LPT stage 3 blades were sent to P&W’s Materials and Process Engineering laboratory for a metallurgical examination. The metallurgical examination showed that blade No. 52 separated from a fatigue crack that had originated from a preexisting intergranular crack near the trailing edge. The fatigue had progressed to the trailing edge and forward to the mid chord area of the blade before the remainder of the blade fractured in overload. The examination of blades Nos. 53 to 57 showed those blades had separated in overload. The metallurgical examination of blade No. 52 showed that the grain size and hardness conformed to the requirements for the specified IN-100 nickel alloy. The energy dispersive spectroscopy (EDS) of blade No. 52 away from the origin of the fatigue crack produced a spectra that was consistent with the requirements for IN-100. However, the EDS of blade No. 52 at the preexisting intergranular crack and the origin of the fatigue crack produced a spectra that was consistent with IN-100, but also had peaks of zirconium and hafnium.

Within the PW1000 engine series, there are engines that have LPT stage 3 blades that are made of beta Ti [titanium] and engines that have LPT stage 3 blades made of IN-100. Although those engines with the beta Ti LPT stage 3 blades have had numerous blade fractures and those blades are in the process of being replaced with the IN-100 blades, this Korean Air LPT event is the only reported IN-100 LPT stage 3 blade fracture.

The LPT stage 3 blade is made of cast IN-100 nickel alloy. According to the manufacturer, Arconic, the nickel alloy is melted in a zirconia crucible and then poured into molds. Each mold contains several blade castings. According to Arconic’s records, the mold that contained the casting of the fractured blade was the first one cast. Furthermore, the Arconic’s records revealed that MAR-M-247 alloy had been melted in the crucible just prior to it being used to melt the IN-100 nickel alloy to cast the blades that included the LPT stage 3 blade that would become No. 52. Hafnium is however, one of the alloying elements of MAR-M-247. Arconic performed several post-casting inspections of the blades including visual, fluorescent penetrant inspection (FPI), and x-ray; no anomalies were detected. The metallurgical examination of the fractured blade showed that the fatigue crack originated from a subsurface intergranular crack. Since the origin of the fatigue crack was a subsurface intergranular crack, presuming it was even cracked at the time of the post-casting visual inspections, it would not have been detectable since the visual and FPI inspections require a defect to be surface breaking to be detectable. The blades also undergo an X-ray inspection. Although a subsequent review of the X-ray inspection records that was focused on the area where the fatigue crack originated detected a casting anomaly, the manufacturer stated that the casting anomaly lacked contrast to be detectable by the typical production level of inspection.