N411LF

ATA 7200

UPON START PILOT HEARD HIGH PITCH HOWLING NOISE EMANATING FROM ENGINE/EXHAUST AREA. PILOT ELECTED TO SHUTDOWN TO NOTIFY MAINTENANCE. UPON RECEIPT, PHI CONDUCTED A BORESCOPE INSPECTION AND NOTED DAMAGE ON THE 1ST STAGE TURBINE WHEEL. THE 1ST STAGE TURBINE WHEEL, NOZZLE, & 2ND STAGE TURBINE NOZZLE WERE SUBMITTED TO THE MATERIALS LAB IN INDIANAPOLIS, IN FOR ANALYSIS. THE PRIMARY FINDINGS OF THE METALLURGICAL INVESTIGATION. A SINGLE FIRST STAGE TURBINE WHEEL AIRFOIL FRACTURED AND LIBERATED IN HIGH CYCLE FATIGUE (HCF). A SPHERICAL GAS HOLE APPROXIMATELY 0.009 INCH IN DIAMETER OBSERVED AT THE ORIGIN AREA. THE OVERALL COMPOSITION AND MICROSTRUCTURE OF THE FIRST STAGE TURBINE WHEEL MET THE COMPONENT DEFINITION REQUIREMENTS. OVERALL VIEWS OF THE FIRST STAGE TURBINE WHEEL IN THE AS-RECEIVED CONDITION ARE SHOWN IN FIGURE 1 (LEFT). THE FRACTURED AIRFOIL IS LABELED AS NR 1. 30 AIRFOILS EXHIBITED DAMAGE ON THE TRAILING HALF OF THE AIRFOILS, CONSISTENT WITH SECONDARY DAMAGE THAT OCCURRED AFTER THE LIBERATION OF AIRFOIL #1. A DETAILED VIEW OF AIRFOIL SURFACE IS SHOWN IN FIGURE 1 (RIGHT), EXHIBITING RADIATING LINES AND BEACH MARKS CONSISTENT WITH FATIGUE PROGRESSION OUTWARD FROM THE FRACTURE ORIGIN (ORANGE BORDER, AT THE SUCTION SIDE CROWN ROOT AREA AS SHOWN IN THE BLACK ARROWS) THEN TRANSITIONS TO OVERLOAD, INDICATED BY THE YELLOW DASHED LINES. THE FRACTURE ORIGIN WAS VISUALLY CONSISTENT WITH HIGH CYCLE FATIGUE INITIATION.FIGURE 1, TURBINE WHEEL AS RECEIVED (LEFT). DETAILED VIEW OF AIRFOIL NR 1 FRACTURE SURFACE (RIGHT) JURISDICTION EXPORT CONTROL CLASSIFICATION (RATING) USE OR DISCLOSURE OF THIS DATA IS SUBJECT TO THE RESTRICTION ON THE FIRST PAGE OF THIS DOCUMENT. AIRFOIL #1 WAS EXTRACTED FROM THE WHEEL AND THE FRACTURE SURFACE SUBJECTED TO SCANNING ELECTRON MICROSCOPIC (SEM) EXAMINATION. DETAILED IMAGES OF THE ORIGIN AREA AND FATIGUE REGION ARE SHOWN IN FIGURE 2. THE ORANGE ARROWS INDICATE DIRECTION OF FATIGUE PROGRESSION; THE DASHED YELLOW BORDER INDICATES THE FATIGUE FACETS. THE SOLID YELLOW BORDER INDICATES THE GAS HOLE AT THE ORIGIN. A PORE CONSISTENT WITH A GAS HOLE WAS LOCATED AT THE FATIGUE ORIGIN. THE GAS HOLE WAS APPROXIMATELY 0.009 INCH IN DIAMETER AND LOCATED APPROXIMATELY 0.009 INCHES FROM THE SUCTION SIDE OF THE AIRFOIL. FIGURE 2: DETAILED SEM IMAGES, ORIGIN LOCATION CONCLUSION BASED ON COMPREHENSIVE LABORATORY ANALYSIS, IT IS CONCLUDED THAT THE MOST PROBABLE CAUSE OF THE BLADE LIBERATION WAS THE PRESENCE OF A SPHERICAL GAS PORE LOCATED IN THE CROWN ROOT OF THE AIRFOIL. THIS SUBSURFACE GAS PORE SERVED AS THE INITIATION SITE FOR FATIGUE CRACKING, ULTIMATELY LEADING TO CRACK PROPAGATION AND OVERLOAD FRACTURE OF THE AFFECTED AIRFOIL. THE M250 FAMILY OF STAGE 1 TURBINE BLISKS, INCLUDING THE SUBJECT PART (PN M250-10226), ARE MANUFACTURED USING A CENTER-OUTWARD CASTING PROCESS. THIS METHOD IS CONSISTENT WITH AEROSPACE INDUSTRY STANDARDS & IS SPECIFICALLY DESIGNED TO DRIVE RESIDUAL CONTAMINANTS AND CASTING ANOMALIES TOWARD THE OUTER DIAMETER, THEREBY MINIMIZING THE RISK OF SUCH DEFECTS IN CRITICAL BORE REGIONS. A THOROUGH REVIEW OF THE FIXED MANUFACTURING PROCESS REVEALED NO SYSTEMATIC ISSUES. ALL PARTS UNDERGO RIGOROUS VISUAL, DIMENSIONAL, FLUORESCENT PENETRANT, AND X-RAY INSPECTIONS. ADDITIONALLY, A LEVEL III NDT-CERTIFIED SPECIALIST IS CURRENTLY RE-EXAMINING THE HISTORICAL X-RAY RECORDS FOR THIS COMPONENT AT THE CASTING FACILITY. A REVIEW OF HISTORICAL DATA FOUND NO EVIDENCE OF SIMILAR OCCURRENCES IN STAGE 1 BLISKS. GIVEN THAT THE GAS PORE IS ASSESSED TO BE AN ISOLATED, RANDOM ANOMALY, NO FLEETWIDE ACTIONS ARE RECOMMENDED AT THIS TIME.