Incident
On May 14, 2024, a United Airlines (UAL) Boeing 787-9, serial number 36409, registration N27957, powered by two GE Aerospace GEnx-1B76A/P2 turbofan engines, experienced an uncontained high pressure turbine (HPT) engine failure and inflight shutdown (IFSD) of the No. 1 (left) engine shortly after reaching assigned flight altitude FL310 following departure from Singapore Changi Airport, Singapore. The aircraft was operating a regularly scheduled passenger flight under Title 14 CFR Part 121 from Singapore to San Francisco International Airport (SFO).
The flight crew reported hearing a pop sound and feeling the airplane shudder, followed by a No. 1 engine EICAS message for left engine fire warning. The flight crew pulled the No. 1 engine fire handle, secured the engine, and conducted an air turnback to Singapore Changi Airport, where an uneventful single-engine landing was performed. There were no reported injuries.
Damage
Upon landing, ground inspection revealed damage to the airplane, missing material from the left side thrust reverser cowling, and a penetration of the engine's HPT case. Documented by UAL and GE Aerospace representatives, the initial on-scene engine examination found two exit holes in the HPT case centered at approximately the 9:00 o'clock position. The HPT stage 1 blades and stage 2 stators were visible through the holes, and the HPT stage 1 aft blade retainer appeared to have missing material. Localized heat distress was observed on electrical wiring harnesses, fire loop grommets/isolators, and some HPTACC/LPTACC rubber bellows. The left side thrust reverser cowl had a large section of material, approximately 3 feet by 6 feet, missing from the translating sleeve, and metal debris was lodged into the inner diameter of the thrust reverser. The left side thrust reverser inner fixed structure contained an exit hole approximately 13 inches by 15 inches centered at about the 8:30 o'clock position.
Investigation
Since the event occurred in international waters, the National Transportation Safety Board (NTSB) led the investigation as the State of Registry under ICAO Annex 13 guidelines. Additional parties included the Federal Aviation Administration (FAA), Boeing, United Airlines, and GE Aerospace. The engine was removed from the airplane, the fan module removed, and the engine propulsor shipped to the United States for disassembly. The left side thrust reverser cowl, other components, and recovered metal debris were also shipped to GE for examination.
Disassembly at GE's Evendale, Ohio facility revealed significant hard body damage and missing material throughout the HPT module. The HPT case had two exit holes with outward distortion. No signs of fuel nozzle streaking, hot spots, or burn throughs were found in the combustion chamber. Multiple HPT stage 1 nozzle segments were disengaged, and all HPT stage 1 and stage 2 blades were fractured transversely across the airfoils at various span lengths. Downstream damage included one fractured LPT stage 1 blade, but LPT stage 7 blades appeared undamaged.
The HPT stage 1 aft blade retainer was found lodged in place on the forward side of the HPT rotating interstage seal (RIS), fractured, with the outboard portion of the retainer arm missing. The retainer presented a 360-degree circumferential fracture surface adjacent to the rabbet undercut fillet. Fractography revealed the crack initiated in the upper collet radius at a single point of origin corresponding to a surface-connected anomaly measuring 0.005 by 0.015 by 0.0005 inches. The anomaly was a cluster of particles enriched with Titanium, Aluminum, Zirconium, Magnesium, and Oxygen, consistent with Oxide/Carbonitride/Nitride/Carbide (OCNC) particles. The fracture surface exhibited intergranular fracture morphology with beach marks, consistent with hold-time fatigue growth, and the remaining surface was consistent with overload fracture.
Findings and Corrective Actions
The investigation supplemented ongoing investigations into two other cracked HPT stage 1 aft blade retainers. GE's analysis suggested that surface-connected OCNCs reduce crack incubation/nucleation life at high stress and do not follow traditional crack growth threshold behavior. Material sensitivity studies focusing on low cycle fatigue (LCF) and sustained peak low cycle fatigue (SPLCF) found that activating surface-connected OCNCs can reduce material fatigue life capability, with unknown bimodal behavior.
The stress conditions for the GEnx HPT stage 1 aft blade retainer collet feature were uniquely high due to assembly interference fits between the HPT stage 1 disk, retainer, and RIS. The collet feature had a significant SPLCF debit under hold-time fatigue.
Corrective actions included service bulletins for removal/inspection/replacement of HPT stage 1 aft blade retainers at the piece part and module levels, and a data analytic tool monitoring core engine vibration trends for all GEnx engines. As the report neared completion, at least 477 retainers had been removed from service for inspection and at least 55 HPT modules ultrasonically inspected, with zero crack findings. In February 2025, the data analytic tool alerted on an operational engine, and removal and ultrasonic inspection resulted in a positive crack indication.
Long-term corrective action involves re-establishing the manufacturing process for the previously approved HPT stage 1 aft blade retainer configuration made of Rene 88 alloy. The Rene 65 retainer will remain certified but will no longer be available as a spare part, reducing its population through attrition.