Flight Event
On February 21, 2019, at approximately 0729 eastern standard time, United Airlines Flight 1768, a Boeing 737-924 (registration N30401) powered by two CFM International CFM56-7B26 turbofan engines, experienced a No. 2 (right) engine failure during initial climb from Orlando International Airport (MCO), Orlando, Florida. At about 7,000 feet altitude, the flight crew reported an "abrupt loud grinding noise and instantaneous boom," followed by a loss of No. 2 engine power and an uncommanded engine shutdown. The crew executed quick reference handbook procedures, closed the No. 2 engine fuel shutoff valve, declared an emergency, and returned to MCO, where they performed an uneventful overweight single-engine landing. Airport rescue and firefighting (ARFF) crews met the airplane on the high-speed taxiway, but no fire or smoke was visible, and the airplane taxied to the gate under its own power. No passenger or crew injuries were reported. The flight was a regularly scheduled Part 121 operation from MCO to George Bush Intercontinental Airport (IAH), Houston, Texas.
Aircraft and Engine Examination
There was no damage to the airplane structure. A preliminary visual inspection of the No. 2 engine by United Airlines maintenance crews revealed high pressure compressor (HPC) case burn through. The engine was removed and shipped to General Electric (GE) Aviation in Evendale, Ohio, where an examination and disassembly was conducted from February 26 to March 2, 2019, with party members from United Airlines, GE Aviation, CFM International, Boeing, the FAA, and the NTSB.
The engine showed no visible undercowl fire or high-energy uncontainment. Fan blades were intact, and the fan spun smoothly with concurrent low pressure turbine rotation. HPC case burn through was observed around approximately 60% of the circumference, with case material intact around rub button pads and stator case split flanges. Surfaces and accessories near the burn through holes were discolored, sooted, and coated with metal spray. The forward sump magnetic chip detector (MCD) plug had metal debris accumulation.
After removal of external components, an HPC stage 1 variable stator vane (VSV) trunnion stem was found missing a washer and retaining nut at the 1:30 position. The VSV alignment mark was positioned at a different angle relative to other vanes in the stage. During disassembly, eight HPC stage 2 rotor blades, including dovetails, were found separated and missing, with corresponding disk post corner separations. Secondary impact damage was observed throughout the gas path aft of HPC inlet guide vanes. The high pressure turbine stage 1 blades and low pressure turbine stage 1 nozzle vanes exhibited thermal damage. Metal flakes and debris were collected in the aft sump near the No. 4 bearing.
Electronic Engine Control Testing
The electronic engine control (EEC) was removed and tested at BAE Systems in Fort Wayne, Indiana. No faults were recorded, indicating the EEC functioned properly during the flight.
Maintenance History
The last engine shop visit was an overhaul at the GE Aviation-Celma MRO facility in Petrópolis, Brazil, in July 2014. Photographs from the overhaul could not evaluate the HPC VSV actuation hardware due to obstructions. Records indicated that 22 of the 82 HPC S1 VSVs were replaced, and the remaining were overhauled, but installation positions were not recorded.
Materials Analysis
The HPC rotor assembly, cases, and debris samples were analyzed at the GE Aviation Materials Laboratory. Examination of the HPC stage 1 VSV #33 trunnion stem confirmed that the missing washer and retaining nut resulted in disengagement of the lever arm from the trunnion D-head, allowing the vane to go off-schedule approximately 31 degrees. The trunnion stem had a uniform coating of dirt and debris consistent with extended operation without the washer and nut. Witness marks indicated that a washer and nut were present at some point, but the timing of separation could not be determined.
The HPC stage 2 disk post fracture morphology was consistent with high cycle fatigue or mixed-mode low cycle fatigue/high cycle fatigue mechanisms. Primary initiation occurred adjacent to the forward right corner of the disk posts. No material anomalies or stress risers were identified. Metal flakes from the aft sump primarily consisted of bearing materials (RBD modified steel and silver plating), while forward sump debris was identified as M50Nil bearing material.
Corrective Actions
Following the event, the GE Aviation-Celma CFM56 MRO facility implemented additional control measures for VSV lever arm installation. The revised procedure requires a trunnion nut torque verification check by a second technician using a torque wrench, followed by a seating check with a 0.001-inch feeler gauge to prevent false torque indications. The GE Aviation-Strother MRO facility revised their installation instructions to include a mechanic stamp requirement confirming proper thread protrusion on each VSV trunnion. GE Aviation and CFM International are evaluating new torque wrench technology that measures torque and angle to alert technicians if the pre-set angle is not applied. The finding of the missing VSV nut and washer was reviewed at all CFM56 MRO shops and presented at the June 2019 Operators Symposium in Brussels and the November 2019 All Middle East Operators Conference in Muscat. It was also featured in the first quarter 2020 edition of GE Fleet Highlites.