On February 20, 2021, about 1309 mountain standard time, a Boeing 777-222, N772UA, operated by United Airlines (UAL) as flight 328, experienced a right engine fan blade separation and subsequent engine fire shortly after takeoff from Denver International Airport (DEN), Denver, Colorado. The two pilots, eight crew members, and 229 passengers onboard were not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 121 scheduled passenger flight.
We determined the probable cause(s) of this incident to be: The fatigue failure of the right engine fan blade. Contributing to the fan blade failure was the inadequate inspection of the blades, which failed to identify low-level indications of cracking, and the insufficient frequency of the manufacturer’s inspection intervals, which permitted the low-level crack indications to propagate undetected and ultimately resulted in the fatigue failure. Contributing to the severity of the engine damage following the fan blade failure was the design and testing of the engine inlet, which failed to ensure that the inlet could adequately dissipate the energy of, and therefore limit further damage from, an in-flight fan blade out event. Contributing to the severity of the engine fire was the failure of the “K” flange following the fan blade out, which allowed hot ignition gases to enter the nacelle and imparted damage to several components that fed flammable fluids to the nacelle, which allowed the fire to propagate past the undercowl area and into the thrust reversers, where it could not be extinguished.
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