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The National Transportation Safety Board released an update Friday on the Feb. 20 United Airlines Boeing 777 that caught fire and showered Broomfield with scraps of plane debris.

Upon initial examination, the right engine fire was primarily contained to the engine’s accessory components, thrust reverser skin and composite honeycomb structure of the inboard and outboard thrust reversers.

All fan blade roots were in place in the fan hub, but two blades were fractured, the investigation update states. One fan blade was fractured 7.5 inches above the base at the trailing edge, and the fracture surface was consistent with fatigue. The second fractured blade showed signs of overload failure, consistent with secondary damage.

The initial review of maintenance and inspection data of the blade with the fatigue fracture showed it had experienced 2,979 cycles since its last inspection, the investigation states. The blade underwent thermal acoustic image inspections in 2014 and 2016, and data collected from the 2016 inspection was examined again in 2018 following a Feb. 13, 2018, incident involving a Boeing 777 with the same Pratt and PW4077 engines.

There was no evidence of a fuel-fed fire, since the spar valve was closed. The spar valve stops fuel flow to the engine when the fire switch is pulled from the cockpit. The spinner and spinner cap of the right engine fan were in place and appeared undamaged.

The fan blade with the fractures was examined at the metallurgical laboratory at Pratt & Whitney, where “findings from the scanning electron microscope examination identified multiple fatigue fracture origins on the interior surface of the cavity within the blade,” the investigation states.

BROOMFIELD, CO - Feb. 20, 2021: ...
The National Transportation Safety Board released an update Friday on the investigation into the Feb. 20 plane engine failure that showered Broomfield with plane parts. (Cliff Grassmick / Staff Photographer)

Efforts to further characterize the fracture surface, including identifying the primary origin and counting striations, are ongoing, and additional work is underway to further characterize secondary cracks identified through fluorescent penetrant inspection, the update states.

The National Transportation Safety Board also plans to analyze the blade’s chemical composition and microstructure near the fracture surface.

The flight crew stated they advanced power while climbing altitude to minimize turbulence. Immediately after the throttles were advanced, a “loud bang was recorded on the (cockpit voice recorder)” and the flight data recorder indicated the engine “made an uncommanded shutdown and the engine fire warning activated shortly after.”

The flight crew declared an emergency and returned to Denver International Airport for an emergency landing.

The investigative update does not contain analysis and does not discuss probable cause in the ongoing investigation, the update notes.

This story will be updated.