The United States (US) National Transportation Safety Board (NTSB) concluded that inadequate inspections, as well as insufficient frequency of inspection intervals, contributed to an uncontained engine failure, or a fan blade out (FBO) event, on a United Airlines flight in 2021.
On February 20, 2021, a United Airlines Boeing 777-200, registered as N772UA, was operating flight UAL328 from Denver International Airport (DEN) to Daniel K. Inouye International Airport (HNL), Honolulu, Hawaii, when shortly after takeoff, the aircraft experienced a failure of the right-hand engine, a Pratt & Whitney PW4000 (PW4077).
The failure resulted in a subsequent fire, with the NTSB detailing that the engine inlet lip skin, fan cowl support beam, and components of the inlet, fan cowls, and thrust reversers (TR) separated from the aircraft.
Reacting to the engine failure, the United Airlines crew declared an emergency, with the captain being the pilot flying (PF), while the first officer was the pilot monitoring (PM). The pilots landed the Boeing 777-200 back at DEN 24 minutes after taking off from the airport. While none of the 293 people onboard, including 10 crew members, sustained injuries, the NTSB said that “a vehicle and a residence sustained damage when impacted by the inlet lip skin and fan cowl support beam, respectively”.
Post-accident PW4000 engine inspection
NTSB investigators found that a separated fan blade and other debris impacted the fan case but were successfully contained. This impact caused damage to the nacelle inner and outer barrels, resulting in the deflection of the fan case and contact with the nacelle doors and hinges. As a consequence, the inlet aft bulkhead and the fan cowl support beam failed, allowing these structures and the inlet lip skin to separate from the engine.
The NTSB discovered that the engine’s carbon fiber reinforced plastic (CFRP) honeycomb structure was unable to handle the loads imposed during this event…