What happened
On February 20, 2021, at approximately 13:09 MST, a Boeing 777-222, registration N772UA, operated by United Airlines as flight 328, was climbing through 12,500 feet MSL after departing Denver International Airport (DEN). While the crew was increasing engine power to minimize time in forecasted moderate turbulence, a loud bang was heard in the cockpit. The flight data recorder indicated an uncommanded shutdown of the No. 2 (right) engine, followed by an engine fire warning on the EICAS.
As the aircraft flew over Broomfield, Colorado, several engine components, including the engine inlet, fan cowls, and thrust reversers, separated from the aircraft. Debris was scattered over approximately 40 acres, impacting a public park and residential areas. There were no injuries to the 239 people on board and no ground injuries reported, though a vehicle and a residence sustained damage from falling debris.
The flight crew declared an emergency and performed the engine fire checklist, discharging both right engine fire extinguishing bottles. Despite these actions, the engine fire warning remained active until shortly before landing. The crew landed the aircraft safely at DEN at 13:28, where emergency responders applied water and foam to the engine for 40 minutes.
The investigation
Investigators examined the right engine and found that fan blade No. 19 had suffered a full-span separation. The adjacent blade, No. 18, showed evidence of overload failure. The examination of the engine nacelle revealed that the impact of the blade separation caused the failure of the inlet aft bulkhead and the fan cowl support beam, leading to the separation of the inlet lip skin and cowls.
Regarding the fire, the investigation found that the engine's "K" flange bolts failed, allowing high-temperature compressed gases to escape the engine core. This provided an ignition source for fuel released when the engine-driven pump supply shutoff valve failed to close properly due to silicone contamination in its motor. The fire spread to the thrust reversers, where it was not extinguished by the onboard systems.
Metallurgical analysis of the fractured fan blade revealed a fatigue crack that originated from a discontinuity introduced during the manufacturing process. The analysis also identified surface carbon contamination within the blade's internal cavities, which likely reduced the blade's fatigue resistance.
Findings
- The primary cause of the event was the fatigue failure of the right engine fan blade.
- A 2016 thermal acoustic imaging (TAI) inspection had identified low-level indications in the blade, but these were incorrectly interpreted as sensor noise or grit, and the blade was returned to service.
- The engine inlet design was unable to adequately dissipate the energy of the fan blade out event, leading to the separation of the inlet and cowl structures.
- The failure of the "K" flange allowed hot gases to enter the nacelle, providing an ignition source for the fire.
- The engine fire was exacerbated by the failure of the engine driven hydraulic pump supply shutoff valve to close, which allowed hydraulic fluid to feed the fire.