Investigation Summary
The investigation centered on the failure of fan blade #11, which experienced a full-length separation due to a fatigue crack. This crack had been present during a 2016 inspection but was misidentified as a low-level indication and allowed to remain in service. The failure was the fourth such event involving PW4000-series engines on Boeing 777s. The incident was particularly severe because the engine inlet, made of carbon fiber reinforced plastic (CFRP), could not dissipate the energy of the fan blade out (FBO) event as effectively as traditional aluminum structures, leading to the total separation of the inlet and fan cowls. Furthermore, the high dynamic loads caused the 'K' flange bolts to fail, allowing hot gases to escape the engine core and ignite a fire that was fed by leaking fuel and hydraulic fluid.
Final Conclusions
The NTSB determined the probable cause was the fatigue failure of the right engine fan blade, exacerbated by inadequate inspections and insufficient inspection intervals by the manufacturer. Contributing factors included the engine inlet design, which failed to limit damage during a fan blade out event, and the failure of the 'K' flange, which allowed an unextinguishable fire to propagate into the thrust reversers. Additionally, a contaminated hydraulic shutoff valve failed to close, providing further fuel to the fire.
Photographic Evidence (1)
Video Analysis
UNITED 328 Engine Failure! WHAT CHECKLISTS did the Pilots use? — A detailed breakdown of the United 328 engine failure, the mechanics of the fan blade out event, and the emergency procedures followed by the crew.
If playback is blocked, the owner has disabled embedding — use the link above.
Airframe & Maintenance
Systems & Failure Modes
Pratt & Whitney PW4077
The turbofan engine that suffered a fan blade out event due to metal fatigue.
Engine Inlet (CFRP)
The carbon fiber reinforced plastic structure that failed to dissipate energy and separated from the aircraft.
K Flange
A structural connection between the main gearbox and the engine core that failed, allowing hot gases to ignite a fire.
Hydraulic Shutoff Valve
A safety valve that failed to close due to electrical contact contamination, allowing hydraulic fluid to feed the fire.
Interesting Facts
- 01The aircraft was the fifth Boeing 777 ever produced, manufactured in 1995.
- 02Debris damaged a residential roof and a parked pickup truck, but caused no injuries on the ground.
- 03The engine fire was fed by high-pressure fuel after the 'K' flange failure caused a servo fuel heater to fracture.
- 04The hydraulic pump supply shutoff valve failed to close due to silicone lubricant contamination.
- 05The fan blade fatigue crack had been visible in Thermal Acoustic Imaging (TAI) records from 2016 but was not caught.
Safety Actions & Advisories
Emergency Airworthiness Directive
The FAA issued an emergency AD requiring immediate thermal acoustic imaging (TAI) inspections of all PW4000-series fan blades before further flight.
Inlet Structural Modifications
Boeing developed modifications to add metallic structure and increase the strength and ductility of the 777 engine inlets to prevent separation during FBO events.
K Flange Redesign
Pratt & Whitney began evaluating actions to improve the strength of the 'K' flange to prevent core gas leakage during high-vibration events.