Flight History
On February 13, 2018, at about 1200 Hawaiian standard time, United Airlines flight 1175, a Boeing 777-222 (registration N773UA), was en route from San Francisco International Airport, California, to Daniel K. Inouye International Airport, Honolulu, Hawaii. The aircraft was in level cruise at flight level 360 over the Pacific Ocean when the flight crew heard a loud bang followed by violent shaking and compressor stall warnings. The crew shut down the No. 2 (right) engine, a Pratt & Whitney PW4077, declared an emergency, and diverted to Honolulu, landing without further incident. There were no injuries among the 374 passengers and crew. The aircraft sustained minor damage.
Three pilots were on the flight deck: the captain (pilot monitoring), the first officer (pilot flying), and an off-duty United Airlines 777 first officer in the jump seat. After the initial jolt, the captain took over as pilot flying. About 15 to 30 seconds later, engine instruments indicated failure of the No. 2 engine. The crew performed the Severe Engine Damage checklist and shut down the engine. The jump seat rider observed the engine from the cabin, noting oscillation and missing cowling.
Aircraft and Engine Information
The Boeing 777-200 is a long-range, twin-engine aircraft with a maximum takeoff weight of 545,000 pounds. The incident aircraft had accumulated 89,723 hours and 16,339 cycles since new. The No. 2 engine, a Pratt & Whitney PW4077, had 77,593 hours and 13,921 cycles since new, with 8,579 hours and 1,464 cycles since last overhaul. The fan blade was a hollow core titanium alloy (6% vanadium, 4% aluminum) measuring about 40.5 inches long and weighing up to 34.85 pounds.
The engine is certified under FAR Part 33, which requires demonstration of containment and safe shutdown after intentional fan blade fracture at redline speed. The test inlet used an aluminum aft bulkhead, while production inlets use a carbon fiber reinforced polymer (CFRP) aft bulkhead. The inlet and fan cowls are certified under FAR Part 25 by Boeing.
Damage and Examination
The fan blade in position No. 11 fractured transversely across the airfoil about 1.44 inches above the fairing. Metallurgical examination revealed a fatigue fracture originating from a subsurface region of micro texturing on the interior surface of the hollow core. The fan case sustained gouging and cracking, but no debris penetrated. The Kevlar environmental wrap had a 35-inch axial tear and bulging. Most of the inlet assembly and fan cowl halves were missing, having fallen into the sea. The aircraft fuselage had two small dents and punctures near seat rows 20 and 21, with embedded titanium and vanadium particles.
Inspection Process Issues
The fractured fan blade had undergone two thermal acoustic imaging (TAI) inspections (in 2010 and 2015). Review of records showed that a small indication was present in 2010, and a larger indication in 2015. Both times, inspectors attributed the indication to paint defects, allowing the blade to return to service. At the time of the incident, Pratt & Whitney had no defined training or certification regimen for TAI inspectors; training consisted of about 40 hours of on-the-job training. The TAI was categorized as a new and emerging technology, despite over 9,000 blades having been inspected. Environmental factors, such as sunlight and workers entering the TAI room, could cause false thermal images. After the incident, Pratt & Whitney developed a TAI curriculum and improved room controls.