On February 13, 2018, around noon local time, a Boeing 777 operating as United Airlines Flight 1175 (UA1175), experienced an in-flight separation of a fan blade in the No. 2 (right) engine while over the Pacific Ocean en route from San Francisco International Airport to the Daniel K. Inouye International Airport, Honolulu, Hawaii. During level cruise flight shortly before beginning a descent from flight level 360 (roughly ), and about from the destination, the flight crew heard a loud bang, followed by a violent shaking of the airplane, followed by warnings of a compressor stall. The flight crew shut down the failed engine, declared an emergency, and began a drift-down descent, proceeding direct to the Daniel K. Inouye International Airport where they made a single-engine landing without further incident at 12:37 local time. There were no reported injuries to the 378 passengers and crew on board and the airplane damage was classified as minor under National Transportation Safety Board (NTSB) criteria.
NTSB investigators traveled to the scene to begin an incident investigation. They found a full-length fan blade fracture in the No. 2 (right) engine, a Pratt & Whitney (P&W) PW4077 turbofan. Its installed set of hollow-core fan blades had undergone two previous overhauls at P&W that included a thermal acoustic imaging (TAI) internal inspection that is intended to prevent this type of failure. The right engine nacelle lost most of the inlet duct and all of the left and right fan cowls immediately after the engine failure. Two small punctures were found in the right side fuselage just below the window belt with material transfer consistent with impact from pieces of an engine fan blade. The damage was eventually repaired and the aircraft returned to service. Improved procedures for TAI inspection were implemented by P&W, increased frequency of TAI inspection was required by regulators, and a redesign of the inlet duct was also initiated by Boeing, all as a result of this incident and investigation.
Aircraft The aircraft involved, manufactured by Boeing in 1995, was a Boeing 777-222, the United Airlines specific variant of the original 777-200 series, registered as N773UA, ( 26929) and line number 4. It was powered by two Pratt & Whitney PW4000 engines. Boeing stopped building the 777 with P&W PW4000 series engines in 2013, and the engine is no longer in active production. Later in 2021, N773UA was used to fly the passengers of Flight 328 from Denver to Honolulu after the aforementioned flight, operated by sister ship N772UA, suffered a similar problem.
The Boeing 777 is a long-range, wide-body, twin-engine aircraft. At the time of the incident it had a relatively low accident fatality rate. The only two 777 accidents with total loss of aircraft, passengers and crew are Malaysia Airlines flight MH17 that was shot down over Ukraine in July 2014 and flight MH370 that disappeared over the Indian Ocean in March 2014. The other fatal accidents, Emirates Flight 521 and Asiana Airlines Flight 214, were both attributed to pilot error. Two other hull losses with passenger injuries occurred: EgyptAir Flight 667 had a cockpit fire while parked at the gate at Cairo Airport, and British Airways Flight 38 crashed on landing at Heathrow Airport. The latter was attributed to a design defect in its Rolls-Royce Trent 895-17 engines, not the P&W engines on Flight 1175.
Engine The original 777-200 was distinctive for its Pratt & Whitney PW4000 engines that are about as wide as a 737 fuselage. The PW4077 variant used on the United 777-222 nominally produces of thrust. It is a dual-spool, axial-flow, high bypass turbofan engine, that is a higher bypass version of the PW4000-94 engine originally fitted to the Boeing 747-400. It was re-designed exclusively for the 777 with a larger diameter fan section using 22 hollow core fan blades. The PW4000-112 fan blade is a wide chord airfoil made of a titanium alloy, about long and about wide at the blade tip. A PW4000-112 fan blade can weigh a maximum of .
Incident
The flight departed SFO on time and the push back, taxi, takeoff, and climb were normal. There were three pilots on the flight deck: Captain Christopher Borzu Behnam (57), who was the pilot monitoring, First Officer (FO) Paul Ayers (60), who was the pilot flying, and a jump seat rider, who was off-duty United Airlines 777 First Officer Ed Gagarin. The captain reported a total of 13,592 hours total time, with 360 hours in the Boeing 777. The first officer reported a total of 11,318 hours total time, with 10,087 in the Boeing 777.
At the time of the fan blade out engine failure event, 11:58 Hawaiian standard time (HST), the flight was about from HNL at flight level (FL) 360 (roughly ) when there was a violent jolt and very loud bang that both pilots stated was followed by extreme airframe vibrations. The pilots reported that immediately after the jolt and loud bang, the autopilot disconnected, and the airplane began to roll to the right. A positive exchange of controls was accomplished with the captain becoming pilot flying. The pilots stated that about 15 to 30 seconds after the jolt and loud bang, the Engine Indicating and Crew Alerting System (EICAS) showed that there was no engine pressure ratio (EPR), N1, or oil pressure. After completing the Severe Engine Damage checklist, the crew shut down and secured the right engine. The jump seat rider stated that after the right engine was shut down, the vibration subsided although the controllability of the airplane was not normal. The crew declared an emergency and began a drift down descent to FL 230 (roughly ).
The captain directed the jump seat rider to go back into the cabin to assess the condition of the engine. The jump seat rider noted that the engine was oscillating and that the cowling was missing. He took a video of the engine to show the captain and the FO the engine's condition. The pilots reported that concurrently, the purser had come to the flight deck and the captain briefed her about the emergency and that they would be landing at HNL. The crew decided the most suitable airport in time, distance, and familiarity was HNL. The airplane continued to HNL and made a visual approach and landed on Runway 8R without further incident. The pilots stated that the aircraft rescue and firefighting (ARFF) personnel inspected the airplane and when the airplane was determined to be safe, they taxied the airplane to the gate where the passengers deplaned normally. The 363 passengers, 3 pilots, and 12 flight attendants board deplaned normally at the gate and there were no injuries.
Cockpit voice recorder timeline
Selected events from the CVR (all times HST).
11:58:27 Sound of bang. 11:58:58 The flight crew told flight attendants to take their seats. 12:00:27 United 1175 declared mayday. 12:05:28 The captain noted a lot of vibration on the controls. 12:05:48 The captain asked the jumpseat occupant to go into the cabin and visually inspect the engine. 12:07:47 The jumpseat occupant returned and reported the entire outer case of the engine was gone. The captain wondered if debris had struck the stabilizer due to the vibration on the controls. 12:08:10 The captain asked the jumpseat occupant to go back again and take a couple pictures of the damage. 12:10:37 The first officer reported the situation to dispatch. 12:17:41 The crew discussed crossfeeding fuel and decide to wait until passing 10,000 feet. 12:21:05 The crew discussed a flaps 20° approach at about 145 knots 12:27:50 The captain briefed flight attendants on the situation. 12:29:51 The crew initiated fuel crossfeed. 12:30:36 The crew briefed arrival procedures into Honolulu. 12:34:00 The crew reported Honolulu Airport in sight. 12:34:20 The crew lowered landing gear. 12:36:12 The crew finished the landing checklist. 12:37:15 The aircraft touched down. 12:37:34 The crew told passengers to remain seated. 12:38:55 The crew asked ARFF to visually inspect the engine for leaks and risks of fire. 12:41:57 ARFF reported a minor hydraulic leak. The crew stated their intention to taxi to the gate. 13:02:34 The aircraft reached the gate and the crew performed the engine shutdown checklist.
Investigation
The examination of the airplane revealed a small hole along with several dents and gouges in the fuselage adjacent to the No. 2 engine. There were two small dents and punctures in the right side of the fuselage, below the window belt in the vicinity of seat rows 20 and 21. Subsequent laboratory examination of the skin surrounding the puncture found embedded particles of largely titanium and vanadium, which along with aluminum are the alloying elements of the fan blade material. There were also several dents in the right wing and the right-hand horizontal stabilizer.
The majority of the right engine inlet assembly was missing. All the inlet lip skin, the forward bulkhead, most of the inner and outer barrels, and about half of the rear bulkhead were not recovered. The majority of both inner and outer halves on the fan cowl were also missing. The missing parts were lost at sea. The left and right side thrust reversers, and the exhaust cowl were in place and intact.
Engine
There was extensive damage to the interior surface of the fan case in the form of gouging and cracking. Although there were cracks in the case and the outer layer of the Kevlar wrap was split, there was no penetration of debris. The maximum deformation of the Kevlar environmental wrap was about at about 3 o'clock. The location of the maximum deformation of the wrap was coincident with the approximately long crack on the inside of the fan case.
Interior surfaces of the fan case and the remains of the inlet duct showed scratches and gouges that were in a spiral pattern across the fan case and front flange onto the inlet duct to the broken edge of the duct on the inboard area of the nacelle. The examination of the fan case showed that there were three distinct patterns of tracks along the flow path that appeared to spiral forward from the plane of the fan blades' leading edge across the A-flange on to the inlet duct's inner barrel.
Fan blade No. 11 was fractured transversely across the airfoil directly above the fairings that are between the base of each blade. The blade's fracture surface was flat with elliptical-shaped marks across the internal ribs and along the convex surface of the airfoil. The other fan blade, which was identified as fan blade No. 10 and was the adjacent trailing blade, was fractured across the airfoil about above the fairings. Laboratory examination of fan blade No. 11 revealed a low cycle fatigue (LCF) fracture that originated on the interior cavity wall directly below the surface.
The metallurgical examination of the fractured fan blade was accomplished at P&W's Materials Laboratory in the presence of members of the Powerplants Group as well as an NTSB metallurgist. The examination revealed a fatigue fracture that had initiated from a subsurface origin on the interior surface of the hollow core fan blade. The origin of the crack was in an area where the basal planes of the crystals were all similarly aligned and were almost perpendicular the localized stress field when the fan blade was formed. The examination also revealed that the fan blade's material conformed to the specified titanium alloy's requirements.