United B772 over Pacific on Feb 13th 2018, fan blade, engine cowl and inlet separated in flight, blade debris impacted fuselage

Last Update: February 23, 2021 / 11:08:37 GMT/Zulu time

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Incident Facts

Date of incident
Feb 13, 2018

Classification
Incident

Airline
United

Flight number
UA-1175

Aircraft Registration
N773UA

Aircraft Type
Boeing 777-200

ICAO Type Designator
B772

A United Boeing 777-200, registration N773UA performing flight UA-1175 from San Francisco,CA to Honolulu,HI (USA) with 363 passengers and 10 crew, was enroute at FL360 over the Pacific Ocean about 45 miutes prior to estimated landing in Honolulu when parts of the right hand engine's (PW4077) separated from the engine. The aircraft continued to Honolulu for a safe landing on runway 08R about 45 minutes later.

Passenger Maria Falaschi reported there was a loud bang about 45 minutes prior to landing followed by the whole aircraft shaking, she saw parts of the engine cowl at the engine inlet missing (and took photos, see below).

The NTSB have dispatched investigators on site and opened an investigation into the occurrence described as "in-flight loss of an engine cowling".

On Mar 6th 2018 the NTSB reported there had been an inflight separation of a fan blade and subsequent loss of the engine inlet and fan cowls of the right hand engine. Shortly after initiating the descent, at about FL327, the flight crew received a warning of an engine compressor stall, shut down the engine and proceeded to Honolulu without further incident. The aircraft received minor damage. The occurrence was rated an incident and is being investigated by the NTSB.

On Jun 30th 2020 the NTSB released their final report and their docket (links updated on Feb 23rd 2021 after major changes of the NTSB website) concluding the probable causes of the incident were:

the fracture of a fan blade due to P&W's continued classification of the TAI inspection process as a new and emerging technology that permitted them to continue accomplishing the inspection without having to develop a formal, defined initial and recurrent training program or an inspector certification program. The lack of training resulted in the inspector making an incorrect evaluation of an indication that resulted in a blade with a crack being returned to service where it eventually fractured.

Contributing to the fracture of the fan blade was the lack of feedback from the process engineers on the fan blades the inspectors sent to the process engineers for evaluation of indications that they had found.

The NTSB described the sequence of events:

At the time of the event, there were three pilots on the flight deck: the captain, who was the pilot monitoring, the first officer (FO), who was the pilot flying, and a jump seat rider, who was an off-duty United Airlines 777 FO.

The flight departed SFO on time and the push back, taxi, takeoff, and climb were normal. The flight was about 120 miles from HNL at flight level (FL) 360 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 instruments indicated a failure of the number 2 engine. After accomplishing the Severe Engine Damage checklist, the crew shut down and secured the engine. The jump seat rider stated that after the right engine was shutdown, 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. 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 what they were dealing with. 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 airplane continued to HNL and made a visual approach and landed on Runway 8R without further incident.

The NTSB analysed:

The airplane, a Boeing 777-222, experienced a full length fan blade fracture in the No. 2 (right) engine, a Pratt & Whitney (P&W) PW4077 turbofan, while in cruise flight shortly before top of descent. The examination of the No. 2 engine revealed most of the inlet duct and all of the left and right fan cowls were missing. 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 examination of the engine's fan blades revealed fan blade No. 11 was fractured transversely across the airfoil directly above the fairings that are between the base of each blade. The other fan blade, which was identified as fan blade No. 10 and was the adjacent trailing blade, was fractured across the airfoil at about midspan. 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 entire fan blade set, including fan blade No. 11 had last been overhauled by P&W's Overhaul & Repair (O&R) facility in July 2015. As part of the overhaul process, the blades underwent a fluorescent penetrant inspection (FPI) and a thermal acoustic imaging (TAI) inspection. The records for the TAI inspection in July 2015 as well as an earlier TAI accomplished in March 2010 revealed a thermal indication in the same location as where the LCF crack occurred. The records for the fractured fan blade's July 2015 TAI inspection was annotated 'paint' that, according to the inspector, was consistent with him accepting the indication because he thought it was an issue with the paint.

P&W developed the TAI inspection process in about 2005 to be able to inspect the interior surfaces of the hollow core PW4000 fan blade. P&W in keeping with NDI industry practice when implementing a new inspection process classified the TAI as a new and emerging technology and therefore did not have to develop a formal program for initial and recurrent training, certify the TAI inspectors, or have a Level 3 inspector on staff, as is done in other established NDI techniques. But in 2015, and still in 2018 when the incident occurred, P&W was still categorizing the TAI as a new and emerging technology after having inspected over
9,000 fan blades. At one point, P&W did provide training on the TAI, however, neither of the two inspectors were permitted to attend the training so that they could work to clear out a backlog of blades in the shop.

The TAI inspector who worked on the incident fan blade stated that they never got any feedback from the engineers about the blades that they had rejected. When they would reject a blade, it would go to an engineer for further evaluation. However, they never got any feedback from the engineers if the rejection was a valid rejection or if it was a false positive.

After it was determined that the two previous TAIs of the fractured fan blade showed thermal indications at the location of the fatigue crack, P&W initiated an over-inspection of all of the digital images of the TAIs accomplished on PW4000 112-inch fan blades.

Because the aluminum versus the CFRP structure has the ability to yield while absorbing the same amount of energy, it can redistribute the FBO loads between the fan case and the inlet without causing failure to the inlet, or the fan case to inlet interface. The inlet and fan cowl structural analyses showed that the CFRP aft bulkhead design was less capable than the aluminum bulkhead that was tested during engine certification test and determined that multiple possible scenarios could have led to their separation; 1) the inlet aft bulkhead load path damage caused by the unanticipated magnitude of the displacements induced by the displacement wave following the FBO combined with the anticipated inner barrel fragment induced damage progressed under rundown loads, resulting in portions of the inlet departing within one second following the FBO, 2) the departure of portions of the inlet including the lower aft bulkhead caused the static and/or dynamic loads to increase beyond the fan cowls capability, that lead to the departure of large portions of the fan cowl, 3) the fan cowl honeycomb core strength was reduced below its capability to react rundown loads due to moisture ingression at the hinge points leading to large portions of the fan cowl departing prior to the inlets departure.
Incident Facts

Date of incident
Feb 13, 2018

Classification
Incident

Airline
United

Flight number
UA-1175

Aircraft Registration
N773UA

Aircraft Type
Boeing 777-200

ICAO Type Designator
B772

This article is published under license from Avherald.com. © of text by Avherald.com.
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