Description

On 20 February 2021, a Boeing 777-200 (N772UA) being operated by United Airlines on a scheduled domestic passenger flight from Denver to Honolulu as UA 328 had been airborne for about four minutes and was in day VMC when there was an uncontained failure of the right engine following which the associated fire did not fully extinguish in response to the prescribed drill. Large pieces of engine debris fell on a populated area of the city but there were no injuries. An emergency was declared and a return to land at Denver was followed by the engine fire being fully extinguished before the aircraft was towed off the runway and the 239 occupants were disembarked. The airframe sustained only minor damage. 

Investigation

An Investigation is being carried out by the NTSB. The CVR and FDR were removed from the aircraft and their relevant data were successfully downloaded.

What Happened

With the Captain acting as PF approximately four minutes after taking off from runway 25 at Denver when climbing through approximately 12,500 feet for FL 230 at around 280 KIAS, a selected thrust increase made in order to minimise time exposed to turbulence was immediately followed by a loud bang and the uncommanded shutdown of the right engine. An engine fire warning was annunciated soon afterwards and an emergency was declared to ATC advising the flights intention to return to Denver for an emergency landing. 

The engine fire checklist was commenced but although both fire shots were activated, the engine fire warning did not subsequently extinguish until the aircraft was on an extended downwind leg for landing. Additional “critical checklists” were then completed and performance for an overweight landing (without fuel dumping for “safety and time reasons”) checked. The Captain completed the single engine approach to and landing on runway 26 without further event. The aircraft was brought to a stop on the runway and the left engine was shut down. Airport RFFS vehicles attended and applied water and foaming agent to the right engine, which had experienced a “flare up” which was quickly extinguished. Once the RFFS had confirmed it was safe to do so, the aircraft was towed clear of the runway and the occupants were disembarked via air stairs and bussed to the terminal.

Initial examination of the remains of the fire-damaged right engine (see the illustrations below) found that it had been mainly confined to “the engine accessory components, the thrust reverser skin and the composite honeycomb structure of the inboard and outboard thrust reversers”. It appeared that both halves of the aft cowl were “intact and undamaged” and noted that all four pressure relief doors were in the open position. The spar valve, which stops fuel flow to an engine when its fire handle is pulled in the flight deck, was found in the closed position with no evidence that there had been a fuel-fed fire once it was shut. An examination of the engine accessories found multiple severed fuel, oil and hydraulic lines and the gearbox was fractured.

Fire damage to the outboard side of the right engine. [Reproduced from the Official Report]

An immediate operation to recover all the fallen engine debris, which included some large pieces of metal, some of which had landed very close to occupied residential property and onto public amenity areas, was commenced by local law enforcement and safety agencies accompanied by locally-based NTSB personnel and continued for several days. Eventually, most of the ejected fan cowl, fan cowl doors and other fallen debris was recovered, laid out in a hangar and identified. 

Fire damage to the inboard side of the right engine. [Reproduced from the Official Report]

It was noted that all recovered debris will be subject to further examination to establish the cowl failure sequence after the fan blade failure and to examine the subsequent progression of fire in the thrust reversers.

An initial examination of the right engine fan found that the spinner and spinner cap were in place and appeared to be undamaged (see the illustration below) but although the fan hub was intact, it could not be rotated by hand. All the fan blade roots were in place in the fan hub but two adjacent blades were fractured. One of these had fractured transversely across the aerofoil about 13 cm above the base of the blade at the leading edge and about 19 cm above the base of the blade at the trailing edge. This blade fracture surface was assessed as consistent with fatigue. A second fan blade was also fractured transversely across the airfoil about 66 cm above its base at the leading edge and very slightly less distance from its base at the trailing edge. The fracture surfaces of this second blade were found to have “shear lips consistent with an overload failure”. All the other fan blades were full length but all had, to varying extent, impact damage to their aerofoils.

A front view of the damaged engine. [Reproduced from the Official Report]

All fan blades were removed from the hub and the blade which was considered to show fracture consistent with fatigue was sent to OEM metallurgical laboratory for further examination led by a senior NTSB metallurgist. The preliminary findings from a Scanning Electron Microscope (SEM) examination identified “multiple fatigue fracture origins on the interior surface of a cavity within the blade” (see the illustration below). Work to further characterise the fracture surface and identify the primary origin of the failure are ongoing.

The fan blade fracture surface with the suspected origin of the fatigue indicated. [Reproduced from the Official Report]

Fluorescent penetrant inspection subsequently identified multiple secondary crack indications within 2.5cm of the fracture surface in the same cavity from where the fatigue failure had originated from and further SEM examination confirmed these to be potential secondary cracks. Work is ongoing “to further characterise the size and depth of these secondary cracks before attempting to open at least two of them for further examination” and the chemical composition and microstructure near the fracture surface will also be analysed. 

As an immediate response to the event under investigation, it was noted that two days after it had occurred, Pratt & Whitney had issued “Special Instruction 29F-21” detailing a reduction in the Thermal Acoustic Image (TAI) inspection intervals to 1000 cycles for first stage Low Pressure Compressor (LPC) blades on the engine variant involved. The following day, the FAA issued EAD 2021-05-51 requiring that Pratt & Whitney PW4077 and similar type engines should, before further flight, be subject to a TAI inspection of the first stage LPC blades for cracks and the immediate removal from service and replacement of any blade which does not pass this inspection.

An initial review of maintenance and inspection data for the suspect blade found that it had accumulated 2,979 cycles since its most recent inspection and had undergone TAI inspections in 2014 and 2016. It was noted that the TAI inspection data from 2016 was examined again in 2018 because of a 2018 in flight fan blade failure in another PW4077 engine installed on another United Airlines Boeing 777-200 which was attributed to Pratt and Whitney’s failure to adequately specify fan blade integrity checking procedures. The Investigation intends to review the 2016 inspection records to examine the presence and disposition of any anomalies in their TAI data in the vicinity of the identified fracture initiation point.

An Investigative Update detailing initial findings from the Investigation and on which this summary is based was released on 5 March 2021. 

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