B772, vicinity Denver USA, 2021
B772, vicinity Denver USA, 2021
On 20 February 2021, a PW4077-powered Boeing 777-200 was climbing through 12,500 feet after departing Denver when a sudden right engine fan blade failure occurred leading to an engine fire which could not be fully extinguished until after landing. The Investigation found that the aircraft operator’s blade inspection process had been deficient and that the engine manufacturer’s prescribed blade inspection intervals were inadequate with both contributing directly to the in-service failure. It found that the design and testing of both the engine inlet and the main gearbox support structure contributed to the severity of the engine damage following the fan blade failure.
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 in day VMC when sudden failure of the right engine was followed by an associated fire which could not be fully extinguished. Engine debris fell over 40 acres of a residential area including large pieces which damaged a vehicle and residential property but there were no injuries. An emergency was declared with a return to land at Denver 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 Accident Investigation was carried out by NTSB and the CVR and FDR were removed from the aircraft and their relevant data successfully downloaded. An initial ‘Investigative Update’ was released on 5 March 2021 prior to the subsequently complex investigative work.
The 60 year-old Captain had a total of 28,062 hours flying experience including 538 hours on type of which 414 hours had been within the previous 12 months. The 53 year-old First Officer had a total of 18,612 hours flying experience including 4,190 hours on type of which 355 hours had been within the previous 12 months.
What Happened
When cleared to climb to FL230 and passing 12,500 feet at around 280 KCAS approximately four minutes after takeoff from runway 25 at Denver, a selected thrust increase made in order to minimise time exposed to expected 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” that 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.
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.
A still image from a passenger in-flight video showing engine nacelle damage and the undercowl fire about 11 minutes after the fan blade separation. [Reproduced from the Official Report]
Initial examination of the right engine found that one fan blade was missing and that part of an adjacent one was also missing (see the illustration below). It was found that the observed fire 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 it was 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. However, examination of the engine accessories found multiple severed fuel, oil and hydraulic lines and that the gearbox was fractured.
A front view of the two fractured fan blades and damage to the remaining intact blades. [Reproduced from the Official Report]
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 Imaging (TAI) inspection intervals to 1,000 cycles for first stage 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.
Why It Happened
A more detailed examination of the remains of the fire-damaged right engine confirmed that the initiating event had been the separation of a fan blade with the resulting debris successfully contained by the fan case. An evaluation of damage to the nacelle inner and outer barrels found that “the displacement wave of the impact had resulted in a deflection of the fan case and contact with the nacelle doors and hinges”. This then led to failure of the inlet aft bulkhead and the fan cowl support beam and the failure of this bulkhead combined with the damage to the inner and outer barrels had resulted in these structures and the inlet lip skin to separate from the engine. The air loads which followed the separation of the inlet then caused the separation of the fan cowls and the fan cowl support beam.
In respect of the initiating fan blade failure, Scanning Electron Microscope (SEM) examination identified “multiple fatigue fracture origins on the interior surface of a cavity within the blade” prompting further detailed investigative work. The completely separated fan blade was eventually determined to have fractured as the result of a fatigue crack which had “originated at the surface of an internal radius in a hollow cavity within the blade”. This blade had accumulated only 2,979 cycles since overhaul which when the event occurred was required every 6,500 cycles. During such overhauls, blades were inspected for both external and internal cracks using a proprietary Thermal Acoustic Imaging (TAI) process. The most recent such inspection of the failed fan blade was found to have occurred in 2016 when inspection imagery had revealed multiple low-level indications, two of which were in the same area where the eventual failure had originated but were incorrectly interpreted as being generated by “camera sensor noise or loose contamination within the cavity”. Given the observed indications and the inspection criteria in place at the time, the Investigation concluded that either the blade should have received a second TAI inspection or the images should have undergone a team review but “there was no record that either of these occurred” and the blade was approved for continued service.
After a further in-service fan blade failure of a PW4077 engine on another United Airlines Boeing 777-200 in February 2018, the data from the previous inspection of the blade involved in the 2016 event were reviewed but again the indications “were not identified as anomalous and the blade continued in service”. The Investigation found that two of the findings of the 2016 TAI inspection were likely to be associated with the fatigue crack that eventually resulted in the 2021 blade failure by which time the failed blade had accumulated “less than a quarter of the expected life for a nominal blade” and only 2,979 cycles since its 2016 overhaul, the latter being less than half the prescribed inspection interval at the time.
Metallurgical examination during the current Investigation “identified two conditions which contributed to the reduced fatigue life of the accident blade - a surface carbon contamination and a geometric discontinuity that occurred during manufacturing". It was estimated that two thirds of the reduced fatigue capability has resulted from the surface carbon contamination and one third from the increased stress from the geometric discontinuity.
In respect of the secondary consequences of the fan blade failure, simulation studies indicated that the CFRP honeycomb structure of the engine inlet and inlet aft bulkhead had been unable to dissipate and redistribute the energy of the loads imposed by the fan blade failure in the same way as the aluminium inlet that was used during certification tests had done. It was noted that separation of the inlet and fan cowls when a fan blade failure occurs is not permitted by the applicable certification standards.
Closing Action taken on engine fan blade integrity
Pratt & Whitney issued an SB introducing Ultrasonic Testing (UT) of the fan blades at regular intervals and a requirement for TAI inspections every 1,000 cycles instead of every 6,500 cycles. This SB was mandated by FAA AD 2022-06-09 which was issued on 15 April 2022. In addition, as the new UT inspection had demonstrated the ability to detect small fan blade cracks not detectable by TAI inspections, blades are now also being inspected by UT every 275 cycles. It was concluded that the combination of increased frequency TAI inspections and the new UT inspections should provide “multiple opportunities to detect cracks in the high-stress areas” of the engine.
Closing Action taken on engine failure containment
Boeing modified the design of the engine inlet to ensure that inlets and fan cowls remain in place during a fan blade failure that may damage the aft bulkhead, inner barrel, or outer barrel by incorporating additional metallic structure. Boeing also developed procedures for inspection and repair for moisture ingression damage to the fan cowls which can degrade the strength of the cowls. These modifications were then mandated by FAA ADs 2022-06-10 and 2022-06-11 also issued on 15 April 2022. However following completion of the Investigation, further modifications to the fan cowl are now envisaged noting that the investigated event was the fourth in-service fan blade failure due to fatigue cracking recorded for PW4000-powered Boeing 777s and had resulted in the most nacelle damage of the four events. The first event in 2010 had led to the release of approximately 50% of the blade with full-span separations then following in 2018, 2021 and during this event.
The Probable Cause of the event was determined as “the fatigue failure of the right engine fan blade”.
Contributory Factors in respect of the event were identified as:
- The inadequate inspection of the fan blades, which failed to identify low-level indications of cracking.
- The insufficient frequency of the manufacturer’s fan blade inspection intervals, which permitted the low-level crack indications to propagate undetected and ultimately resulted in the fatigue failure.
- The design and testing of the engine inlet, which had failed to ensure that the inlet could adequately dissipate the energy of, and therefore limit further damage from, an in-flight fan blade loss event.
- The failure of the “K” flange following loss of the fan blade which allowed hot ignition gases to enter the nacelle and impart damage to several components that fed flammable fluids to the nacelle and allowed the fire to propagate past the undercowl area and into the thrust reversers, where it could not be extinguished.
The Final Report of the Investigation was published on 7 September 2023.