United B772 at Denver on Feb 20th 2021, engine inlet separates from engine, engine fire

Last Update: March 5, 2021 / 18:46:25 GMT/Zulu time

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Photo of United N772UA, Boeing 777-200 — United N772UA, Boeing 777-200 (Photo credit: InSapphoWeTrust / Flickr / License: CC by-sa)

Incident Facts

Date of incident
Feb 20, 2021

Classification
Incident

Airline
United

Flight number
UA-328

Departure
Denver, United States

Destination
Honolulu, United States

Aircraft Registration
N772UA

Aircraft Type
Boeing 777-200

ICAO Type Designator
B772

A United Boeing 777-200, registration N772UA performing flight UA-328 from Denver,CO to Honolulu,HI (USA) with 229 passengers and 10 crew, was in the initial climb out of Denver's runway 25 when the right hand engine's (PW4077) inlet separated associated with the failure of the engine. The crew declared Mayday reporting an engine failure. The aircraft stopped the climb at about 13000 feet, the crew requested to return to Denver after running the checklists. ATC offered any runway, they would make it happen. The aircraft returned to Denver for a safe landing on runway 26 about 23 minutes after departure. The aircraft stopped on the runway for a check by emergency services. Emergency services advised of an active fire within the right hand engine and extinguished the fire a few minutes later. The aircraft was subsequently towed off the runway to a remote parking stand, where passengers disembarked and were bussed to the terminal. There were no injuries.

A replacement Boeing 777-200 registration N773UA reached Honolulu as flight UA-3025 with a delay of 6 hours.

Beyond the damage to the right hand engine itself the aircraft received a puncture of the right hand wing root fairing below the right hand wing.

The engine inlet fell into the neighbourhood of Broomfield,CO, located about 16nm west of Denver near 13th and Elmwood Street, the debris also struck through the roof of an adjacent house.

Broomfield police reported that although debris impacted the neighbourhood and damaged a number of homes, there were no injuries on the ground. The debris field expands over a nautical mile.

Ground observers reported hearing the sound of an explosion like bang, smoke and saw the debris falling down. The aircraft continued flying.

Another ground observer, John Sloan (also see reader comments below), reported: "I was walking home from lunch and heard a boom. I thought it might be a fighter going supersonic, but when I looked up I noticed a 2-engine commercial plane on a roughly west-bound heading. There was a fairly low cloud deck (maybe a few thousand feet) and he was just below it, so it was difficult to discern visual details, but I suddenly heard his engine noise go from silent to moderately loud (that might have been what made me notice him) - not sure if he applied power or if it was a trick of acoustics. Looking behind him, there was what looked like a large black puff of smoke (but it might have just been clouds). Similarly, it looked like there was a thin trail of black smoke coming from the starboard engine, but again, it was hard to make out and might have just been a contrail. As he passed overhead the smoke trail seemed to go away. Once past me, he might have begun a turn to port (back towards the airport), but he vanished into the clouds so it was hard to tell. It looked like the aircraft was under control the whole time it was visible." The following day John Sloan wrote in his reader comment: "Two things: 1) I was giving Simon a heads up to look for a report out of DEN - if it was anything, I thought it was just something like a (big) compressor stall. That was why so many caveats were there - I wasn't sure at the time that the aircraft had had a problem. I didn't realize it was going to be widely reported until my mother texted me a couple of hours later asking if any of the plane parts came down close to me. 2) Something I omitted that I should have put in: After I got home I imagined how many thumb-widths at arm's length it would take to occlude it (from memory), and plugged that into similar triangles and the length of a 737 (didn't realize it was a 777) to get an estimated altitude and got 2-4 thousand meters. I thought the cloud deck was only 2000 feet AGL or so, so I didn't believe the numbers. I did change "a couple" to "a few" though. Turns out it was ~8000 feet AGL. I should have believed more in the math!"

The FAA reported the aircraft experienced a right hand engine failure and is aware of debris on the ground along the flight path of the aircraft. The FAA as well as the NTSB have opened investigations, the NTSB is leading the investigations.

On Feb 21st 2021 the FAA Administrator stated that an Emergency Airworthiness Directive (EAD) is to be prepared, that requires immediate or stepped up inspections of Boeing 777 aircraft with certain PW4000 engines likely resulting in a number of aircraft being removed from service. The Administrator said: "We reviewed all available safety data following yesterday's incident. Based on the initial information, we concluded that the inspection interval should be stepped up for the hollow fan blades that are unique to this model of engine used solely on Boeing 777 airplanes."

Late Feb 23rd 2021 the FAA released their Emergency Airworthiness Directive EAD-2021-05-51 for all PW4074, PW4074D, PW4077, PW4077D, PW4084D, PW4090, and PW4090-3 model turbofan engines. The FAA argues: "This emergency AD was prompted by the in-flight failure of a 1st-stage low-pressure compressor (LPC) blade on a PW4077 model turbofan engine resulting in an engine fire during flight. This condition, if not addressed, could result in 1st-stage LPC blade release, damage to the engine, and damage to the airplane." The EAD requires operators to "Before further flight, perform a thermal acoustic image (TAI) inspection of the 1st-stage LPC blades for cracks using a method approved by the FAA." If any 1st-stage LPC blade fails the inspection, remove the blade from service and replace with a part eligible for installation before further flight.

The NTSB stated they have opened an investigation into the occurrence, "Denver-based NTSB investigators are responding".

On Feb 21st 2021 the NTSB reported a senior NTSB investigator as well as three further NTSB investigators, all four based in the Denver area, responded and began working with local law enforcement officials to coordinate the recovery of components that separated from the engine and fell into residential areas. The NTSB investigator in charge along with a powerplant specialist travelled from Washington to Denver on Feb 21st 2021. The NTSB wrote:

The initial examination of the airplane indicated most of the damage was confined to the number 2 engine; the airplane sustained minor damage. The examination and documentation of the airplane is ongoing.

The initial examination of the Pratt & Whitney PW4077 engine revealed:

- The inlet and cowling separated from the engine
- Two fan blades were fractured:
+ One fan blade was fractured near the root
+ An adjacent fan blade was fractured about mid-span
+ A portion of one blade was imbedded in the containment ring
+ The remainder of the fan blades exhibited damage to the tips and leading edges

Investigators continue to examine the engine, airplane and the photographs and video taken by passengers aboard United flight 328.

The cockpit voice recorder and flight data recorder were transported to the NTSB laboratory in Washington where each will be downloaded and analyzed.

On Feb 22nd 2021 the NTSB reported in a virtual media briefing that the right hand engine suffered the failure of one fan blade at about 12,500 feet MSL, the fractured blade caused an overload to the neighboured blade which fractured about mid span. Following the failure the crew pulled the fire handle and discharged both fire bottles. Fuel was turned off. The NTSB is looking into what caused the fire to continue. There was additional damage to a composite fairing, the damage was not structural. One of the blades was found in the soccer field on the ground. Following landing the crew assessed no emergency evacuation was necessary. Metal fatigue played a role and is being looked at. The engine's maintenance history is being reviewed. Additional debris was found Monday morning (Feb 22nd 2021) in Arvada about 20nm westsouthwest of Denver Airport. First inspections of the engine do not suggest that anything pierced the containment ring and engine case. At this point the NTSB does not consider the engine failure as uncontained, the containment ring did contain the fractured fan blades, the failure thus was contained.

On Feb 21st 2021 United, Japan Airlines and All Nippon Airways grounded their active Boeing 777 aircraft equipped with PW-40xx engines.

On Feb 13th 2018 a fractured and separated fan blade had caused the engine inlet and cowl to separate from the right hand engine of B772 registration N773UA, see Incident: United B772 over Pacific on Feb 13th 2018, fan blade, engine cowl and inlet separated in flight, blade debris impacted fuselage.

On Mar 5th 2021 the NTSB released an investigation update stating:

Facts gathered to date in the investigation, and provided in the update, include:

- Initial examination of the right engine fire damage found it was primarily contained to the engine's accessory components, thrust reverser skin, and composite honeycomb structure of the inboard and outboard thrust reversers.

- The spar valve, which stops fuel flow to the engine when the fire switch is pulled in the cockpit, was found closed – there was no evidence of a fuel-fed fire.

- Initial examination of the right engine fan revealed the spinner and spinner cap were in place and appeared undamaged.

- All fan blade roots were in place in the fan hub, two blades were fractured.

- One fan blade was fractured 7.5 inches above the base at the trailing edge. The fracture surface was consistent with fatigue.

- The second fractured blade exhibited indications of overload failure, consistent with secondary damage.

- Initial review of maintenance and inspection data for the blade with the fatigue fracture, revealed it had experienced 2,979 cycles since its last inspection. This blade underwent thermal acoustic image inspections in 2014 and 2016. Inspection data collected from the 2016 inspection was examined again in 2018 because of a Feb. 13, 2018, incident involving a Boeing 777 with Pratt & Whitney PW4077 engines.

The engine fan blade with the fractures consistent with fatigue was sent to the metallurgical laboratory at Pratt & Whitney for further examinations led by a senior NTSB metallurgist. Preliminary findings from the scanning electron microscope examination identified multiple fatigue fracture origins on the interior surface of a cavity within the blade. Efforts to further characterize the fracture surface, including identifying the primary origin and counting striations, are ongoing. Additional work is underway to further characterize secondary cracks identified through fluorescent penetrant inspection. The NTSB metallurgy group also plans to analyze the blade's chemical composition and microstructure near the fracture surface.

Related NOTAM (Japan FIR):
Q0396/21 NOTAMN
Q) RJJJ/QAFXX/IV/NBO/E/000/999/3310N14118E999
A) RJJJ B) 2102211257 C) 2105210830 EST
E) ALL AIR CARRIERS OPR(TAKING-OFF/LANDING AND OVER FLT) WI THE
TERRITORY OF JAPAN ARE REQUESTED TO AVOID USING BOEING 777 EQUIPPED
WITH PW4000 SERIES ENGINES UNTIL FURTHER NOTICE

Metars:
KDEN 202053Z 36019KT 10SM BKN100 BKN200 11/M04 A2967 RMK AO2 SLP031 T01111039 56036=
KDEN 201953Z 18005KT 10SM FEW085 SCT130 SCT200 13/M09 A2970 RMK AO2 SLP040 T01331094=
KDEN 201853Z VRB04KT 10SM FEW090 FEW200 12/M09 A2973 RMK AO2 PRESFR SLP052 SHSN OMTNS DSNT W-NW T01171089=
KDEN 201753Z 21015KT 10SM FEW070 FEW100 FEW200 11/M07 A2979 RMK AO2 SLP077 T01111072 10111 21044 58018=
KDEN 201653Z 21017KT 10SM FEW070 FEW100 FEW200 09/M07 A2982 RMK AO2 SLP091 T00891072=
KDEN 201553Z 21015G23KT 10SM FEW070 FEW100 FEW200 06/M07 A2984 RMK AO2 PK WND 20026/1513 SLP103 T00611072=