Saturday, February 20, 2021

Boeing 777-222, N772UA: Incident occurred February 20, 2021 in Denver, Colorado

National Transportation Safety Board Number: DCA21LA085
Status:  In work
Event Date:  02/20/2021
Location:  Denver, Colorado
Federal Aviation Administration / Flight Standards District Office; Denver, Colorado

Aircraft departed and declared emergency due to right engine failure. 

Date: 20-FEB-21
Time: 20:20:00Z
Regis#: N772UA
Aircraft Make: BOEING
Aircraft Model: 777
Event Type: INCIDENT
Highest Injury: NONE
Aircraft Missing: No
Flight Phase: TAKEOFF (TOF)
Operation: 121
Aircraft Operator: UNITED AIRLINES
Flight Number: UAL328

The midair breakdown of a jet engine that spewed a trail of metal over a Colorado town is putting regulatory focus on the design and strength of engine coverings.

Four minutes into the Honolulu-bound United Airlines flight from Denver on Saturday, pilots and passengers heard a loud bang, and the plane’s right engine began vibrating, according to the National Transportation Safety Board’s preliminary analysis of the cockpit’s voice recorder. Within 20 minutes, the plane had landed on the runway back at Denver International Airport, with no injuries to anyone on the ground or aboard the plane.

However, large pieces of debris plunged from the sky, landing in yards and a soccer field. Photos and videos taken by passengers show the airplane’s right engine, stripped of its front coverings, oscillating and in flames.

Investigators have homed in on a fan blade that had weakened over time and broke off at its base. That fast-spinning blade remnant appears to have sheared a second blade roughly in half. But investigators are still trying to understand why the engine’s front external cover, known as the cowling, was then ripped away, according to NTSB Chairman Robert Sumwalt. Something similar happened in a handful of other engine failures caused by broken fan blades in recent years.

“Certainly, we don’t expect the cowling to separate like that,” Mr. Sumwalt said this week, during a briefing with reporters. He added that the safety board’s investigators would examine the engine’s maintenance and inspection records.

Such damage to external engine sections isn’t supposed to happen under current manufacturing and inspection systems, which have ushered in an era of record airline safety in the U.S. and globally. The incident raises concerns about whether engine coverings are too vulnerable to damage from engine components flying forward at great speed, in addition to questions about why the fan blades broke, some safety experts said.

Alan Diehl, a former U.S. aviation accident investigator, said investigators and regulators will need to think about the integrity of the structures that house aircraft engines and how they are designed and certified. If one were to fail, that can pose a danger to the rest of the aircraft. “They’re going to have to reexamine that,” he said.

Concerns about engine coverings also came to the fore after a Southwest Airlines flight in April, 2018, when remnants of engine blades broke off and battered front engine coverings. When airflow and severe vibration then broke up the damaged cover, fragments were hurled into the wings and body of the Boeing 737. One piece blasted out a cabin window, partially sucking out the passenger.

After that deadly Southwest engine failure, then-acting Federal Aviation Administration chief Dan Elwell said the agency would conduct a broad safety review focused on potential threats from structural failures of front engine coverings. Airborne breakups of such ancillary parts typically haven’t been considered in safety analyses or mandatory certification standards for modern jet engines.

In responses last year to recommendations the NTSB made after the Southwest incident, FAA Administrator Steve Dickson said the agency was working with Boeing Co. on a design change for the type of engine involved in the incident, and was reviewing design and certification standards for engine casings to determine whether broader changes were needed beyond that engine type.

The FAA is working on additional responses to the NTSB, a spokesman said Tuesday.

The engine failure Saturday was the third in a string of strikingly similar incidents involving the same type of Pratt & Whitney-made engine on Boeing 777 aircraft. In all three cases, a fan blade showing signs of fatigue largely broke off, and a second blade was also fractured. In all three cases, damaged engine coverings broke off.

Pratt & Whitney, a unit of Raytheon Technologies Corp., declined to comment Tuesday. On Sunday the engine maker said it was working with regulators and investigators.

The FAA said it had been in the process of evaluating a Dec. 4 engine failure on a Japan Airlines flight to Tokyo when the weekend incident happened.

A shielding ring surrounding the engine is designed to prevent rapidly spinning internal parts from shooting through the engine’s outer casing.

Ray Valeika, a former maintenance and engineering chief at Delta Air Lines Inc., said the prospect that a broken fan blade can shoot forward out of the engine rather than through the sides of the casing necessitates a new approach to how these elements are designed and approved by regulators. It is possible, he said, that more-efficient fan blades aren’t behaving in the way that regulators expected when the testing systems and regulations were designed.

“We have to think differently about containment,” he said. “I think there needs to be a design review and a fault analysis review to see if we need to change the design criteria,” he said.

Engines are tested to make sure they can survive if a fan blade breaks off. But those tests differ from conditions during flight in key ways, according to a safety board report following a 2018 engine failure on another United flight to Hawaii, also linked to a weakened fan blade that broke off. In that incident, fan blade fragments spiraled forward and engine covering pieces broke off.

During the tests, an inlet was attached to the engine, but it wasn’t made of the same material as the inlet used on the aircraft, the NTSB report said. And coverings that surround the fan aren’t required to be on during the tests, according to that report.

The NTSB’s report last year didn’t recommend changes to the testing procedure, and the safety board didn’t respond to questions Tuesday about whether it will advise changes now.



  1. Video taken looking out the 777 window:

    1. Here is video from the ground that shows a black cloud from the sudden anomaly. Blew the cowling off, apparently.

    2. The twitter account limits access on who can view that video, so not all can see that, especially those without a twitter account.

      Here's a version on YouTube:

  2. Fairing opening for access video:

  3. CNN is funny at this point.... they are playing the mayday call and the announcer things that "United 328 heavy engine failure" is "heavy engine failure" rather than "United 328 heavy, engine failure."

  4. CNN is nothing but fake news. They don’t have a clue to what they are talking about. They are programmed to report their narrative and slant that information anyway they want to. They will probably blame climate change or President Trump this incident. MANY KUDOS TO THE FLIGHT and CABIN CREW for a suburb job in handling this situation.

    1. That's CNN for ya. Remember their "The 777 will struggle to stay in the air with no fuel" remark a few years ago.

  5. Good thing that engine didn't fail halfway over the Pacific. Don't know if it would have made it.

    1. It's certified for at least 3 hours of single engine operations, so it'd be interesting, but otherwise a non-event.

    2. The vibration from the windmilling unbalanced rotor was stress testing the pod mount in fatigue mode. Three hours of that and the galley would run out of complimentary adult beverages.

  6. This video shows missing blade view in the first 20 seconds:

  7. suggest crew was aware of the engine issue prior to the explosion ...
    "out for a walk with his family when he noticed a large commercial plane flying unusually low and took out his phone to film it.

    “While I was looking at it, I saw an explosion and then the cloud of smoke and some debris falling from it."

    1. N772UA was not any lower on the incident day than when that same aircraft and flight number passed over Broomfield on 14 February.

      Turn on speed/altitude ADS-B labeling by clicking the "K" tab in those two tracks below and compare the data points as it passed by Broomfield.

      20 Feb N772UA incident flight:

      14 Feb N772UA normal flight:

  8. Two photos below show that only the forward cowling blew off initially, then fire burned through the cowling portion behind it:

    Early photo before burn through:

    Later photo with burn through underway:

  9. NHK news is reporting that the Japanese transport ministry has issued an order to halt domestic operation of Boeing 777 aircraft following the United Airlines flight 328 engine failure.

    On December 4, 2020 Japan Airlines 777-289 JA8978 (flight JAL/JL904) suffered engine failure six minutes after takeoff. Broken fan blades similar to the 2018 N773UA failure (NTSB DCA18IA092) were noted. All Nippon Airways and Japan Airlines have fleets of 19 and 13 of the 777-200 series jets.

    Photos and text (in Japanese) of the JA8978 failure in JTSB press release:

  10. Ramp photos of hole found in unpressurized lower fuselage N772UA:

    Wide view:

    Close view:

  11. These engines are marvels of aerospace engineering that we tend to take for granted. A second marvel is that the Denver plane was able to land safely with no physical harm to passengers, crew or anyone on the ground. The disturbing part of the story was the human failure to inspect engines properly.

    1. I respectfully disagree. The most disturbing part of the story is that there is obviously a fundamental design and/or manufacturing defect in these “marvelous” engines which PW has apparently not addressed directly but just transformed it into a sloppy inspection problem.

    2. Only a person who is unaware of the 2018 N773UA failure investigation findings would claim "fundamental flaw" in 9,606 blades that have been in service since the mid 90's.

      The difficulties and shortcomings related to inspection of those blades is the story to go read about.

    3. In 2000, over 2,000 PW4000 engines had accumulated over 40 million hours of service with 75 operators. In 30 years between June 1987 and 2017, more than 2,500 engines have been delivered, logging more than 135 million flight hours.

      Now about your car- no problems driving the same model for 30 years and running it every day at the local 1/4 mile drag strip before driving to work ?

      And simply checking it every 5 years or so ?

  12. NTSB Report from the 2018 N773UA failure (NTSB DCA18IA092):

    Powerplants Group Chairman's factual report:

  13. In pictures you can see one of the fan blades missing. This is a 112" diameter fan meaning each fan blade is ~50" long. I believe this fan spins at ~4000 RPM at maximum power output so when one of them 'comes undone' at speed, the 'check underpants' light starts flashing pretty quick. It will be interesting to see the forensic metallurgy results on the blade components. Most likely a fatigue failure. So very thankful the fugitive blade went a different direction than the passenger compartment.

  14. My house and neighborhood are directly over one of the departure climb out waypoint paths of Atlanta's Hartsfield-Jackson. This has always been my nightmare scenario as they are accelerating and climbing some 7-10,000' overhead. I can hear freight dogs like Kalitta Air 744s and international flight 777s on climb power in the late night/early morning if I'm still awake (and check the Flightaware app to see what it is). This area is very fortunate - and United specifically - there were no ground injuries or deaths. Yikes.

  15. No panic necessary. This is what FAA does. This is why there are two engines on these aircraft. And over time any design weakness will reveal itself. The mfg, Boeing and FAA will slice and dice this thing down to the serial number, date and location and who was on the clock during final assembly and the last inspection cycle. Yes, those kinds of records are kept. The last time it was a metallurgical issue with fan blades on the Rolls Royce engine. Determined it to be part of a batch run, date specific, and with each blade having a serial number, all were replaced. There was microscopic contamination causing the vibration which, at those rpm’s, led to failure. Not noticeable by visual inspection, which when an engine is in service is usually how they are inspected in each maintenance cycle [to the best of my knowledge, I only talk to these mechanics, I’m not one of them]. This is what the industry designs for and the detail of tracking will result in a fix, or we wouldn't pilot them.

    1. The PW4000 and its contemporaries at GE and RR were the first engines to really heavily utilize composites and advanced alloys, as well as innovative machining techniques producing hollow and curved fan blades. These engines are now getting on 20-25 years. We really don't have a lot of data on reliability and failure frequency of these first -generation advanced components. But three uncontained failures on PW4000s in as many years is a red flag.

  16. What are the chances that the blade debris will be found? I'm surprised they haven't offered a serious reward for it.

    1. They don't need the portions that departed because the base remnant of the missing blade and the lower portion of the second blade are still in their running position and the fracture faces can be seen.

      The NTSB reporting of the 2018 N773UA blade failure linked a few comments above provides all the details from that investigation. Changes were made after N773UA to improve blade inspection. The Denver blade failure investigation is likely to focus mostly on blade inspection history and process after the basic fracture failure is compared to the N773UA incident.

  17. Glad all parties are safe, WoW.. What a Failure that was. Guess their going to need more than a Broom in Broomfield, Sorry i couldn't resist...

  18. Boeing has had its problems but this isn't really one of them. Boeing doesn't build or maintain engines and the customers tell Boeing what engines to install. Of course, Boeing is the integrator and has bottom line responsibility. Unfortunately, most people don't know that. Most engines are made by GE or Rolls Royce or Pratt and Whitney.