Pratt & Whitney Contained Engine Failure

26 Feb 21 14 Comments

On the 20th of February 2021, a Boeing 777-200, flying as United Airlines flight UA328, suffered an engine failure shortly after take off from Denver International Airport for a scheduled domestic flight to Honolulu, Hawaii. There were 229 passengers and ten crew on board.

United Airlines was the launch customer for the Boeing 777 when the company placed an order for 34 Pratt & Whitney-powered aircraft in 1990 to replace its aging DC-10s. The accident aircraft, registration N772UA, was the fifth Boeing 777 ever produced, built in 1994. It is powered by two Pratt & Whitney engines model PW4077.

About four minutes after take off from Denver, climbing through 12,500 feet, the number two engine failed. The aircraft was being flown manually and the flight crew immediately declared an emergency and requested a left turn back to Denver (as an NTSB representative put it, so “as to not turn right over the ‘dead’ engine”.)

Two of the fan blades fractured within the engine. A portion of one of the blades was embedded in the fan blade containment ring. The remaining fan blades had damage on the tips and leading edges. The inlet and cowling separated from the engine and fell to the ground.

A passenger took a video of the engine which immediately became a viral sensation.

The incident is frequently being referred to as an uncontained engine failure, which I suspect is because of this video showing the engine with no cowling. However, this was a contained engine failure, which means that the failed engine pieces (in this case the fan blades) are contained within the engine case or exit via the tail pipe. Much development has been put into aviation technology to contain the fan blades of an engine in the case of a failure, as the potential hazard of high speed debris acting as shrapnel would put the aircraft and the passengers at much more risk than the engine failure itself.

An NTSB representative confirmed that the video made the failure look “not as contained as it actually was”. He said that both fire suppression tanks were deployed into the #2 engine, the thrust was neutralised and the fuel was cut. The fire was likely residual flames from a damaged fuel line.

The debris on the ground was not from the engine, but the inlet and cowling which separated from the engine and fell to the ground over Broomfield, Colorado, a city northwest of the airport. Local news quickly reported that the Broomfield police had received reports of a plane falling from the sky when the debris began to land. A senior FAA investigator who lives in the Denver area immediately started working with local law enforcement to recover the components for the investigation.

This footage shows the debris falling in the distance:

Resident Kieran Cain took this photograph of a piece of cowling which landed on his neighbour’s porch:

He reported that he both heard and saw the debris dropping and that there was property damage to houses but no injuries.

The inlet and cowling which separated were manufactured by Boeing, not Pratt & Whitney. According to the Wall Street Journal, Boeing has been in discussions with the FAA to replace all of the casing from its 777 jets after a similar incident in 2018.

The NTSB has posted photographs of the aircraft in the hangar, including this one showing the damage to the fan blades:

This image taken Feb. 22, 2021, shows the damage to fan blades in the number 2 engine of United Airlines flight 328, a Boeing 777-200, following an engine failure incident Saturday. (NTSB photo)

and this one showing the missing inlet and cowling:

NTSB photo

The naked engine looks a lot less frightening on the ground, I have to admit.

On the 23rd of February, three days after the incident, the FAA released an emergency airworthiness directive:

…prompted by the in-flight failure of a 1st-stage low-pressure compressor (LPC) blade on a PW40077 model turbofan engine resulting in an engine fire during flight. This condition, if not addressed, could result in a 1st-stage LPC blade release, damage to the engine, and damage to the airplane.

The FAA is issuing this AD because the agency has determined the unsafe condition described previously is likely to exist or develop in other products of the same type design.

Required Actions
(1) Before further flight, perform a thermal acoustic image (TAI) inspection of the 1st-stage LPC blades for cracks using a method by the FAA
(2) If any 1st-stage LPC blade fails the inspection required by paragraph (g)(1) of this AD, remove the blade from service and replace with a part eligible for installation before further flight.

High cycle fatigue is a primary cause of fan blade failures, which is why the inspection to search for fatigue cracks is required on a list of Pratt & Whitney engines before the 777 can go back into service.

Pratt & Whitney have confirmed that they will carry out Thermal Acoustic Imaging inspections on all fan blades on the Pratt & Whitney PW4000-112 engines to confirm their airworthiness. They also commended the flight crew operating United Airlines flight 328 for their professionalism.

Boeing’s media release says that this affects 69 PW4000-powered 777s in service and 59 in storage, while Pratt & Whitney’s statement says that their PW-4000-112” engine powers approximately 125 Boeing 777 aircraft.

United Airlines is the only affected US operator, with 24 in service. Japan’s regulator ordered all PW4000-powered 777s grounded, affecting 13 aircraft from Japan Airlines and 19 from All Nippon Airways. The UK CAA has banned flights by aircraft with PW-4000-112 engines, although there are no airlines that currently operate that type to destinations in the UK or indeed anywhere near UK airspace.

On the same day, a Boeing 747-412 flying as Longtail Aviation flight 5504 from Maastrict, NL to NYC, US, suffered an uncontained engine failure in their number one engine. The engine was coincidentally also Pratt & Whitney engine, the PW4056, which is not one of the engines listed in the FAA’s airworthiness directive. The flight was passing through 1,400 feet with the number one engine failed. This was an uncontained failure. Pieces of the fan blades fell over the village of Meerseen, causing two injuries and stabbing one poor car through the heart.

(found on multiple social media sites with no attribution)

The flight crew declared an emergency and then entered a holding pattern to dump fuel before diverting to Liege Airport in Belgium, making a safe landing an hour after its departure from Maastricht.

The engine which failed over the Netherlands was also a Pratt & Whitney, the PW4056, however the failure appears to be unrelated; the PW4056 does not have the same fan blades. Both aircraft went into service over a quarter of a century ago (1991 and 1994), making them older than some of the readers of this website. It appears to be a coincidence that both failed on the same day.


  • “the video made the failure look “not as contained as it actually was””

    There’s an understatement!

    For a cabin containing 229 pax they all seem remarkably unbothered that one of the engines has ripped apart.

    The fire bothers me the most…..I wonder how long it would until the engine burns itself away and then perhaps spread to the wing? :(

    Great piloting.

    Thanks for your thoughtful analysis as ever.

  • Modern fan blades are marvels of engineering, but right at the limits of tech progress. The RB211 composite blades from Rolls Royce bankrupted the company after they proved too brittle in bird strike, and it took decades for them to return to composite blades.

    In this case you can see the internal cooling / lightening channels in the root of the broken blade, and it highlights why there is just so much potential for problems in these designs. With the very limited amount of material, stress concentration can lead to rapid crack propagation once a crack start.

    As an engineer it will be interesting to learn more as information emerges. As a flyer (when that becomes possible again), I remain encouraged that these planes kept flying, and there were no fragments penetrating the fuselage or wings.

    • Looking at the remaining root, it looks like there is a smooth and gradual propagation on the left side of the root, followed by breaking off shown as the rough surface on the right. I wonder whether there might have been vibration/rubbing near the trailing edge which then propagated?

  • When the news first broke, with TV channels showing images of the damaged aircraft… no, I rephrase: When I first saw the obviously very serious damage to the no. 2 engine, both taken from the ground and by a very calm passenger, my first reaction was of course of horror.
    But on second thought, hearing of the fortunate outcome, I had to come to the same conclusion as Sylvia:
    – In how far was the damage “uncontained” ? Semantics perhaps, but it can open an interesting discussion. Colinto already opened it with an expert opinion, from an engineer’s point of view.
    I still remember the bankruptcy of Rolls-Royce, although I cannot remember the exact details of the government bail-out and the separation of the Rolls-Royce aircraft engine division from the luxury motor car builder. A cartoon at the time showed the CEO of Rolls-Royce arriving at his work in pin-striped suit with the obligatory bowler hat and, of course, a folded black umbrella, Times and briefcase, standing on a chauffeured scooter.
    Colinto is quite right when he calls the fan blades “marvels of engineering”. They are large, very large and their rapid rotation subjects them to a high stresses.
    Colinto will be able to tell us if a small piece of debris, maybe a pebble or some other relatively small object, ingested with the engine running at high power, could cause stresses that are not immediately obvious, perhaps even invisible?
    These stresses, I imagine, could accelerate fatigue that over time might lead to the blade separating. Even if one blade separates, it is likely to hit the next one. There would be an immediate domino effect. Even if a blade looses only a piece without it hitting the next, it will cause vibrations that very rapidly will result in engine failure.
    The engines of the 747 losing parts over Meerssen may not have been identical to the ones fitted to the 777, but the result is the same. My – hopefully educated – guess is that the age and number of cycles of the engine will be a more important factor than the actual model. Make? I am not an engineer so I do not know if engine components from different manufacturers have the same or similar metallurgical properties.
    – Coming back to my original conclusion: Yes, the pictures make it look very scary, but the flight was probably not in any great danger.
    – The crew acted professionally. Without doubt, they deserve praise. But then, how many times would they have practiced a similar scenario during their training? The aircraft would have remained perfectly controllable. When I was still an active pilot we were subjected to simulator sessions every year at a minimum, in some companies pilots even undergo annual proficiency checks AND in between “type recurrent training”. When it does happen in the actual aircraft there is of course a heart-stopping moment, but pilots employed by major airlines are very highly trained. A reaction, a comment I once heard was “It was just like in a real simulator”.
    – Contained or not, the flight had just departed from a major airport. The crew do not seem to have dumped fuel, maybe they accepted an overweight landing or perhaps the 777 was within reasonable landing weight limits? Anyway, the fire would not have had long enough time to burn itself through structural components.
    Don’t forget: A jet engine, when operating, is on fire constantly. Only this time missing bits exposed the flames to view.

  • It’s mostly hidden behind the engine in the hanger photo but there was damage to the fairing by the wing root of the 777. That’s a fair distance horizontally from the engine. I don’t know how much danger the debris that caused the damage would actually pose, but I would assume this sort of damage is not supposed to happen in an engine failure. Another photo here:

  • The pictures reminded me of AF66, an A380 which lost the front of its Engine Alliance engine over Greenland in 2017. Damage to the rest of the aircraft was minimal, and they landed safely in Goose Bay.

    There are a lot of Boeing aircraft in the sky, and thus a lot of them can develop engine problems. There were five engine problems in a short time span recently, the two that Sylvia mentioned and the blog, and these less spectacular ones:

    On February 17th, a Garuda Boeing 737-800 was climbing out of Makassar when the right hand engine (CFM56) began to pull black smoke behind. The crew stopped the climb at 4000 feet, shut the engine down reporting the engine was damaged and returned to Makassar for a safe landing.
    On February 20th, a United Boeing 737-800 from Cancun (Mexico) to Houston was enroute at FL380 about 400nm southsoutheast of New Orleans,LA (USA) when the crew needed to shut the right hand engine (CFM56) down. The aircraft drifted down to FL240 and diverted to New Orleans for a safe landing.
    On February 22nd, a Delta Airlines Boeing 757-200 was enroute at FL380 about 110nm northeast of Salt Lake City,UT (USA) when the crew reported a left hand engine (PW2037) failure and decided to divert to Salt Lake City.

    Looks like abit of bad luck.

  • Yes, usually an in-flight engine failure is an inconvenience. Other than that more a non-event. It is very rare in turbine engines, be they jets or turbo-props. I flew a Cessna 310 for 8 years, about 4000 hours. It had, of course, piston engines but they never missed a beat. We were in the 3rd set of engines when it was sold. Of course it really helps when you fly not just the same type, but even the same aircraft – from new – over a long period, you get to know every creak and clatter. It happened to me in a Fokker F27 with cargo, on our way from somewhere in Spain (Salamanca perhaps) to Paris. My co-pilot was the pilot flying. He looked down through the side window and asked me what the town was we had just passed. I checked the charts: “Bordeaux”. “I have never been in Bordeaux”, he replied. “Well, today is your lucky day”, I replied. Right at that moment a red annunciator light told us that the oil pressure of no. 2 engine had dropped below the acceptable minimum value. We shut it down. He expected me to take over controls, but (lazy as I am) I decided that he was doing a good job; we were all trained to the same standard and so I did not see any reason to take over.
    Okay, there was nothing dramatic. No fire, no bits and pieces landing in gardens, porches or on roofs of cars, but the principle was the same: go through the drills, follow the “abnormal procedures” in the checklist and make a precautionary landing at the nearest suitable airport. In our case: Bordeaux.
    The cause was simple: an engine accessory had somehow worked itself loose from the engine’s casing. Which caused an oil leak. A mechanic solved it in a few hours, so the next morning we were ready to depart again. We did not even get to see the city of Bordeaux itself.

  • Sylvia, you disappoint us all ! The answer is obvious.
    On the other hand, in the ‘sixties the German Luftwaffe (Air Force) operated Lockheed F 104 Starfighters.
    I forgot the details, but if I remember correctly the electronics were made by a German company (Becker?).
    The technology was less advanced than the standard American equipment. It worked, but the units were heavier.
    This meant that the aircraft gained some weight.

    Still with me?

    In order to compensate for the extra weight, the engines had to be modified to give a bit more power.
    It affected the German F104’s reliability.
    The standard joke at the time was:
    How does a German acquire a Starfighter?
    Answer: buy a plot of land and wait.

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