On Saturday, about five minutes after taking off from Denver International Airport for a flight to Honolulu, United Airlines flight 328 experienced an uncontained engine failure on its starboard engine. Fragments pierced the wing-to-body fairing and debris from the engine rained down over a one-mile area of a Denver suburb, with the almost intact engine inlet narrowly missing a home.
Following the incident, the Boeing 777-200 immediately returned to Denver and landed safely about a half hour after it departed. No injuries were reported among the 231 passengers and 10 crew on board the aircraft, or for anyone on the ground.
Images of the failure were captured by passengers on the plane and observers on the ground. A video from one passenger that was shared widely on Twitter shows the engine missing most of its nacelle (the housing that holds and protects the engine) and fire from what's likely leaking fuel. Video and photos from the ground show the engine appearing to explode and then trailing smoke.
According to a preliminary investigation from the National Transportation Safety Board, two fan blades in the engine were fractured. While the agency prepares its final report, the FAA has called for closer inspection of all 777s with similar engines, and United has grounded its 777 aircraft that use it. Here's what we know so far about what happened.
What is an uncontained engine failure?
A jet engine failure occurs when parts within the engine fragment or disintegrate while the engine is running. In a contained failure, the broken parts do not pierce the engine's nacelle and/or they are ejected through the engine's nozzle.
In an uncontained failure, the parts explode with such force that they penetrate the nacelle or destroy it completely. Though they're less common than contained failures, uncontained failures are far more dangerous. They can be so violent -- remember that the turbines of a jet engine spin at a tremendously high speed of thousands of revolutions per minute -- that the parts rip into the airplane's fuselage, causing much bigger problems or what's sometimes called a catastrophic failure.
For example, on April 17, 2018, an engine on a Southwest Airlines Boeing 737 flight from New York LaGuardia to Dallas Love field failed so violently that cowling fragments damaged the plane's fuselage, causing explosive depressurization. One passenger was sucked out of the aircraft and killed.
And in a more notable incident, a United McDonnell Douglas DC-10 was flying from Denver to Chicago on July 19, 1989, when the fan on its tail-mounted engine explosively disintegrated. Parts from the engine severed flight control lines in the tail assembly, leaving the pilots unable to control the aircraft using conventional methods (instead the flight crew maneuvered the plane by applying alternating thrust to the remaining wing-mounted engines). Though 112 people died in a crash landing Sioux City, Iowa, 185 passengers and crew survived.
What causes an engine failure?
A few things, including undetected wear and tear or defects in the engine, metal fatigue, errors made during maintenance and fuel contamination. External factors like a bird strike, ice or flying through volcanic ash can cause an engine failure, as well.
Failure can also occur because of blade separation, where one of the blades from the engine's fan (that's the spinning thing at the front of the engine) breaks off while the engine is running. It can then ricochet through the engine like a bullet, causing extensive damage.
It's also the likely cause here. In its preliminary report issued Sunday, the agency found that two of the engine's 22 fan blades were fractured. Investigators found one of broken blade on the ground and the other was embedded in the engine's containment ring, which is a casing that surrounds the engine (see more images in an NTSB tweet).
Are engines tested for failure?
Yes. When new jet engines are designed, the FAA and other aviation safety agencies require that they pass certain safety tests before they're certified. Those tests include shooting ice balls and dead birds into a running engine while it's secured to a testbed facility on the ground. Manufacturers also run a blade-off test in which a small explosive is rigged to one of the blades to separate it from the shaft.
The desired outcome of all of these tests is a contained failure, and engines are designed to deliver that outcome as much as possible. But sometimes things happen that exceed design parameters.
In the passenger video, why is the engine still turning?
Following the failure, the flight crew would have cut power and fuel lines to the engine. So while it may look like it was still running, that's simply a "windmilling" effect of air flowing through the fan at high speed, causing it to turn. It's just like a breeze blowing through a wind turbine.
What kind of engine was involved?
It was a PW-40077 turbofan, produced by Connecticut-based Pratt & Whitney for almost 20 years. It's just one version of the Pratt & Whitney PW-4000 engine family. The 777-200, a long-range, wide-body airliner, flies on two of the engines.
The PW-4077 is one of the largest and most powerful engines on a commercial airliner. Its fan has a diameter of 9.3 feet, or almost as wide as the passenger cabin of a Boeing 737, and it measures almost 16 feet long. It produces a maximum of 98,000 pounds of takeoff trust, which is equivalent to two engines used on the early 747 models.
Boeing doesn't make engines?
No. Boeing, like Airbus and all other modern aircraft manufacturers, doesn't design or build jet engines. It builds the airplane, and airline customers choose an engine made by a third party. Besides the PW4077, the 777 also flies with engines made by Rolls-Royce and General Electric. The 777X are the largest commercial engines ever.on the new
How old was the engine?
It's unclear right now. Though the 777-200 was 26 years old (which is old, but not ancient for an airliner), engines are regularly replaced on aircraft during maintenance overhauls.
Have there been failures with engines in the PW-4000 family before?
Yes. In December, a Japan Airlines 777 flying from Okinawa to Tokyo suffered fan blade damage and lost part of one engine cover shortly after takeoff. The aircraft returned to Okinawa and landed safely.
On Feb. 13, 2018, another United 777 with PW-4077 engines experienced an uncontained engine failure about 40 minutes from landing on a flight from San Francisco to Honolulu. The NTSB found that a fractured fan blade was the cause. The aircraft also landed safely without any injuries.
Can the 777 fly one engine?
Yes. Like all multiengine aircraft, the 777 can still fly safely if one of its engines fails. The plane will fly at a slower speed, of course, and thrust on only one side of the aircraft will affect how it handles. But pilots are well-trained to manage such incidents by using the rudder to compensate. Boeing also designed a flight control system that will adjust the rudder automatically. Pilots are trained for all kinds of engine failures in simulator sessions.
What would have happened if the engine failure occurred over the Pacific Ocean?
The 777 would have continued on to Hawaii or turned back to an airport on the West Coast, whichever was closer. That's because it's certified for ETOPS, which stands for Extended Twin Engine Operations. That rating means a twin-engine aircraft can fly on one engine for as long as it takes to get to an airport. The maximum distance depends on the aircraft, but it can range from 2 hours to as long as 5.5 hours.
The NTSB will continue its investigation. Depending on its findings, the FAA may issue an airworthiness directive that would order airlines to repair or modify all PW-4000 engines.
Pratt & Whitney said immediately following the incident that it dispatched a team to work with investigators.
On Sunday, FAA administrator said in a statement. "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." The agency followed with a more complete order on Tuesday.ordered increased inspections of all 777 aircraft with PW4000 engines. "We reviewed all available safety data following yesterday's incident," Dickson
In a statement issued the same day, Boeing said it's actively monitoring the investigation. "While the NTSB investigation is ongoing, we recommended suspending operations of the 69 in-service and 59 in-storage 777s powered by Pratt & Whitney 4000-112 engines until the FAA identifies the appropriate inspection protocol."
Also on Sunday, United said it was temporarily removing 24 of its 777 aircraft powered by PW4000 engines from its schedule. "We will continue to work closely with regulators to determine any additional steps and expect only a small number of customers to be inconvenienced," the airline said. "Safety remains our highest priority, which is why our crews take part in extensive training to prepare and manage incidents like UA328."
And in other developments:
- The Japan Civil Aviation Bureau ordered Japan Airlines and All Nippon Airways to ground its 777 aircraft that use the engines.
- Grant Shapps, the UK Secretary of State for Transport, has banned affected 777s from entering British airspace.
- Korean Air and Asiana Airlines said they will temporarily ground their aircraft.