What we know about the explosion of an engine on a Boeing 777 flying over Denver in the US

A Boeing 777 operated by United Airlines had to make an emergency landing in Denver at the weekend after one of its engines blew apart.

The explosion resulted in huge chunks of wreckage crashing down across suburban neighbourhoods.

Passengers captured video of the crippled engine, wobbling and still on fire, as pilots made a safe return to the airport minutes after the plane bound for Hawaii took off.

Now the investigation into the explosion is focusing on broken fan blades — a similar development to what followed a fatal failure on another plane in 2018.

This is what we know so far.

So, what do authorities think happened?

US officials say two fan blades on the Boeing 777’s Pratt & Whitney engine broke off.

Experts say it’s likely one blade snapped first and chopped off the second.

US National Transportation Safety Board (NTSB) chairman Robert Sumwalt says a preliminary assessment suggests one of the fan blades had a metal fatigue crack.

Federal Aviation Administration head Stephen Dickson says inspectors have quickly concluded that inspections should be done more frequently on the type of hollow fan blades in certain Pratt & Whitney engines that are used on some Boeing 777s.


As a result, 69 planes — and an additional 59 in storage — have been grounded in the US, Japan and South Korea. They are the only countries with planes using this particular engine.

United Airlines, the only US carrier with affected planes, says it has grounded 24 Boeing 777s and 28 others will remain parked.

Japanese regulators have ordered Japan Airlines and All Nippon Airways to ground 32 planes, and South Korea’s Korean Air and Asiana Airlines say they will ground their Boeing 777s.

What are investigators looking into?

Safety experts say the investigation will focus on why the fan blades snapped.

It will probe whether mistakes were made in manufacturing or maintenance, or if problems were missed during inspections.

The investigators will also look at whether blade inspections need to be done differently or more often.

They will compare the weekend’s incident with similar ones in December in Japan and in 2018 on another United flight to Hawaii.


Investigators will also look at why the cowling, which covers the front of the engine, broke off along with other parts. Photos purported to show a large gash to the lower side of the plane.

“That was a substantial hit,” John Goglia, a former member of the NTSB, which is investigating Saturday’s incident, says.

“If that had hit the wing, things might have been different because the wing is full of fuel.”

Todd Curtis, a safety consultant and former Boeing engineer, says the fact the engine remained on fire, even after pilots presumably shut off the fuel supply, suggests there might have been a fuel leak.

How much danger were the passengers in?

None of the 231 passengers or 10 crew members were hurt in the incident at the weekend.

However, the incident is being called an “uncontained” failure because debris blew off the disintegrating engine, creating shrapnel that can damage key systems like hydraulic lines or hit the passenger cabin.

The last accident-related death on a US airline flight was in 2018, when a broken fan blade triggered an engine break-up on a Southwest Airlines Boeing 737.

Part of the engine housing struck and broke a window.

The passenger in the window seat was blown halfway outside and she died of her injuries.

The engine involved in that incident was made by a different company, CFM International, a joint venture between General Electric and France’s Safran SA.

Debris from the aircraft landed in the Colorado town of Broomfield.(Broomfield Police Department)

Have there been similar incidents?

In December, a Japan Airlines Boeing 777 with the same series Pratt & Whitney engines suffered fan blade damage and lost a large panel, but the aircraft was able to land safely.

In 2018, another United Airlines Boeing 777 suffered an engine failure that caused parts of the housing to break off and fall into the Pacific Ocean as the plane flew from San Francisco to Honolulu.

In a report last year on the incident, the NTSB said Pratt & Whitney missed signs of cracking in previous inspections of the fan blade that broke, and it faulted the company’s training.

The company told the NTSB it was fixing the shortcomings.