An explosive engine failure that showered debris over a Denver suburb prompted authorities in Japan to halt flights using Boeing Co. 777 aircraft and spurred U.S. regulators to order emergency inspections of engine fan blades.

The U.S. Federal Aviation Administration ordered the checks after examining a hollow fan blade that failed, the agency said in an emailed statement February 21 evening. The inspections apply to 777s equipped with PW4077 engines made by Raytheon Technologies Corp.’s Pratt & Whitney division.

The incident on United Airlines Flight 328 from Denver to Honolulu took place shortly after it took off with 231 passengers and 10 crew members. The plane landed safely back at Denver and nobody was injured by the falling debris. Footage of the burning engine was filmed by a passenger aboard the flight, while people on the ground captured scenes of the plane overhead and scattered aircraft parts near houses.

The scare comes at an extremely sensitive time for the global aviation industry, which has been plunged into crisis by the coronavirus pandemic and its impact on travel. Boeing is only just dusting itself off from the nearly two-year grounding of its best-selling 737 Max following fatal crashes in Ethiopia and Indonesia, while Pratt has faced separate issues with its geared turbofan engines on Airbus SE A320neo jets, particularly in India.

Two fan blades were fractured on the United flight, the National Transportation Safety Board said. Most of the destruction was contained to the engine and the plane suffered only minor damage. United said it will voluntarily halt operations of 24 of its planes while the FAA order is carried out.

While the incident doesn’t suggest broader problems with the 777, it adds another urgent issue to Boeing’s to-do list only just after the 737 Max was cleared to fly again in markets including the U.S. and Europe. The company has halted deliveries of its 787 Dreamliners to check for manufacturing flaws.

“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,” Boeing said in a statement, adding that it supports decisions by the FAA and Japan’s Civil Aviation Bureau to temporarily ground aircraft powered by the engines.

Airlines in the U.S., Japan and South Korea operate 777s with the PW4000 family of engines. United is the only U.S. carrier with that combination. A Japan Airlines Co. 777-200 with the engines suffered a similar failure on Dec. 4.

Spokespeople for Japan Airlines and ANA Holdings Inc. said on February 22 they’ve grounded their 777s following the transport ministry’s order and are using other aircraft, while South Korea’s transport ministry said it was looking into the issue.

Korean Air has PW4090 engines on 12 777-200s and four 777-300s. More than half of those planes are in storage due to the pandemic. Asiana Airlines has nine 777s with the Pratt engines, but most of them are also grounded due to COVID-19.

Only 60 Pratt-powered 777-200 and -300 models are still flying globally, according to analysis of Cirium data by George Ferguson at Bloomberg Intelligence. Another 67 of the jets are sitting in storage, although they haven’t officially been retired.

The latest inspections could hasten the end of the earliest 777 models if the repairs turn out to be costly, Ferguson said. “They are already out of favor because of their size and the pandemic.”

The 777 is distinctive for its hulking turbofans that are about as wide as a 737 jetliner cabin. The PW4000-112 debuted with the first of the 777s to fly in 1995, and was also available to customers of the later -200ER and -300 models. In 1999, Boeing awarded General Electric Co. an exclusive contract to power newer, longer-ranger versions of the 777 with its GE90 engines and eventually phased out the Pratt offering.

“Pratt has not had any market share in this space for a long time,” Ferguson said. “It’s all GE.”

The crack that led the fan blade to break on the United flight was similar to one that occurred on a 2018 United flight, said a person familiar with the preliminary investigation results who wasn’t authorized to discuss them.

In the latest failure, one fan blade cracked and broke off near where it attached to a rotating hub, according to the person. A second blade was also broken, apparently after it was struck by the first blade.

The fan blades on this type of PW4000 are hollow and made of titanium. The cracks appear to start from within the surface, making them hard to detect. Airlines can use technologies such as ultrasound to find cracks beneath the surface. The blades are only used on some 777 planes, said the FAA, which is stepping up the frequency of inspections.

Pratt has dispatched a team to work with investigators and is coordinating with airlines and regulators to support inspections. “Any further investigative updates regarding this event will be at the discretion of the NTSB,” the company said in a statement.

FAA Administrator Steven Dickson said in an emailed statement that the aggressive inspections “will likely mean that some airplanes will be removed from service.” The exact details of what type of inspections will be needed and how quickly they must be done are still being worked out, he said.

“After consulting with my team of aviation safety experts about yesterday’s engine failure aboard a Boeing 777 airplane in Denver, I have directed them to issue an Emergency Airworthiness Directive that would require immediate or stepped-up inspections of Boeing 777 airplanes equipped with certain Pratt & Whitney PW4000 engines,” Dickson said.

United has 52 of the planes in its fleet. Of those 28 are in storage, the airline said. Many carriers have stopped using planes during the pandemic because of a steep drop in air travel, particularly on international routes serviced by the 777.

“Since yesterday, we’ve been in touch with regulators at the NTSB and FAA and will continue to work closely with them to determine any additional steps that are needed to ensure these aircraft meet our rigorous safety standards and can return to service,” the company said in an emailed statement.

A flaw in a fan blade on a United 777 in 2018 was blamed by the NTSB on inadequate test standards at Pratt. An inspector had seen a possible sign of a crack during a test years before the failure, but attributed it to paint, the NTSB said.

The company said last June that it had taken corrective actions to address the cause of the failure. After the incident during the San Francisco to Hawaii flight on Feb. 13, 2018, the company re-inspected all 9,600 fan blades and didn’t find any others with potential safety problems, the NTSB said.

Separately, Dutch investigators are looking into another engine failure that occurred on Saturday, when a Boeing 747-400 cargo plane shed engine parts after taking off from Maastricht, according to reports. The Pratt model was a PW4056, which is from the same range as the PW4077.

“The PW4000 family has been a very popular and reliable engine now for decades across multiple aircraft types,” said Sanjiv Kapoor, a former top executive at SpiceJet Ltd. and Vistara, the Indian affiliate of Singapore Airlines. “These recent incidents on the 777 and a 747 do suggest a closer look needs to be taken at the older PW4000 engines and their inspections and maintenance.”

An engine failure on a Southwest Airlines Co. 737-700 in 2018 that sent debris into a window on the plane resulted in the most recent fatality of a passenger on a U.S. flight.