Airlines grounded dozens of older Boeing Co. 777 aircraft after the failure of a Pratt & Whitney engine showered debris onto a Denver suburb and prompted U.S. regulators to order emergency inspections.

United Airlines Holdings Inc. halted operations of 24 of its planes in the wake of the incident involving one of its fleet over the weekend, after the U.S. Federal Aviation Administration ordered fan-blade checks on PW4077 engines. Japan’s transport ministry grounded aircraft with the engine variant on Monday, while Korean Air Lines Co. and Asiana Airlines Inc. idled theirs and the U.K. banned such jets from its airspace.

The incident on United Airlines Flight 328 from Denver to Honolulu took place shortly after it took off on Saturday with 231 passengers and 10 crew members on board. The Boeing 777 landed safely back at Denver and nobody was injured by the falling debris. Footage of the burning engine was filmed by a passenger, while people on the ground captured scenes of the plane overhead and scattered aircraft parts near houses.

The scare comes at a sensitive time for the global aviation industry, which is trying to emerge from a year in crisis caused by the coronavirus pandemic’s impact on travel. For Pratt, the United incident came on the same day as a separate emergency in the Netherlands on a 747 cargo jet using the same family of engines. Boeing is only just dusting itself off from the nearly two-year grounding of its best-selling 737 Max jet following fatal crashes in Ethiopia and Indonesia.

The Pratt engines are installed on a small portion of the twin-aisle 777s in service, and many such long-distance planes have been idled as the pandemic decimated international air travel.

Fan Blades

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.

While the Denver event doesn’t suggest broader problems with the 777, a twin-engine wide-body plane typically used on intercontinental routes, it adds another urgent issue to Boeing’s to-do list just after the 737 Max was cleared to fly again in markets including the U.S. and Europe. The company also 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.

The European Union Aviation Safety Agency said Monday it was in touch with the FAA about the Denver incident and an engine failure in the Netherlands on Saturday. On the Dutch flight, a 747 cargo plane had to make an emergency landing shortly after takeoff from Maastricht.

Pratt engines were used on the earliest versions of the 777, with the United example making its debut in 1995. While the manufacturer doesn’t make engines for newer wide-body aircraft, it collects service and parts revenue. Many older, larger planes are already in storage or retired due to a lack of demand for long-distance flights during the pandemic.

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

Over the life of the 777, Boeing has delivered 174 of the jetliners with Pratt engines, according to the planemaker’s website, although some engines and aircraft may since have been retired. Boeing had shipped more than 1,000 of the planes with General Electric Co. engines through January and a couple hundred powered by Rolls-Royce Holdings Plc.

While 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 fitted with the engines suffered a similar failure on Dec. 4.

Japan Airlines and ANA Holdings Inc. said Monday they’re using other aircraft following the transport ministry’s order. South Korea’s transport ministry said it was looking into the issue.

Korean Air halted operations of 12 777-200 and four 777-300 jets with PW4000 engines, more than half of those already in storage. Asiana has nine 777s with the Pratt engines, mostly sitting unused.

The 777 is distinctive for its hulking turbofans that are about as wide as a 737 jetliner cabin. When the 777 debuted in the mid-1990s, buyers could choose engines from Pratt, Rolls-Royce or GE. In 1999, Boeing awarded GE an exclusive contract to power newer, longer-range versions and eventually phased out the Pratt offering.

2018 Flight

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

The earlier incident was blamed by the NTSB on inadequate test standards at Pratt. An inspector had seen a possible sign of a crack years before the failure, but attributed it to paint, the NTSB said.

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 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 them beneath the surface. The blades are only used on some 777 planes, said the FAA, which is stepping up the frequency of inspections. The regulator said its action was limited to the hollow titanium fan blades on this specific engine.

Pratt has dispatched a team to work with investigators and is coordinating with airlines and regulators. The exact details of what type of inspections will be needed and how quickly they must be done are still being worked out, FAA Administrator Steven Dickson said in an emailed statement.

United has 52 of the planes in its fleet. Of those, 28 are in storage. The airline will work closely with the NTSB and FAA “to determine any additional steps that are needed” to return the aircraft to service, it said.

Dutch Emergency

Pratt said last June that it had taken corrective actions to address the cause of the 2018 failure. After that incident, Pratt re-inspected all 9,600 fan blades and didn’t find any others with potential safety problems, the NTSB said.

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

“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,” said Sanjiv Kapoor, a former top executive at SpiceJet Ltd. and Vistara, the Indian affiliate of Singapore Airlines.