Airlines are grounding Boeing (NYSE:BA) 777 aircraft powered by Raytheon Technologies (NYSE:RTX) Pratt & Whitney engines after a weekend incident in which one of them on a United Airlines Holdings (NASDAQ:UAL) aircraft failed shortly after takeoff.

United Flight 328 from Denver to Honolulu experienced engine failure on Sunday, but returned safely. The engine broke into pieces that scattered over suburban Broomfield, Colorado, and passengers filmed footage of the engine in flames during the flight.

United responded by halting operations of 24 of its planes, and Boeing late Sunday recommended suspending all planes similar to the one involved in the failure until a proper engine-inspection protocol can be established.

The 777 assembly line.

The Boeing 777 assembly plant. Image source: Boeing.

The 777 is one of the primary international workhorses for airlines. But with the number of transoceanic flights reduced due to the pandemic, the plane is not in heavy use right now. By Boeing's estimate, about half of the global fleet is currently already out of service, with 69 in service and 59 in storage.

Engines are the most expensive single component of the plane, and are manufactured separate from Boeing. The aircraft involved in the United incident was one of the older generation 777s still in service, and the engines have likely been through several major maintenance cycles and overhauls.

Still, even if Boeing isn't directly to blame, the incident continues a difficult period for the manufacturer. Its 737 MAX was grounded for 20 months after a pair of fatal accidents, and deliveries of its 787 Dreamliner have been halted while the company works out a potential manufacturing deficiency.

A newer version of the 777 has been delayed until 2023 due to several development issues.

A Boeing cargo 747 experienced an engine failure over the Netherlands on Saturday. That plane was powered by a smaller version of the Pratt & Whitney engine on the United flight.

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