The fix, expected to be completed as soon as early June, would strengthen the engine covers to prevent plane parts from detaching midair and striking the aircraft or falling to the ground below, these people said.
After a United 777's engine failed shortly after takeoff from Denver in February, its external cover broke apart, raining metal on a nearby suburb. No one was injured, and the flight landed safely.
The progress in developing a fix for the engine covers comes as United Airlines Holdings Inc. hopes to return to service certain Boeing 777 jets that are powered by Pratt & Whitney engines. Boeing's fix is expected to include metal components that would strengthen the engine covers, people familiar with the matter said.
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United took 24 of the jets out of service after the Denver incident, and U.S. air-safety regulators ordered immediate inspections of those engines' blades for cracks that could potentially lead to failures and their covers detaching. United has said it had another 28 of the jets with Pratt & Whitney engines in storage. Other carriers that operate 777s with those engines include South Korean and Japanese airlines.
United's Pratt & Whitney engine blades are undergoing inspection, people familiar with the matter said. Pratt & Whitney is a unit of aerospace company Raytheon Technologies Corp. A spokeswoman for the engine maker said it was inspecting all affected fan blades before those jets return to service.
United has hoped to resume flying the affected jets by the busy summer season as travel demand picks up after a sharp decline due to the Covid-19 pandemic, people familiar with the matter said. But the timing of their return remains unclear.
"It's just too premature for us to outline what that schedule looks like, " United Chief Operating Officer Jon Roitman said on an earnings call in April. Citing "really productive collaboration" with Boeing, Pratt & Whitney and regulators, Mr. Roitman added, "We're really looking forward to getting the aircraft back in the air safely."
A Boeing spokeswoman said the company continued to work with the Federal Aviation Administration on potential design changes for certain Pratt & Whitney engine covers. She said that the work is exacting and time-consuming and that Boeing was making sure its specialists have the time they need.
FAA chief Steve Dickson said in congressional testimony Wednesday that the agency was working with Boeing and Pratt & Whitney to prevent engine-cover breakups from damaging aircraft or raining down debris. "We are requiring the manufacturers to address strengthening" engine covers, Mr. Dickson said.
The FAA said the exact timing and requirements of its expected order will depend on the completion of the design and engineering work on the engine-cover fix.
The planemaker, federal regulators and U.S. safety investigators have been focusing on the engine covers of Pratt & Whitney-powered Boeing 777s since at least early 2018, when an incident similar to the recent Denver episode occurred on another United flight en route to Hawaii.
The Wall Street Journal previously reported that Boeing outlined to FAA officials in August last year its initial plans to replace components of those 777s' engine covers. The fix wasn't rolled out before another engine cover broke apart on a Japan Airlines Co. flight late last year or the February incident over Colorado.