United Air Lines accelerated a program to replace defective bearings on its Pratt & Whitney PW-4090-powered Boeing 777s following a 14 December 2007 incident in which the cabin of a 777-200 filled with smoke on a flight inbound to Chicago from Shanghai.
That information was part of a recently-released final report on the incident by the US National Transportation Safety Board (NTSB).
Cabin crew of Flight 836 had alerted pilots that the cabin was filling with smoke when the mid-afternoon flight was 13 miles from the Chicago O'Hare International Airport. Pilots declared an emergency and then landed at O'Hare, turning off the runway at a high-speed taxiway and initiating an emergency evacuation of the 264 passengers and crew onboard. One passenger fractured a vertebra during the evacuation, according to the report.
A detailed teardown of the right engine revealed that its number two bearing had failed, causing the oil smoke to enter the cabin through the bleed air system for pressurization. NTSB notes that Pratt & Whitney had "released an improved bearing design prior to the accident".
United at the time had decided to replace the bearings "on an attrition basis when the original bearings were no longer serviceable," according to the report.
"However, [United] has revised that policy and is proactively replacing the original bearings, regardless of the condition, with improved bearings," the NSTB says.