NTSB Identification: CHI08FA050
Scheduled 14 CFR Part 121: Air Carrier United Airlines Inc
Accident occurred Friday, December 14, 2007 in Chicago, IL
Probable Cause Approval Date: 12/24/2008
Aircraft: Boeing 777-222, registration: N220UA
Injuries: 1 Serious,263 Uninjured.

NTSB investigators either traveled in support of this investigation or conducted a significant amount of investigative work without any travel, and used data obtained from various sources to prepare this aircraft accident report.

On approach for landing, about 13 miles from the airport, the flight crew was informed that the cabin was filling with smoke. The cabin crew was unable to determine the source of the smoke. The captain declared an emergency in order to get the airplane on the ground as soon as possible. The first officer performed a normal landing and turned off on the first high-speed taxiway, at which time he noticed a low oil indication on the right engine. The engine was subsequently shut down. The captain elected to initiate an emergency evacuation due to the uncertainty of the situation and the possibility of a fire on-board. During the evacuation, one passenger sustained a fracture of a vertebra. Post accident examinations revealed a low oil quantity in the right engine, with metallic deposits on one of the engine chip detectors. A detailed teardown inspection of the right engine revealed the #2 bearing had failed. The compressor gas path exhibited oil deposits consistent with oil migration into the airplane's environmental system. The cabin pressurization system utilized 8th and 15th stage engine compressor bleed air. No evidence of an engine or cabin fire was observed. The engine manufacturer had released an improved bearing design prior to the accident. At that time, the operator began replacing the bearings on an attrition basis when the original bearings were no longer serviceable. However, the operator has revised that policy and is proactively replacing the original bearings, regardless of the condition, with the improved bearings.

The National Transportation Safety Board determines the probable cause(s) of this accident to be:

Emergency evacuation due to dense smoke in the cabin and the possibility of an on-board fire, which resulted in a serious injury to one of the 264 passengers and crew. An additional cause was the failure of an engine bearing, allowing oil to migrate into the environmental system, and resulted in the smoke in the cabin.

Full narrative available

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