NTSB Identification: CHI95IA119.
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Scheduled 14 CFR (D.B.A. UNITED AIR LINES )
Incident occurred Friday, April 07, 1995 in CHICAGO, IL
Probable Cause Approval Date: 07/11/1996
Aircraft: BOEING 737-222, registration: N9090U
Injuries: 116 Uninjured.
NTSB investigators used data provided by various sources and may not have traveled in support of this investigation to prepare this aircraft incident report.
THE LEFT OUTBOARD MAIN WHEEL SEPARATED FROM THE AIRPLANE ON TAKEOFF. THE AIRPLANE WAS LANDED WITHOUT FURTHER INCIDENT. INVESTIGATION REVEALED THAT THE OUTER BEARING FAILED. HEAT AND MECHANICAL DAMAGE WERE CONSISTENT WITH INADEQUATE LUBRICATION. PRE-EXISTING MECHANICAL DAMAGE COULD NOT BE DETERMINED. THE WHEEL BEARING HAD ACCUMULATED 349 LANDING CYCLES SINCE INITIAL INSTALLATION, AND 219 CYCLES SINCE A BRAKE CHANGE. IT COULD NOT BE CONFIRMED THAT THE BEARING HAD BEEN REPACKED WITH GREASE SINCE INITIAL INSTALLATION. AN UNKNOWN QUANTITY OF GREASE HAD MIGRATED AWAY FROM THE BEARING CAVITY DURING OPERATION. AVERAGE CYCLES BETWEEN 737-200 WHEEL CHANGES PERFORMED BY THE OPERATOR WERE 259, AND WORLDWIDE ACCORDING TO BOEING WERE 200. AN OPTIONAL OUTER GREASE SEAL TO RETAIN GREASE WITHIN THE BEARING WAS NOT INSTALLED. THE BEARING AND WHEEL ASSEMBLY MANUFACTURERS STATED THAT AFTER 150 CYCLES, LUBRICATION PROPERTIES OF GREASE RAPIDLY DEGRADE.
The National Transportation Safety Board determines the probable cause(s) of this incident to be: the inadequacy or deterioration of lubricating grease in the wheel bearing, which led to the total bearing failure and subsequent loss of the wheel. Factors were the insufficiently defined procedures for repacking the bearing, along with an insufficient method of retaining lubricant within the bearing. Full narrative available
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