CHICAGO, IL, USA
N9090U
BOEING 737-222
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.
HISTORY OF FLIGHT On April 8, 1995, at 1710 central daylight time(cdt), a Boeing 737-222, N9040U, aircraft number 9640, operated under 14 CFR Part 121 as United Airlines (UAL) flight #536 to Atlanta, Georgia, experienced a left outboard (#1 main) wheel separation on takeoff from runway 09L at Chicago O'Hare International Airport (ORD). The left inboard wheel was unaffected and remained installed. The airplane returned to ORD and landed on runway 32L without incident at 1846 cdt after circling to burn fuel. The 111 passengers and 5 crewmembers were uninjured, and deplaned via the stairs. The wheel damaged two fences and several rental cars in parking lots located on the airport. Detailed laboratory inspection revealed a failure of the outboard wheel bearing. Indications of grease migration outboard, out of the bearing cavity, was evident. The recovered bearing components exhibited mechanical deformation and heat distress. This damage precluded a definite determination of the initiating failure. The December 5, 1995, engineering report by the Timken Company on "The Investigation of Landing Gear Parts, Aircraft #9640" states that a likely cause of the bearing failure was inadequate lubrication. The Timken report was written at the request of the National Transportation Safety Board, and parties to the investigation were the NTSB, Timken, UAL, Allied Landing Systems (ALS), and Boeing Aircraft Company. All parties were present at the Timken facility during the physical inspection, and all have made comments which are incorporated in or appended to the Timken report. Boeing comments state "...no definitive cause was established, but the hardware condition was consistent with a failure stemming from inadequate lubrication." Twenty five of twenty seven rollars were recovered. No fragments of the two unrecovered rollers were apparent (Reference paragraph #3, page 6, under "General Discussion" of Timken report 12/5/95). The bearing cage was destroyed during the failure. Timken stated "there was no evidence for there having been one or more cracked rollers, and that the damage in the failed bearing was not consitent with such a condition." HISTORY OF THE FAILED BEARING A review of maintenance records on the wheel assembly indicated that the #1 main wheel had been installed on February 1, 1995, at 55,443 aircraft cycles. A brake change was performed on February 25, 1995, at 55,573 cycles. The wheel loss occurred at 55,792 cycles. This is a total of 219 cycles since the brake change, and 349 cycles since installation. It could not be determined if the bearing was repacked with grease during the brake change. The outboard bearing cup exhibited date code "ZD" which indicates a manufacturing date of February 1993 (approximately 26 months). Timken (the bearing manufacturer) stated this age is not exceptional. BEARING LIFE EXPECTANCY Bearings are installed with wheel assemblies after buildup, and remain installed until the wheel is removed and disassembled. This normally occurs as a result of tire removal due to tread wear or tire damage. There is no high cycle limit placed on tires or wheel assemblies. Removals are "on condition" based on tire tread. Timken stated that "...with rare exceptions, rolling contact fatigue is not a condition observed as a life limitation of aircraft wheel bearings." Boeing indicated a worldwide 737-200 fleet average of 200 cycles between tire changes (which would require repacking or installation of a new bearing). UAL provided a sample of 19 737-200 tire changes. The high life tire in the sample was removed at 326 landing cycles. The average (mean) tire life was 259 cycles, with a standard deviation of 39.9 cycles. The failed bearing remained installed to 349 cycles. LUBRICATION PROCEDURES At initial installation, the wheel assembly has a freshly grease packed new or re-used and inspected bearing. During a brake change, if the wheel assembly is reinstalled, the bearing may not be repacked if it visually appears to have grease. UAL Maintenance Procedures for 737-200 Wheel and Tire Assembly - Removal / Installation, 32-41-01, paragraph 2. D (4), states "Install outer bearing (packed with GRE4500-8A)." The procedures for Main Gear Wheel Brake - Removal / Installation, 32-43-01, paragraph 7. J. states, "Install main gear wheel (Ref. 32-41-01/201)." The Safety Board investigator asked UAL maintenance personnel to clarify these procedures. Line maintenance supervisors from UAL San Francisco Engineering stated that personnel may interpret this to mean that if the outer bearing was removed from the wheel, or a new one installed, that the bearing would be repacked with grease. If the wheel was removed as an assembly for a brake change, and then reinstalled as an assembly, the bearing would not necessarily be repacked. Interviews with three FAA airworthiness inspectors, two of whom had B-737 experience, indicated that during a brake change where a wheel was removed and reinstalled, they would expect a mechanic to visually inspect the wheel. They stated that if grease was visually present, they would not expect a mechanic to repack the bearing. TYPE OF LUBRICANT UAL repair shop working documents specify Aeroshell-5 as an approved lubricant by Allied Landing Systems in CMM 32-40-01. Aeroshell-5 was confirmed by lab testing to be the grease in use. Timken stated that they and ALS have recommended Aeroshell-22 as a superior grease for aircraft wheel applications, but that Aeroshell-5 has shown acceptable performance. No discrepancy with the use of Aeroshell-5 was noted. RETENTION OF LUBRICANT UAL Maintenance Procedures for 737-200 Wheel and Tire Assembly - Removal / Installation, 32-41-01, paragraph 2. D (4), states "Outer seal (adjacent to axle nut) no longer required." This refers to a grease dam, part number 2602576, depicted on Allied Landing System drawing 2601571-1. This grease dam is designed to retain lubricant within the bearing cavity, and is optional as per ALS. The failed wheel assembly exhibited radial streaks of grease which had migrated out of the bearing cavity. Boeing stated that the majority of 737-200 airplanes in service worldwide operate without the optional grease dam installed. AGE OF LUBRICANT The UAL procedures in place specify for a brake change to refer to the wheel removal/installation procedures. These procedures refer to the installation of a packed bearing. Statements by various FAA inspectors, and UAL, indicate that if grease was visually present, a bearing may or may not actually be repacked with fresh grease by a mechanic on the line. The wheel may be reinstalled with the original grease. The failed wheel bearing had grease which was either 219, or 349 cycles old. Allied Landing Systems and Timken indicate that after approximately 150 cycles, lubricant degradation begins to accelerate in aircraft wheel bearings.
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.
Source: NTSB Aviation Accident Database
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