NTSB Identification: CHI86FA119.
The docket is stored on NTSB microfiche number 34465.
Scheduled 14 CFR operation of UNITED AIRLINES
Accident occurred Tuesday, April 08, 1986 in CHICAGO, IL
Probable Cause Approval Date: 01/25/1988
Aircraft: BOEING 737-222, registration: N9054U
Injuries: 1 Minor,113 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.

THE LEFT MAIN LANDING GEAR COLLAPSED MOMENTS AFTER TOUCHDOWN. WINDS AS REPORTED BY THE TWR WERE FROM 350 DEGS AT 19 KTS GUSTING TO 29 KTS. THE CAPTAIN AND COPILOT REPORTED THE TOUCHDOWN WAS NORMAL. THE 3 FLIGHT ATTENDANTS SAID THE AIRCRAFT LANDED HARD AND STARTED SHAKING VERY BADLY. THE FLIGHT ATTENDANTS ALSO REPORTED THAT THE PREVIOUS 2 LANDINGS HAD BROUGHT A GREAT DEAL OF VIBRATING/SHAKING TO THE AIRCRAFT. THE LEFT MAIN LANDING GEAR TEE BOLT WAS FOUND SEPARATED. METALLURGICAL EXAMINATION OF THE TEE BOLT SHOWED THE FRACTURE EMANATED FROM A SHALLOW PREEXISTING CRACK IN THE DRAG BRACE BEARING SURFACE RADIUS THAT THE METALLURGIST FOUND TO BE SHARPER THAN DESIRED. THE COLLAPSE OF THE LANDING GEAR IS CONSIDERED RELATED TO THE FRACTURED TEE BOLT. THE SAFETY BOARD IS UNABLE TO DETERMINE WHETHER OTHER EVENTS, CIRCUMSTANCES OR CONDITIONS WERE PRESENT THAT CONTRIBUTED TO THE TEE BOLT FAILURE IN THIS INSTANCE. THE OPERATOR HAS AMENDED THEIR REPAIR SPECIFICATIONS FOR THE TEE BOLTS.

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

LANDING GEAR,MAIN GEAR..FATIGUE

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

PROCEDURES/DIRECTIVES..NOT FOLLOWED..COMPANY MAINTENANCE PERSONNEL

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