A SWEARINGEN SX300 ON TOUCH DOWN L MAIN LNDG GEAR WHEEL TURNED SIDEWAYS AND ACFT DEPARTED THE RWY CAUSED BY A BROKEN UPPER SCISSORS TRUNION MOUNT.

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SHORT FINAL DOWN TO 90 KTS; SLIGHT TURB; NO WIND; THEN UPON TOUCHDOWN IT FELT LIKE A TIRE BLEW OUT. LOUD THUMP; NOISE AND VIBRATION WITH A DROP IN THE L WING AND THE ACFT TURNED L IMMEDIATELY. ACFT TURNED TO THE L IMMEDIATELY; HIT HARD R RUDDER AND BRAKE. HELD R AILERON TO KEEP L WING UP. ACFT DEPARTED RWY TO THE L AT A SLIGHT ANGLE. PWR OFF. ACFT CONTINUED OFF THE L SIDE OF THE RWY FOR APPROX 1000 FT. FLEW OVER A RWY TURN OFF INTXN. HIT ONE RWY LENS AND BULB (ONLY PROPERTY DAMAGE). L GEAR CYLINDER STUB DUG A LITTLE GRASS PRIOR TO THE INTXN. AFTER THE INTXN THE DRAINAGE DITCH DEEPENED AND THE NOSE GEAR AND THE R MAIN GEAR ROLLED INTO SOFT MOIST GRASS AND SOIL; DUG IN AND COLLAPSED. ACFT SLID TO A STOP APPROX 50 FT LATER AND ENDED UP NEAR THE PARALLEL TXWY. PLT AND PAX DEPARTED THE ACFT. NO INJURY. PRELIMINARY ANALYSIS SEEMS TO INDICATE THE WELD BROKE THAT HOLDS TIRE ASSEMBLY TO STRUT CYLINDER. WHEN THE GEAR IS EXTENDED IT COMES DOWN WITH AUTHORITY. AT THIS POINT WITH THE STRUT FULLY EXTENDED; THE SCISSORS ASSEMBLY ARE NOT FULLY EXTENDED. IF THE WELD BREAKS THE TIRE ASSEMBLY CAN COME DOWN ANOTHER ONE INCH. GEAR ASSEMBLY STUB IS ONLY 1 1/2 INCH LONG; SO IT CAN COME ALMOST OUT. AT TOUCHDOWN; WHEEL ASSEMBLY BROKE OUT; PEELED BACK STRUT CYLINDER AND TURNED SIDEWAYS AFTER THE SCISSORS UPPER TRUNION SPLIT IN HALF. SCRAPED RWY BRIEFLY AND GROUND OFF WHEEL RIM AND PAVEMENT; THEN DEPARTED ACFT. AFTER RPTING THIS TO ONE OF SX300 OWNERS; HE RPTED HIS ACFT WHILE TAXIING HAD A WHEEL TURN SIDEWAYS ALSO. UPON INSPECTION HE FOUND THE UPPER TRUNION MOUNT SPLIT IN HALF THE SAME WAY THIS ONE DID. HE MADE A NEW ASSEMBLY OUT OF STEEL AND NEVER PASSED THE WORD. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE ACFT WAS A HOME BUILT SWEARINGEN SX300 WITH 123 HOURS TOTAL TIME AIRFRAME AND ENG. THE RPTR SAID FAILURE WAS NOT THE FUSION WELD AS HIS RPT HAD INDICATED; BUT THE UPPER SCISSORS TRUNION MOUNT CRACKING IN HALF. THE RPTR SAID A PREVIOUS SX300 OWNER IN HIS CLUB HAD HIS WHEEL TURN SIDEWAYS WHILE TAXIING AND DISCOVERED THE CRACKED TRUNION. THE RPTR STATED THE CLUB ALERTED ALL KNOWN OWNERS OF THE SX300 OF THIS TRUNION MOUNT PROB. THE RPTR SAID THE PROPELLER; L WINGTIP AND L GEAR TRUNION MOUNTS WERE DAMAGED.