A C310 ATX PLT HAS THE L GEAR COLLAPSE AS HE TRIES TO MAKE A TURN ON THE RWY. HE HAD; PREVIOUS TO THE LNDG; HAD PROBS WITH GEAR RETRACTION ON THE L GEAR; IT STAYING EXTENDED.

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I DEPARTED ON A VFR PART 91 FLT FROM STL TO 3SQ IN A C310R. AFTER DEPARTING RWY 30R; I APPLIED THE BRAKES AND RAISED THE LNDG GEAR LEVER TO THE UP POS. I HEARD THE GEAR MOTOR ENGAGE AND FELT THE USUAL VIBRATION AS THE GEAR WAS RETRACTING. HOWEVER; I NOTICED THAT THE L MAIN GEAR INDICATOR LIGHT WAS STILL ILLUMINATED; INDICATING A DOWN AND LOCKED CONDITION. AT THIS POINT; I MADE SURE I WAS STILL BELOW THE LNDG GEAR MAX OPERATING SPD AND MOVED THE GEAR LEVER TO THE DOWN POS AGAIN. I FELT THE GEAR MOTOR ENGAGE AND THE NOSEWHEEL AND R MAIN WHEEL INDICATORS ILLUMINATED; GIVING ME AN INDICATION THAT ALL WHEELS WERE DOWN AND LOCKED. I TRIED AGAIN TO RAISE THE GEAR WITH THE SAME EFFECT. THE NOSEWHEEL AND R MAIN GEAR LIGHTS WENT OUT WHILE THE L MAIN STILL GAVE ME A DOWN AND LOCKED INDICATION. I LOWERED THE GEAR AGAIN AND GOT A POSITIVE INDICATION THAT ALL 3 WHEELS WERE DOWN AND LOCKED. THE LNDG WAS SMOOTH AND UNEVENTFUL. THE LNDG GEAR FELT STURDY AND I HAD NO INDICATION THAT THERE WAS A PROB ON THE GND. I ALLOWED THE AIRPLANE TO ROLL THE LENGTH OF THE RWY USING MINIMAL BRAKING TO REDUCE THE EXTRA LOAD ON THE LNDG GEAR. THE PLANE CAME TO A STOP AT THE FAR END OF THE RWY. I BEGAN TO MAKE A L 180 DEG TURN TO BACK TAXI WHEN I HEARD A METALLIC GROANING NOISE. THE PLANE BEGAN TO SHIFT TO THE R; THEN TILT TO THE L. I HEARD THE LNDG GEAR WARNING HORN BEGIN TO SOUND AND I ALSO HEARD THE PROP TIPS STRIKING THE RWY. I PULLED THE MIXTURE KNOBS IMMEDIATELY AND THE ENGS STOPPED. I TURNED OFF THE MAGNETOS; THE ALTERNATORS AND BATTERY. I THEN SWITCHED OFF THE FUEL TANKS. I SURVEYED THE VISIBLE DAMAGE. THE L MAIN LNDG GEAR ASSEMBLY HAD FOLDED OUTWARD (IT IS SUPPOSED TO RETRACT INWARD) AND THE L WING WAS RESTING ON THE RWY. THE PROP TIPS ON THE L ENG WERE BENT. WHEN THE MECHS BROUGHT THE AIRPLANE TO THE HANGAR THEY INSPECTED THE GEAR ASSEMBLY TO DETERMINE THE CAUSE. THEY FOUND THAT IT WAS A MECHANICAL DEFECT THAT COULD NOT HAVE BEEN DETECTED DURING A PREFLT INSPECTION OR EVEN DURING A ROUTINE 100 HR INSPECTION. IT WAS LATER DETERMINED THAT THE PIN HOLDING THE LNDG GEAR TO THE TRUNNION HAD COME LOOSE DUE TO THE SHEARING OF THE RETAINING PINS. THE RESULTING STRESS ON THE PIN CAUSED IT TO FAIL; WHICH ALLOWED THE LNDG GEAR TO COLLAPSE. I SPECULATE THAT A CONTRIBUTING FACTOR TO THE GEAR PIN IS THE CONDITION OF THE RWY AT 3SQ. IT IS QUITE BUMPY AND THE ACFT MAKES ANYWHERE FROM 10-20 LNDGS THERE PER WK. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THAT TO HIS KNOWLEDGE THERE WAS ONLY 1 OTHER SIMILAR INCIDENT OF THIS NATURE ON A C310. HIS INSTRUCTOR PLT MENTIONED THIS FACT TO HIM AFTER THIS INCIDENT HAD OCCURRED. RPTR AGREED THAT; HAD HE KNOWN BETTER AND HAD BEEN WARNED OF THIS POSSIBILITY; THAT HE WOULD HAVE SIMPLY SHUT THE ENGS DOWN AT THE END OF THE RWY AND HAD THE GEAR PINNED. OF THE 2 RETAINING PINS THAT HOLD THE GEAR TO THE TRUNNION; THE AFT ONE SHEARED; ALLOWING THE OTHER PIN TO BEND WHEN A SIDE LOAD WAS PLACED ON THE GEAR. IT WAS AN ABNORMAL/NORMAL SIT WITH THE GEAR INDICATING 3 GREEN LIGHTS; BUT AFTER THE ABNORMAL GEAR INDICATIONS DURING THE RETRACTION ATTEMPT. NO LATER WARNING TO ALERT HIM TO THE DANGERS INVOLVED IN THIS OP.