ACFT EQUIP PROB. LNDG GEAR COLLAPSED AFTER LNDG.

Narrative

I MADE A NORMAL APCH TO LNDG IN LIGHT WIND CONDITIONS. THERE WAS NO INDICATION OF A PROB DURING THE APCH. I PUT THE LNDG GEAR IN THE DOWN POS WHEN I WAS APPROX 2 MI W OF THE FIELD. THE INDICATOR SHOWED THAT THE GEAR WAS EXTENDED FULLY. I ENTERED A L DOWNWIND FOR RWY 35 AND COMPLETED THE PRELNDG CHKLIST FOR THE SECOND TIME WHEN I TURNED A 1.5 MI FINAL APCH. I TURNED FINAL AT 550 FT AGL; THE GEAR WAS DOWN; THE MIXTURE WAS FULL RICH; THE PROP SPD WAS FULL FORWARD; AND I WAS OPERATING OFF THE L MAIN FUEL TANK. I FLEW THE FINAL APCH JUST ABOVE THE VASI GS AND TOUCHED DOWN AT THE 1000 FT MARKERS. EVERYTHING SEEMED NORMAL AT TOUCHDOWN AND I WAS IN THE PROCESS OF REMOVING THE LAST OF THE ENG PWR WHEN A BUZZER SOUNDED BRIEFLY. I THEN REALIZED THAT THE PLANE WAS BEGINNING TO SETTLE TO THE RWY. I HELD THE NOSE OF THE PLANE UP FOR AS LONG AS POSSIBLE AS THE PLANE SLID ON THE RWY. I DO NOT KNOW WHAT CAUSED THE LNDG GEAR TO FOLD UP; BUT AM CERTAIN THAT THE WHEELS TOUCHED AND THE PLANE BEGAN TO ROLL BEFORE THE GEAR FOLDED. WHILE I TOUCHED DOWN AT THE 1000 FT MARKER AND THE PLANE PROGRESSED ANOTHER APPROX 500 FT; THERE WAS ONLY ABOUT 30-50 FT OF SKID/SCRAPE ON THE PAVEMENT. THE PLANE CAME TO A REST RIGHT SIDE UP WITH EACH OF THE MAIN GEARS STILL PARTIALLY EXTENDED. THERE WERE NO INJURIES. A NUMBER OF WITNESSES MENTIONED; AFTER THE INCIDENT; THAT THE LNDG APPEARED TO BE NORMAL UNTIL THE PLANE FELL TO THE RWY. THE CIRCUIT BREAKER FOR THE LNDG GEAR MOTOR WAS BLOWN AFTER THE INCIDENT.