LNDG GEAR COLLAPSES DURING LNDG ROLLOUT ON A PLANNED TOUCH-AND-GO IN AN AC90 AT JLN; MO.

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THIS MORNING ABOUT XA30; I WAS DOING TOUCH-AND-GOES IN THE PATTERN IN TWIN COMMANDER AC90. I HAD DONE A WX CHK. IT WAS A BASIC VFR MORNING. I HAD NOT FLOWN THE ACFT IN 10 DAYS AND FELT THAT I NEEDED SOME MORE LNDG PRACTICE BEFORE A PART 91 PAX TRIP THAT AFTERNOON. IN MY HABIT; I HAD CALLED AND VERIFIED 3 GREEN (BOTH BY LOOKING AT THE MAIN GEAR AND REFLECTION OF THE NOSE GEAR IN THE SPINNER AND THE 3 GREEN LIGHTS) AND CLRED TO LAND AFTER THE TWR HAD CLRED ME TO LAND. I WAS LNDG FOR THE 8TH TIME (THE TOUCHDOWN AND INITIAL LNDG ROLL SEEMED NORMAL). I WAS PWRING UP FOR TKOF WHEN I FOUND MYSELF SLIDING ON THE BELLY OF THE ACFT. I DID APPLY HVY BRAKING AFTER REALIZING THAT WE WERE ON THE BELLY. I HAD DONE THE LAST 3 LNDGS WITH HALF FLAPS; AND HAD NOT BEEN RECONFIGURING PRIOR TO TKOF. I DO NOT KNOW WHAT HAPPENED. I DID NOT MOVE ANY GEAR OR FLAP SWITCHES AFTER THE GEAR COLLAPSED. THE ONLY SWITCHES I TOUCHED AFTER THE ACFT CAME TO A STOP WAS TO SHUT DOWN THE ENGS AND TURN OFF PWR TO THE ACFT PRIOR TO EXITING. ANOTHER PLT WAS RIDING ALONG WITH ME; BUT HAD NOT BEEN OPERATING ANY CTLS OR SWITCHES. WE ARE BOTH PHYSICALLY FINE BUT THERE IS BELLY DAMAGE TO THE ACFT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE PF STATED THAT THE EVENT; CLASSIFIED AS AN INCIDENT; IS STILL UNDER INVESTIGATION BY THE FAA. THE FACTORY REPRESENTATIVE WAS INVOLVED IN THE INVESTIGATION AND HAS INITIALLY CATEGORIZED THE EVENT AS BEING CAUSED BY THE LNDG GEAR BEING 'IMPROPERLY RIGGED.' FURTHER INVESTIGATIONS INTO THE MECHANICAL PROBS OR CONNECTIONS WILL BE ONGOING STARTING IN 1 WK. THE FAA HAS INITIALLY AGREED WITH THE ANALYSIS TO DATE. THE RPTR HINTED AT ADDITIONAL ACFT MAINT DISCREPANCIES THAT MAY BE INVOLVED. SUPPLEMENTAL INFO FROM ACN 608530: I WAS RIDING IN THE COPLT'S SEAT. I WAS RIDING ALONG AND NOT INSTRUCTING OR PLAYING ANY ROLE AS TO THE PLTING OF THE ACFT. I WAS STRICTLY A PAX AND NOT RUNNING RADIOS; FLT CTLS; OR SWITCHES OF ANY TYPE. HE HAD COMPLETED 7 OR 8 GOOD TOUCH-AND-GOES WITH NO PROBS AND NO SIGN OF ANY DIFFICULTY. JOPLIN TWR WAS OPEN. THE LAST LNDG WAS NORMAL AND HAD NO ABNORMAL CONDITIONS OR CTL PROBS OF ANY ACFT SYS. THE LAST 2 LNDGS WERE MADE AT 1/2 DEG FLAPS; EARLIER LNDGS WERE MADE WITH FULL FLAPS. AT 500 FT AGL; THE GUMP ALARM SOUNDED; AND WE BOTH CHKED AND VERIFIED THE GEAR WAS DOWN AND WE HAD 3 GREEN LIGHTS; INDICATING 3 GEAR DOWN AND LOCKED. THE TOUCHDOWN WAS NORMAL WITH THE MAINS TOUCHING DOWN; THEN THE NOSEWHEEL. AIRSPD STARTED TO DECREASE. HE BEGAN TO SLIGHTLY ADD PWR FOR THE NEXT TKOF. WE CROSSED THE INTERSECTING RWY 13/31; WHICH HAS A RISE IN THE PAVEMENT. ON THE DOWNSIDE OF THE RISE; THE ACFT INSTANTLY DROPPED TO THE GND; AND WE WERE SLIDING ON THE PAVEMENT AND VEERING TO THE L. I DID NOT SEE THE PLT RETRACT THE GEAR HANDLE OR THE FLAP HANDLE. WHEN WE CAME TO A STOP ON THE L SIDE OF RWY 36; THE GEAR HANDLE WAS DOWN AND LOCKED AND THE FLAP HANDLE WAS IN THE NEUTRAL POS AND FLAPS WERE AT 1/2 DEG FLAPS. THOSE 2 HANDLES WERE NOT CHANGED DURING THE SKID OR AFTER ACFT EXIT.