PLT OF AN SMA MOONEY EXPERIENCED ELECTRICAL PWR PROB ON FINAL RESULTING IN THE LNDG GEAR COLLAPSING DURING LNDG BECAUSE IT WAS NOT LOCKED IN THE DOWN POS.

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TWR TO ME: 'CALL SIGN CLRED TO LAND.' MY RESPONSE: 'CLRED TO LAND RWY 28; CALL SIGN.' AT THIS TIME I WAS STILL ON L BASE. AT ABOUT THIS TIME I HAD ALREADY DEPLOYED 15 DEGS OF FLAPS AND HAD JUST TURNED ON THE LNDG LIGHT FOR VISIBILITY SAFETY. LNDG GEAR FOLLOWED BY PRE-LNDG CHKLIST; WAS TO BE NEXT. TWR TO ME: 'CALL SIGN I DON'T SEE YOUR GEAR.' RESPONSE TO TWR: 'AFFIRMATIVE; NOT DEPLOYED YET.' AT THAT TIME; SHORTLY AFTER LNDG LIGHT WAS TURNED ON; PANEL STARTED TO GO BLACK (SEVERAL ELECTRICAL PANEL ITEMS INCLUDING #1 RADIO WENT OUT). I TURNED OFF LNDG LIGHT AND PANEL INCLUDING #1 RADIO CAME BACK ON. TO TWR; 'OUR CALL SIGN; I THINK I MIGHT HAVE AN ELECTRICAL PROB.' I DON'T REMEMBER A RESPONSE BUT REMEMBER THINKING THE LNDG LIGHT WAS THE PROB; AND IT'S OFF NOW. I ACTIVATED LNDG GEAR AND STARTED REDUNDANT LNDG CHKLIST. SHORTLY AFTER ACTIVATING GEAR; THE PANEL WENT COMPLETELY BLACK. ALL ELECTRICAL OUT. I HAD JUST TURNED FINAL. ON FINAL I COMPLETED LNDG CHKLIST. NO PANEL GREEN FOR DOWN AND LOCKED GEAR; NOTHING ELECTRICAL ON PANEL. MANUAL GEAR INDICATOR BTWN SEATS SHOWED GREEN; DOWN AND LOCKED. DECISION TIME. DO I TRUST JUST ONE GEAR DOWN INDICATOR OR; GAR AND TRY AND FIGURE OUT THE ELECTRICALT PROB? MY DECISION WAS TO LAND THE ACFT RATHER THAN DEAL WITH THE POSSIBILITY OF THE ELECTRICAL PROB BEING A SHORT THAT MIGHT RESULT IN A FIRE. ON FINAL; I RECHKED THE MANUAL GEAR DOWN INDICATOR 3 OR 4 TIMES AND ASSURED MYSELF ALL WAS OK; DOWN AND LOCKED. NO GEAR WARNING HORN SOUNDED EVEN THOUGH I PUT IN A LITTLE MORE FLAPS (ELECTRICAL) FOR LNDG. SO I HAD SOME ELECTRICAL PWR; MY THINKING WAS IF THE GEAR WEREN'T DOWN AND LOCKED; SURELY I'D HAVE THE GEAR HORN IF I HAD ELECTRICAL FOR FLAPS. CAME OVER THE RWY END AT EXACTLY 80 MPH FOR A PERFECT LNDG. WHEELS (MAINS) TOUCHED DOWN SOFTLY AND COLLAPSED AS SOON AS THE LOAD WAS OFF THE WINGS. AIRPLANE SKIDDED ON RWY AND CAME TO REST IN INFIELD TO R OF RWY. NO WINGTIP DAMAGE; NO MAIN GEAR DOOR DAMAGE; NO STRUCTURAL DAMAGE TO AIRPLANE. AN FAA OFFICIAL HAD JUST LANDED AND WAS AT THE FBO AT TIME ALL THIS TRANSPIRED. SHE HEARD THE TWR XMISSIONS AND MY XMISSIONS UNTIL MY ELECTRICAL WENT OUT KNOCKING OUT MY RADIOS. SHE ALSO WITNESSED THE LNDG AND WAS THE FIRST PERSON ON THE SCENE. SHE DID NOT CITE ME FOR ANY INFRACTION AND TERMED THIS AN INCIDENT. FAA INSPECTION OF THE ACFT SHORTLY AFTER THE INCIDENT SHOWED THE BATTERY TO BE ALMOST COMPLETELY DEAD AND THE ALTERNATOR TO HAVE SOME SORT OF A SHORT.