INSTRUCTOR RPT REGARDING A PA30 TWIN COMANCHE WHICH HAD A LNDG GEAR COLLAPSE JUST AFTER TOUCHDOWN.

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AS INSTRUCTOR; I WAS PROVIDING DUAL INSTRUCTION IN A PA30 AT HOUSTON'S DAVID WAYNE HOOKS ARPT. THE ACFT WAS ESTABLISHED ON DOWNWIND; WHEN TWR REQUESTED A SHORT APCH. WE ACCEPTED; AND STARTED THE APCH TO A FULL STOP. STUDENT WAS FLYING ON MID-FIELD DOWNWIND. THE STUDENT LOWERED THE LNDG GEAR; AND VERIFIED THE GEAR WAS DOWN; ALL PROCS BEING VERIFIED BY ME. I TOOK THE CTLS TO FLY FROM DOWNWIND TO FINAL PURSUANT TO THE TWR'S REQUEST; AND LOWERED THE FLAPS TO FULL. WE STARTED A BASE TURN; ROLLING OUT ON FINAL. ESTABLISHED ON FINAL; BEFORE LNDG CHKLIST WAS COMPLETED. FUEL WAS SELECTED ON MAINS; UNDERCARRIAGE WAS DOWN AND GREEN; MIXTURES WERE FULL FORWARD; PROPS FULL FORWARD; FUEL PUMPS ON. I ASKED THE STUDENT ONCE AGAIN TO VERIFY THE GEAR. HE POINTED THAT THE GEAR WAS DOWN AS HE CALLED IT OUT; POINTED TO THE GREEN LIGHT AS HE CALLED THAT OUT AS WELL; AND SAID HE SAW ONE IN THE MIRROR. STUDENT RESUMED FLYING THE ACFT FOR LNDG. THE STUDENT MAINTAINED NORMAL AIRSPD INTO THE FLARE; TOUCHED DOWN SLIGHTLY OFF CTRLINE BUT WELL WITHIN NORMAL PARAMETERS. ACFT TOUCHED DOWN ON BOTH MAIN GEAR; AND WAS IN THE PROCESS OF SETTING DOWN ON THE NOSEWHEEL; WHEN THE R MAIN COLLAPSED; FOLLOWED BY AN ALMOST IMMEDIATE COLLAPSE OF THE OTHER 2 LNDG GEAR. THE ACFT SKIDDED ON ITS BELLY; COMING TO REST ON THE RWY. THERE WAS BLACK SMOKE FORMING IN THE ACFT; AND I ADVISED MY STUDENTS TO EVAC THE ACFT IMMEDIATELY. ONCE THE SMOKE CLRED; I RETURNED TO THE ACFT TO TURN OFF THE MASTER; MAGNETOS; AND FUEL PUMPS. THERE WERE NO INJURIES TO ANY OF THE OCCUPANTS. THERE WAS MINIMAL DAMAGE TO ACFT. HOOKS TWR RPTED VIEWING THE LNDG GEAR EXTENDED. AT LEAST 1 OTHER WITNESS IN THE AREA RPTED TO THE TWR THAT THE LNDG GEAR WAS EXTENDED. BOTH STUDENTS RPTED THE LNDG GEAR EXTENDED. I CONFIRMED AT LEAST TWICE THAT THE GEAR HANDLE WAS SELECTED DOWN WITH GREEN LNDG GEAR EXTENDED LIGHT ILLUMINATED. NORMAL OPERATING PROCS FOLLOWED THROUGHOUT. NO EXPLANATION AS TO THE CAUSE OF THE GEAR COLLAPSE AT THIS TIME. NO SUGGESTIONS AS TO HOW TO PREVENT THIS IN THE FUTURE DUE TO UNEXPLAINED GEAR COLLAPSE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATES THAT HE HAS SPOKEN TO PEOPLE WHO OWN PA30 ACFT AND THERE IS NO HISTORY OF THIS OCCURRING PREVIOUSLY. RPTR ALSO SPOKE TO AN FAA EXAMINER WHO CHKED THE GEAR AND HAD HIM CONTACT THE ANALYST. EXAMINER STATED THAT IT WAS THE LATERAL GEAR BRACE THAT BROKE IN HALF. THIS IS A RARE INCIDENT; NOT ONE SEEN BEFORE. THE ACFT IS A 1965 MODEL BUT DID NOT SHOW SIGNS OF METAL FATIGUE.