GA SMA GND LOOP AFTER LNDG WITH RWY EXCURSION AND GEAR COLLAPSE.

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I WAS APCHED BY COMPANY FOR THE POSSIBLE PURCHASE OF MY SMA FOR THE CONVERSION TO A 210 HP ENG. THEY REQUESTED AN EVALUATION FLT BY ONE OF THEIR TEST PLTS; AND I AGREED; WITH THE STIPULATION THAT I BE THE PIC. THEREFORE; THE PURPOSE OF THE FLT WAS TO DEMONSTRATE TO AN ENGINEERING TEST PLT THE FLT CHARACTERISTICS. I PERFORMED ALL OF THE PREFLT AND TAXI REQUIREMENTS TO THE TKOF POS. WE HAD AGREED TO SHARE SOME FLT DUTIES ON A SELECTIVE BASIS; WITH ME AS THE PIC. I AGREED TO LET HIM MANIPULATE THE CTLS ON TKOF WITH MY DUTIES BEING TO MANIPULATE THE BRAKES FOR AXIAL ALIGNMENT TO THE RWY AND BEING AVAILABLE FOR RUDDER CTL WHILE SUPERVISING HIS ACTIONS. THE TKOF WAS NORMAL. WE FLEW FOR APPROX 1 1/4 HRS AND RETURNED FOR A LNDG. IT APPEARED WINDS WERE GUSTING MODERATELY (APPROX 15 KTS) FROM THE NE. THE RWY IN USE WAS 25. I ALLOWED THE TEST PLT TO SHARE CTL OF THE ACFT IN THE PATTERN AND TOUCHDOWN; WITH MYSELF PROVIDING BRAKES FOR AXIAL ALIGNMENT TO THE RWY AND BEING AVAILABLE FOR RUDDER CTL WHILE SUPERVISING HIS ACTIONS. THE LNDG WAS MADE TAIL LOW ON THE MAIN GEAR NORMAL ATTITUDE. UPON ROLLOUT; THE ACFT STARTED TO YAW TO THE R; AND THE TEST PLT ASKED FOR 'BRAKE'. I APPLIED THE L BRAKE; BUT NOTHING SEEMED TO HAPPEN. THE TEST PLT ASKED AGAIN; 'BRAKE; BRAKE'; AND I APPLIED MORE PRESSURE AND STILL NOTHING SEEMED TO HAPPEN. THE ACFT WENT INTO A GND LOOP AND SKIDDED OFF THE RWY. BOTH MAIN LNDG GEARS COLLAPSED AND THE ACFT CAME TO A STOP. BOTH THE TEST PLTS AND I EXITED THE ACFT.