FLC OF AN MLG ACR ACFT TOOK EVASIVE ACTION BY MAKING AN ABRUPT TURN TO AVOID AN MTR ACFT DOING ACROBATICS.

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WE WERE LETTING DOWN IN A GRADUAL DSCNT TO 11000. OFF AIRWAYS CLRED DIRECT TO VULCAN VOR. PLT IN JUMPSEAT RPTED AN ACFT UP AHEAD DOING AEROBATICS. I SAW A TANDEM SEAT STRAIGHT WING FOREIGN JET TRAINER ZOOM STRAIGHT THROUGH 12000; PULL THROUGH THE TOP OF A LOOP AND DIVE STRAIGHT DOWN TO THE DECK AND DISAPPEAR UNDER THE NOSE. WE GOT A TA WITH NO ALT AT 12 O'CLOCK. I ALSO RPTED THE ACFT TO ZTL. I HELD COURSE AND ALT BECAUSE I DIDN'T KNOW WHAT HE WAS GOING TO DO NEXT. HE THEN REAPPEARED AT 12:30 O'CLOCK ZOOMING STRAIGHT UP. I GRABBED THE CTLS; DISCONNECTED THE AUTOPLT AND ROLLED A 30 DEG BANK TO THE L. AS I LOST SIGHT OF HIM HE WAS ZOOMING STRAIGHT THROUGH OUR ALT. INTRUDER WAS SQUAWKING 1200; MODE C OFF. NO CLRING TURNS; AND MAY HAVE BEEN SUPERSONIC ON THE DIVES. TCASII HAS NO DEFENSE FOR THIS. NO INJURIES. 1 FLT ATTENDANT LANDED IN A PAX'S LAP. EVERYONE ELSE WAS SEATED; SEAT BELT SIGN WAS OFF. MY EVASIVE MANEUVER WAS RAPID; BUT A COORDINATED TURN. I USED TO BE AN AEROBATIC INSTRUCTOR; TOO. ON THE GND AT BHM (OUR DEST) THE SUPVR AT ZTL TOLD ME THE PLT OF THE JET HAD IMMEDIATELY LANDED AT ALBERVILLE; AL; AND PHONED THE CTR. HE IDENTED HIMSELF AS (PLT NAME) (PHONE NUMBER). HE WAS FLYING FROM THE BACK SEAT WITH A STUDENT IN THE FRONT. HIS ACFT WAS MTR. THERE ARE MANY EXCELLENT PLTS WHO ENJOY AEROBATICS AND FLY RESPONSIBLY AND SAFELY. OTHER ACFT PLT IS NOT ONE OF THEM.