THE PLT HAD JUST COMPLETED BUILDING THE ACFT AND WAS ON THE INITIAL FLT. FAMILY MEMBERS WATCHED AS THE ACFT TOOKOFF AND CLIMBED OVER THE ARPT. THEY THEN OBSERVED PIECES FALLING FROM THE ACFT. THE ACFT ROLLED INVERTED AND CRASHED. AN EXAM OF THE ELEVATOR BELL CRANK REVEALED THE PLT HAD MADE AN UNAPPROVED MODIFICATION TO THE CRANK DUE TO A CONTRUCTION ERROR BY THE PLT WHICH CONSISTED OF CUTTING THE BOTTOM OFF THE CRANK AND ATTACHING 2 STRIPS OF ALUMINUM TO THE CRANK WITH POP RIVETS. THE ELEVATORS AND STABILIZERS SEPARATED FROM THE ACFT IN FLT. THE UPPER AND LOWER HALVES OF THE LEFT ELEVATOR HAD SEPARATED AND AN EXAM REVEALED THE BONDING MATERIAL WAS STILL SOFT AND SITCKY TO THE TOUCH. THE DESIGNER OF THE ACFT STATED THAT FLUTTER WAS CREATED IN THE ELEVATORS BY THE MODIFICATION RESULTING IN FAILURE OF THE TAIL COMPONENTS.