THE PLT WAS PRACTICING HIS ROUTINE FOR AN AIR SHOW THAT WAS TO OCCUR LATER THE SAME DAY. ONE WITNESS RPRTD THE ACFT APCHD THE ARPT FROM THE WEST 'LOW & SLOW' AS IF TO LAND, THEN IT WAS MANEUVERED TO AN INVERTED ATTITUDE OVER RWY 8 AT 50 TO 100 FT AGL. AFTER REMAINING INVERTED FOR A SHORT TIME, THE ACFT WAS ROLLED BACK TO AN UPRIGHT ATTITUDE. HOWEVER, AS THE ACFT ROLLED UPRIGHT, IT ANGLED TO THE RIGHT SIDE OF THE RWY, DESCENDED & CRASHED ON A HDG OF 140 DEG. THE WITNESS SAID THE ACFT SEEMED TO 'SLIP' SIDEWAY AS IT ROLLED TO THE UPRIGHT PSN BEFORE CRASHING. AFTER IMPACT, A FIRE ERUPTED & THE ACFT WAS DESTROYED. NO PREIMPACT PART FAILURE OR MALFUNCTION OF THE ACFT WAS FOUND. THE PLT WAS A FORMER ASTRONAUT THAT WAS EMPLOYED BY NASA. HE HAD VISITED WITH A PROMINANT AT-6 PLT & HAD EXPRESSED SPECIAL INTEREST IN THE AT-6 PLT'S ROUTINE INVOLVING INVERTED FLT. THE AT-6 PLT HAD PROVIDED INFO, BUT ADZD THE ASTRONAUT AGAINST PERFORMING ROLLS IN THE AT-6 AT LOW ALT.