WHILE APCHG FOR A LNDG, THE ACFT PITCHED UP, ROLLED INVERTED & IMPACTED THE TERRAIN TO THE LEFT OF THE RWY. NO PREIMPACTPART FAILURE/MALFUNCTION WAS FND; HOWEVER, AN INV REVEALED THE ACFT HAD BEEN LOADED WITH THE CENTER-OF-GRAVITY (CG) WELLBEHIND THE AFT LIMIT. THE ACFT HAD BEEN WEIGHED ON 9/8/84. LATER, ON 1/2/86, WEIGHT & BALANCE DATA HAD BEEN RECOMPUTED AFTER REMOVAL OF A RADAR & AN AUTOPLT, BUT AN ERROR HAD BEEN MADE DURING THE WEIGHT & BALANCE PREPARATION. SUBSEQUENTLY,THE ACFT WAS OPERATED IN A TAIL HEAVY CONDITION. WHETHER USING THE EXISTING OR THE CORRECT DATA, CALCULATIONS FOR THIS FLT SHOULD HAVE SHOWN THE CG WAS BEHIND THE AFT LIMIT. THE ELEV TRIM TAB WAS FND TRIMMED TO A FULL NOSE DOWN POSITION; 200 LBS OF BALLAST HAD BEEN INSTALLED IN THE VICINTY OF THE AFT CARGO DOOR. TESTS OF LUNG TISSUE SHOWED THE PLT & COPLT HAD ALCOHOL LVLS OF 0.044% & 0.171%, RESPECTIVELY. THE PLT'S URINE TESTED 0.035% FOR ALCOHOL. ALSO, PHENYLTOLOXAMINE, ACETAMINOPHEN & CHLORPHENIRAMINE (MEDICATIONS COMMONLY USED IN DECONGESTANTS/ANTIHISTAMINES) WERE DETECTED IN HIS URINE.