THE PLT MADE A VFR APCH TO LND AT NGT ON RWY 31. ACCORDING TO WITNESS(ES), THERE WAS A TSTM SOUTH OF THE ARPT, BUT AT THE ARPT, THE WND WAS LGT & ONLY LGT RAIN WAS FALLING. AS THE PLT WAS LNDG, THE ACFT IMPACTED ON THE APCH END OF THE RWY, THEN BECAME AIRBORNE & IMPACTED AGAIN, FURTHER DOWN THE RWY. AN EXAM REVEALED THAT INITIAL IMPACT WAS 29' FM THE THRESHOLD. THE INITIAL SCRAPE MARK WAS ANGLED 10 DEG RIGHT OF THE RWY HDG. THERE WAS EVIDENCE THAT THE LEFT WING TIP CONTACTED THE RWY NR THE APCH END, THEN THE TIPS OF THE LEFT PROP BLADES STRUCK THE RWY. THE ACFT THEN BECAME AIRBORNE & CRASHED INVERTED AFTER TRAVELING ABOUT 613' DOWN THE RWY. THE BLADE TIPS OF BOTH PROPS WERE TWISTED & CURLED; THERE WERE SLASH MARKS ON THE RWY FROM BOTH PROPS AT THE 2ND IMPACT POINT, WHERE THE ACFT HAD IMPACTED IN AN INVERTED ATTITUDE. OF 62 RWY EDGE LGTS, 20 WERE INOP OR BROKEN & 2 WERE NOT VISIBLE DUE TO TALL VEGETATION. THE LAST RECORDED NGT TIME IN THE PLT'S LOG WAS DATED 5/5/81.