DRG IFR DEP, PLT RCVD PROGRESSIVE ALT CLNCS TO CLIMB TO 15,000'. AT 1607 EDT, HE WAS VCTRD FOR 'A GOOD RATE OF CLIMB' THRU 14,000' WITH CLNC TO PROCEED ON COURSE AFTER LVG 14,000' FOR 15,000'. RADAR DATA INDCD A STEADY CLIMB TIL ACFT WAS ABV 13,000'. AS IT CLIMBED FM 13,300' TO 13,900' (MAX RECORDED ALT), ITS SPD SLOWED FM ABT 115 KTS TO BLO 80 KTS. AT 1613 EDT, PLT WAS CLRD TO PROCD DRCT & CHG FREQ. RADAR DATA SHOWED THAT AFTER RCHG 13,900', ACFT DEVIATED FM COURSE & ENTERED STEEP DSCNT. RADAR CTC WAS LOST & INFLT BREAKUP OCCURRED. PIECES OF WNGS & STABILIZERS WERE FND UP TO 1.5 MI FM FUSELAGE. TRAJECTORY STUDY DISCLOSED BREAKUP OCCURRED BTN 6000' & 9000' MSL. EXAM OF FRACTURES ON MAJOR CMPTS REVEALED CHARACTERISTICS TYPICAL OF OVERSTRESS; NO PREEXISTING CRACKS WERE FND. NO AUTOPLT FAILURE OR BIRD STRIKE WAS FND. CLDS WERE LAYERED TO 20,000'; FRZG LVL WAS ABT 12,500'. THERE WAS EVIDENCE ACFT WAS IN OR NR CONVECTIVE PRECIP ABV FRZG LVL FOR ABT 1.5 MIN BFR RAPID DSCNT. FND PITOT HEAT SW 'OFF' & INDUCTION AIR DOOR IN ITS PRIMARY PSN. (SEE: NTLS/SIR-92/03)