A FOKKER 100 ON FINAL APCH AT 400 FT EXPERIENCES EXPLOSIVE SOUND BELOW COCKPIT SEATS ON GROUND; COCKPIT AND CABIN FILLED WITH SMOKE. SMOKE CAUSED BY RUPTURED LEFT PACK HEAD EXCHANGER.

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FLYING THE ILS APCH AND AT APPROX 400 FT AGL; I FELT A LOUD EXPLOSIVE SOUND BENEATH MY COCKPIT SEAT. THE CONCUSSION REVERBERATED IN THE COCKPIT. AIR CONDITIONING FLOW INTO THE FLTDECK WAS REDUCED CONSIDERABLY. A CHK OF ALL WIND SCREENS WAS MADE AND FOUND INTACT. ALL ENG INSTRUMENTS WERE NORMAL; ON TOUCHDOWN; IDLE REVERSE WAS SELECTED (NORMAL PROCEDURE FOR US) AND BOTH THRUST REVERSERS INDICATED NORMAL DEPLOYMENT. WHEEL BRAKING COMMENCED AT 100 KIAS WITH NORMAL DECELERATION. THE ACFT EXITED THE RWY ON A HIGH SPEED TXWY AT APPROX 30 KTS. UPON CLEARING THE RWY THE FLTDECK BEGAN FILLING WITH SMOKE. THE SMOKE WAS NOT ELECTRICAL IN NATURE. THE COCKPIT DOOR WAS OPENED TO CHK FOR SMOKE IN CABIN. FA'S 'CHIMMED' THE FLTDECK AT THE SAME INSTANT. SMOKE BECAME THICKER IN THE FLTDECK AS WE ROLLED TO A STOP ON PARALLEL TXWY. COCKPIT WINDOWS WERE BOTH OPENED. AT THIS TIME CAPT AND MYSELF MADE THE DECISION TO EVACUATE THE ACFT. CAPT ORDERED 'EVACUATION CHKLIST.' I DONNED MY OXYGEN MASK AND WENT SPEAKER/HOT MIC AND BEGAN READING MY EVAC CHKLIST. WHEN CHKLISTS WERE COMPLETE AND ENGS WERE SHUTDOWN; THE SMOKE BEGAN SUBSIDING. CAPT AND MYSELF UNSTRAPED AND HELPED PASSENGERS EVACUATE THE ACFT. FWD SLIDES WERE DEPLOYED AND ALL PAX '48 AND CREW 5' EGRESSED THE ACFT. I WAS ORDERED BY CAPT TO GATHER ALL PASSENGERS OFF THE ACFTS LEFT FORWARD NOSE ABOUT 30 YARDS. ONLY POSSIBLE INJURIES AT THE SAME TIME WAS A SPRAINED ANKLE FROM ONE OF THE PASSENGERS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE AIRPLANE WAS A FOKKER 100 AND THE CAUSE OF THE EXPLOSIVE SOUND WAS THE LEFT PACK HEAT EXCHANGER RUPTURE. THE RPTR SAID THE PACK HEAT EXCHANGERS ARE LOCATED BENEATH THE COCKPIT FLOOR. THE RPTR SAID THE PACK COMPARTMENT WAS FILLED WITH HOT ENG BLEED AIR MIXING WITH OIL FROM THE AIR CYCLE MACHINE CAUSING THE COCKPIT AND CABIN SMOKE. THE RPTR SAID THE ACFT WAS EVACUATED ON THE GROUND WITH ONLY ONE PAX INCURRING A TWISTED ANKLE.