AN ACR B727 FLC EXPERIENCED SMOKE IN THE CABIN AND FLT STATION AFTER TKOF. THEY RETURNED AND DURING THE POSTFLT MAINT INSPECTION THE TECHNICIANS DETERMINED THAT THE APU FLOW MULTIPLIER WAS THE SOURCE OF THE SMOKE.

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STRONG AND APPARENTLY INCREASING SMOKE IN COCKPIT AND CABIN IMMEDIATELY AFTER TKOF. EMER DECLARED AND CLRNC FOR DOWNWIND PROVIDED. RETURNED TO RWY OF DEP AND LANDED. FUEL WAS DUMPED TO REDUCE GROSS WT TO MINIMUM POSSIBLE BEFORE LNDG. CFR CREW INSPECTED ACFT AND DECLARED 'NO HOT SPOTS...ALL CLR.' RETURNED TO GATE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR WAS THE CAPT ON A B727-200 AND HE SAID THAT THE FLC HAD FOLLOWED NORMAL OPERATING PROC AND HAD CLOSED THE APU AIR BLEEDS AND SHUT DOWN THE APU DURING TAXI OUT. DURING TKOF THEY STARTED TO NOTICE SMOKE COMING THROUGH THE AIR CONDITIONING VENTS AS THEY APCHED ROTATION SPD. AFTER LIFT-OFF THE SMOKE INTENSIFIED AND THEY DECLARED AN EMER AND REQUESTED TO RETURN TO THE ARPT. THE TWR CTLR CLRED THEM TO MAKE A CLOSE-IN PATTERN BACK TO THE SAME RWY. THE CREW DEPRESSURIZED THE ACFT AND LANDED. AFTER RETURNING TO THE GATE AND DEPLANING THE PAX; THE MAINT TECHNICIANS FOUND THAT THE FLOW MULTIPLIER WAS; APPARENTLY; THE SOURCE OF THE SMOKE. THE SYS IS NORMALLY NOT OPERATED OR EVEN IN THE PNEUMATIC FLOW DURING FLT AND THE RPTR SAID THAT THE AIR CONDITIONING SYS WAS IN THE NORMAL CONFIGN AND THE APU WAS SHUT DOWN. HE SAID THAT WHEN HE LOOKED AT THE APU AND THE AIR CONDITIONING AND PRESSURIZATION PANELS DURING THE FUEL DUMP PROC THEY LOOKED NORMAL EXCEPT THAT THE PRESSURIZATION SYS HAD BEEN SHUT OFF AT THIS TIME TO STOP THE SMOKE FROM CONTINUING TO ENTER THE ACFT. AFTER THE FLT THE CREW DISCUSSED THE PRE-SMOKE OP AND EVERYONE AGREED THAT THE OP WAS NORMAL ACCORDING TO THE CAPT. AFTER MAINT FINISHED THEIR INVESTIGATION AND CLRNC OF THE ACFT BACK TO OP THE CAPT QUESTIONED THEIR FINDINGS; BUT THE TECHNICIANS CONFIRMED THAT THE FLOW MULTIPLIER WAS THE SOURCE. THE RPTR SAID THAT THE TECHNICIANS COULD NOT EXPLAIN HOW THE MULTIPLIER COULD HAVE BEEN ACTIVE IF THE SYS HAD BEEN ISOLATED. THE CAPT SAID THAT HE FLEW THE ACFT AFTER MAINT WAS COMPLETED WITHOUT THE SMOKE RECURRING AND HE STILL DOES NOT KNOW HOW THE FLOW MULTIPLIER WAS ABLE TO OPERATE DURING THE ORIGINAL FLT. HE SAID THAT HE HAS SPOKEN TO MAINT AND THE TRAINING DEPT SINCE THIS INCIDENT AND; ALTHOUGH IT IS STILL A MYSTERY; IT IS NOW ANOTHER INCIDENT THAT IS USED AS A TRAINING AID IN RECURRENT GND SCHOOL.