FLC OF LGT DECLARED AN EMER AND RETURNED TO LAND DUE TO SMOKE IN THE COCKPIT AFTER TKOF AND CLB.

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AFTER TKOF FROM RWY 9L AT FLL A FAINT SMELL; LIKE THAT OF JUST LIT MATCHES; WAS EVIDENT IN THE COCKPIT. AT ABOUT 5000 FT THE COPLT DONNED HIS OXYGEN MASK WHILE I CONTINUED TO LFY THE ACFT AND THE ENGINEER BEGAN INVESTIGATING THE SMELL. AFTER CHKING ALL THE CIRCUIT BREAKERS AND TURNING THE COCKPIT LIGHTS ON TO CHK FOR SMOKE; IT WAS DECIDED TO TURN ON THE SECOND AIR CONDITIONING PACK. SOON AFTER THE SECOND PACK (THE R ONE) WAS STARTED; SMOKE BECAME EVIDENT IN THE COCKPIT TO THE EXTENT THAT AN IMMEDIATE RETURN TO FLL WAS DEEMED IN OUR BEST INTEREST. AT APPROX 14000 FT AND 40 MI NE OF FLL; WE DECLARED AN EMER; DONNED OUR OXYGEN MASKS AND REQUESTED; AND RECEIVED; IMMEDIATE VECTORS BACK TO FLL. BECAUSE OF THE UNKNOWN NATURE OF THE SMOKE; THE DESIRE TO LAND ASAP AND OUR PROX TO OUR INTENDED LNDG; I ELECTED TO MAKE AN OVERWT LNDG (APPROX 6000 LBS). AFTER OUR LNDG ON RWY 27R; WE WERE MET BY THE ARPT EMER EQUIP. AFTER EXITING THE RWY AND ASSESSING THE SIT; I DETERMINED WE COULD SAFELY TAXI THE ACFT AND REQUESTED THE EMER EQUIP TO FOLLOW US TO THE RAMP. AFTER SOME INVESTIGATION; MAINT DETERMINED THE CAUSE OF THE SMOKE AND ODOR TO BE A FAULTY GASPER FAN. THE FAN WAS CONFIRMED TO BE THE CAUSE AND WAS DEFERRED. AFTER FURTHER MAINT CHKS FOR THE OVERWT LNDG; WE DEPARTED AGAIN AND HAD NO FURTHER INCIDENTS. OF ALL THE FACTORS INVOLVED IN OUR HANDLING THE SIT; ONE OF THE MOST SURPRISING WAS THE ADVERSE EFFECT THE OXYGEN MASKS HAD ON OUR PERFORMANCE. THIS WAS THE FIRST TIME IN 15 YRS OF COMMERCIAL FLYING I HAD TO USE THE MASKS AND; EVEN THOUGH I REGULARLY CHK THE MASK AND AM FAMILIAR WITH ITS USE; I FOUND USING IT TO BE OTHER THAN SECOND NATURE. I CAN ONLY CONCLUDE THAT ALL THE TRAINING I HAVE BEEN THROUGH WAS DEFICIENT WHEN IT CAME TO USING THE MASKS. ALMOST ALWAYS THE USE OF THE MASKS IN THE SIMULATOR IS 'SIMULATED' FOR COMFORT SAKE AND IS NOT WORN AND BECAUSE OF THAT I FEEL I WAS ILL PREPARED FOR THE ADVERSITY IT PRESENTED US WHILE WE TRIED TO DEAL WITH A REAL SIT.