PLT OF A PVT C310 ACFT RETURNED AND LANDED JUST AFTER TKOF DUE SMOKE IN THE COCKPIT.

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I WAS DEPARTING CPS ARPT IN A C-310L ON RWY 12R. JUST AS THE AIRPLANE ROTATED I SMELLED PLASTIC BURNING (EXTREMELY STRONG). AT 200 FT AGL (600 FT MSL) I SAW SMOKE COMING FROM THE AVIONICS. I IMMEDIATELY DECIDED TO MAKE A LNDG. ALL NON ESSENTIAL AVIONICS WERE TURNED OFF WHILE I MADE AN IMMEDIATE 180 DEG TURN TO ENTER DOWNWIND. ANOTHER PLT WAS ALONG FOR THE RIDE AND I TOLD HIM TO CALL THE TWR AND ADVISE THEM THAT WE HAD A PROB AND WERE GOING TO MAKE AN IMMEDIATE LNDG. AS I WAS MAKING THE 180 DEG TURN TO DOWNWIND AN AEROSPATIALE TAMPICO CLUB WAS ABOUT TO ENTER DOWNWIND. THE OTHER ACFT WAS IN SIGHT BY MYSELF AND THE OTHER PLT WITH ME. WE WERE WELL BELOW AND FAR AHEAD OF THE TAMPICO AND WERE FLYING MUCH FASTER; THE TAMPICO WAS NOT GOING TO BE A FACTOR AND THERE WAS NO ACTUAL CONFLICT. WHEN WE CALLED FOR AN IMMEDIATE LNDG THE TAMPICO STARTED AN IMMEDIATE CLB AND TURNOUT OF THE TFC PATTERN. THE TAMPICO MUST HAVE HAD US IN SIGHT (AND AN INSTRUCTOR MOST LIKELY IN THE AIRPLANE WITH A STUDENT). THE TWR CANCELED LNDG CLRNCS FOR ALL ACFT AND CLRED US FOR LNDG ON ANY RWY OF OUR CHOICE. I TOLD THE OTHER PLT TO ADVISE THE TWR THAT WE HAD SMOKE IN THE CABIN. HE INSTEAD TOLD THEM WE HAD A FIRE IN THE AIRPLANE. TWR IMMEDIATELY CALLED THE LCL FIRE DEPT. A LNDG WAS MADE WITHOUT INCIDENT AND WE RETURNED TO THE RAMP. TWR CANCELED THE FIRE CALL. LATER INVESTIGATION OF THE AVIONICS REVEALED A DEFECTIVE ADF WHICH HAD TO BE REMOVED FROM THE ACFT AND DISCARDED. THERE WAS NO ACTUAL CONFLICT OR INCIDENT. EVERYONE HAD EVERYONE ELSE IN SIGHT AND COULD SEE WHAT THE OTHERS WERE DOING. DUE TO THE EXTREMELY STRONG BURNING SMELL AND ACTUAL SMOKE IN THE CABIN THERE WAS NO ALTERNATIVE BUT TO LAND IMMEDIATELY. THE LONGEST/WIDEST RWY WAS THE ONE I DEPARTED FROM. A 180 DEG TURN BACK TO THE RWY WOLD NOT HAVE BEEN PRACTICAL AND MAY HAVE CONFLICTED WITH OTHER ACFT I DID NOT HAVE IN SIGHT. BEST ALTERNATIVE WAS TO CUT IN FRONT OF AND BELOW THE ACFT IN SIGHT; IT WAS THE ONLY AIRPLANE IN THE PATTERN FOR THE RWY OF INTENDED LNDG.