A PA32-300 AFTER TKOF; AT 1000 FT; THE UPPER ENG COWLING SEPARATED FROM THE ACFT.

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A THOROUGH PREFLT WAS CONDUCTED ON THE ACFT. NO DISCREPANCIES WERE NOTED ON THE ENG COWLING. ALL SCREWS AND FASTENERS WERE IN PLACE. DURING THE RUN-UP; NO UNUSUAL VIBRATIONS ON OR ABOUT THE COWLING WERE NOTED. PNE CTL TWR CLRED ME FOR A TKOF ON RWY 33 WITH A L DOWNWIND DEP. NOTHING UNUSUAL HAPPENED DURING THE TKOF RUN. UPON TURNING DOWNWIND; HOWEVER; THE ENG COWLING FOLDED BACK AGAINST THE WINDSHIELD AND THEN SEPARATED TOTALLY FROM THE ACFT. THE TWR OPERATOR WAS NOTIFIED. I WAS CLRED FOR A LNDG ON RWY 33. I LANDED WITHOUT ANY FURTHER INCIDENT. IT WAS NOTED; AFTER LNDG; THAT A HOLE HAD BEEN PUNCHED IN THE LEADING EDGE OF THE R WING FUEL TANK AND FUEL WAS LEAKING FROM IT. THE TANK WAS DRAINED AND THE ACFT SECURED IN THE HANGAR. I CALL THE CTL TWR AS I WAS REQUESTED TO DO AND GAVE THEM INFO WHICH THEY REQUESTED. I DON'T KNOW WHY THE ENG COWLING SEPARATED FROM THE ACFT. I HADN'T OPENED IT DURING THE PREFLT. I ONLY OPENED THE OIL INSPECTION PORT AND CHKED THE ENG VIA THE AIR INLET OPENINGS. THE ONLY TIME PRIOR TO THIS FLT THAT THE COWLING WAS OPENED WAS WHEN AN ACFT REPAIR STATION INSTALLED OIL SUMP HEATING PADS. THIS WAS APPROX 3 WKS PRIOR TO THIS FLT. I DON'T KNOW IF THE PERSON WHO PICKED UP THE ACFT FROM THE REPAIR SHOP OPENED THE COWLING. THE COWLING HASN'T BEEN FOUND AS OF THIS RPT AND; THEREFORE; NO EVALUATION OF ITS CONDITION HAS BEEN MADE.