FUEL LEAKED INTO THE CABIN OF AN SMT.

Narrative

CAPT AND COPLT PERSONALLY REFUELED ACFT WITH FULL FUEL IN REAR HOST TANK OF THE TWIN OTTER (DHC6-200). 12 PAX WERE BOARDED. ACFT DEPARTED EBOUND STRAIGHT OUT AND LEVELED OFF AT APPROX 700 FT AGL. ALMOST IMMEDIATELY; 9 PAX CAME FORWARD TO THE COCKPIT AND ALERTED THE COPLT OF FUEL SMELL IN REAR OF CABIN. WE ACKNOWLEDGED HER COMMENT AND BEGAN TO DISCUSS POTENTIAL CAUSES OF PROB. PAX IMMEDIATELY RE-ENTERED THE COCKPIT WHILE SHOWING HER HAND WETTED BY FUEL FROM PLACING IT ON REAR CABIN FLOOR BY THE AIRSTAIR. I IMMEDIATELY TURNED BACK TOWARDS THE ARPT AND LANDED APPROX 5 MINS LATER. ACFT TAXIED IN; SHUTDOWN AND UNLOADED PAX WITHOUT EVENT. CREW INSPECTED ACFT AND FOUND CARPET AT TOP OF AIRSTAIR (APPROX 2 SQUARE FT) WET WITH FUEL; AND FUEL DRIPPING FROM REAR BELLY OF ACFT. FUEL CAPS WERE INSPECTED AND FOUND TO BE SECURED AND LOCKED APPROX IN PLACE. HOWEVER; AS REAR CAP WAS REMOVED; IT WAS NOTED THAT IT FELT 'LOOSE' WHILE REMOVING. FUEL WAS NOTED TO STILL BE STANDING IN FUEL FILLER NECK -- INDICATING FULL TANK. ON CLOSER INSPECTION; THE BACKING LOCK NUT ON CAP WHICH CTLS THE 'LOCKED-ON' PRESSURE OF THE CAP APPEARED TO HAVE POSSIBLY LOOSENED; RESULTING IN A LESS THAN IDEAL TIGHT SEAL OF 'O' RING ON CAP. (NOT COMPRESSING ADEQUATELY) CAP WAS SWITCHED; WITH FRONT CAP AND TIGHTENED WITH BOTH CAPS 'FEELING' TIGHTER IN THEIR NEW RESPECTIVE FILLER NECKS. CARPET WAS REMOVED AND CABIN CLRED AND DEODORIZED. PAX COMPLAINED INITIALLY OF SLIGHT HEADACHES FROM FUEL ODOR AND 2 REAR NEST PAX SEATED ON AFT BULKHEAD HAD LIGHT FUEL SPRAY ON SHOES. THE FUEL APPARENTLY FLOWED OUT OF AFT TANK AROUND CAP AND INFILTRATED CABIN THROUGH GASKET AROUND REAR AIRSTAIR DOOR FRAME. THE PROB APPEARED TO BE CAUSED BY A COMBINATION OF FULL FUEL IN TANK; LOOSENED LOCK NUT ON CAP AND POOR GASKET SEAL AROUND DOOR. THIS PROB HAS BEEN NOTED BEFORE IN OUR TWIN OTTER OPS. PERMANENT CURE COULD INCLUDE BETTER FAIL-SAFE CAP SEAL DESIGN OR 'SPOILER' MOUNTED AFT OF FUEL FILLER TO CAUSE FUEL TO FLOW OFF FUSELAGE IN FLT.