WHILE OPERATING ON AN IFR FLT PLAN IN VMC; A CAPT WHO WAS COMPLETING IOE INADVERTENTLY VISUALLY DSNDED WITHOUT A CLRNC FOR A LOWER ALT. THE RPTR STATES THAT HE WAS ACCUSTOMED TO OPERATING ON A VFR FLT PLAN AND FOLLOWING THE COMPANY PROCEDURAL VISUAL DSCNT AT THIS POINT.

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ON AN IFR FLT PLAN INTO SEATTLE; 25 MI OUT; THE CTLR ASKED IF I HAD A VISUAL LANDMARK IN SIGHT NEAR THE ARPT. I ACKNOWLEDGED THAT I HAD THE LANDMARK AND THE CTLR ADVISED ME TO PROCEED DIRECT TO THE LANDMARK. THIS WAS THE FIRST IFR FLT ON THIS RTE DURING IOE AFTER 2 DAYS OF VFR FLTS INTO THIS ARPT. THE VISUAL APCH UTILIZES THE SAME LANDMARK AND COMPANY PROC IS TO BEGIN A DSCNT TO 2000 FT ABOUT 10 MI OUT. FOR SOME REASON I THOUGHT I WAS ON THE VISUAL ARR AND BEGAN A DSCNT FROM 3000 FT. AT 2400 FT THE CHK AIRMAN REMINDED ME THAT WE WERE ON AN IFR CLRNC; NOT A VISUAL ARR. I IMMEDIATELY CLBED BACK TO 3000 FT. I BELIEVE THIS HAPPENED FOR SEVERAL REASONS. I HAVE NOT FLOWN OR OPERATED IN THE IFR ENVIRONMENT DURING MY LAST 3000 HRS OF PART 135 FLYING. WE HAD BEEN DOING THE SAME RTE VFR; UTILIZING THE VISUAL ARR ON ALL PREVIOUS TRIPS. THE WX WAS VMC. THE CTLR TOLD ME TO PROCEED VISUALLY TO THE SAME LANDMARK USED FOR THE VISUAL ARR; CAUSING ME TO MENTALLY BELIEVE WE WERE CLRED FOR THE VISUAL ARR; WHICH WE WERE NOT. CONTRIBUTING WITH THE FACT THAT I WAS FLYING JUST AFTER AN EPISODE WITH THE FLU; WHICH GROUNDED ME FOR 2 DAYS; AND STILL HAD A HEAD COLD AT THE TIME OF THIS INCIDENT; WHICH MAY HAVE DIMINISHED MY ABILITY TO MENTALLY STAY ON TOP OF THE FLT. SUPPLEMENTAL INFO FROM ACN 334475: INCIDENT WOULD HAVE BEEN AVOIDED HAD PLT ADVISED ATC HE WAS LEAVING ALT.