A PA28 PILOT REPORTED THAT HE FAILED TO LOAD AND ACTIVATE A REVISED APPROACH PROCEDURE IN HIS GPS; RESULTING IN A TRACK DEVIATION AND DESCENT BELOW THE PUBLISHED APPROACH SEGMENT ALTITUDE.

Narrative

I WAS FLYING FROM 1A9 TO TCL; VIA OKW. S OF BIRMINGHAM; I WAS CLRED DIRECT TO TCL; BYPASSING OKW. WHEN A FEW MILES SE OF OKW VOR; THE BIRMINGHAM APCH CTLR CHANGED MY CLRNC TO THE TCL VOR RWY 22 APCH VIA THE OKW TRANSITION. I HAD OKW ALREADY IN MY GPS FLT PLAN (GARMIN 430; IFR APCH CERTIFIED); SO SELECTED DIRECT TO OKW FROM THE FLT PLAN PAGE SO I COULD GET THE HDG QUICKLY. I HAD THE CORRECT/CURRENT APCH PLATE OPEN AND CLIPPED TO THE YOKE; BUT FAILED TO ACTIVATE THE VOR RWY 22 APCH IN THE GPS. WHEN THE ACFT REACHED OKW; THE GPS TRANSITIONED TO THE NEXT FLT PLAN WAYPOINT; TCL. I TURNED L DIRECT TO TCL; INCORRECTLY FOLLOWING THE GPS GUIDANCE RATHER THAN THE OKW R-300 PER THE IAP. THIS PUT ME ON A TRACK APPROX ON THE TCL R-092; AND THE OKW R-272. FOLLOWING THE PRINTED IAP; I DSNDED TO 2250 FT MSL (IAP REQUIRES AT/ABOVE 2200 FT FROM OKW TO THE IAF). APPROX 5 MILES SW OF OKW; THE CTLR ASKED ME TO STATE MY ALT AND HDG. I RESPONDED 2250 FT AND 268 DEGS. HE POINTED OUT I WAS DEVIATING FROM THE BROOKWOOD TRANSITION AS CLRED. I REALIZED MY ERROR AND TURNED N TO INTERCEPT THE OKW R-300; RPTED MY CORRECTIVE ACTION; AND ACTIVATED THE APCH IN MY GPS. I FLEW THE REST OF THE IAP CORRECTLY AND WITHOUT INCIDENT; USING VOR AS PRIMARY NAVIGATION AND GPS FOR SITUATIONAL AWARENESS. CONTRIBUTING FACTORS: FAILURE TO LOAD AND ACTIVATE THE GPS APCH; AND RELIANCE ON INCORRECT GPS GUIDANCE INSTEAD OF FOLLOWING THE PUBLISHED IAP. PREVENTION: FOLLOW THE PUBLISHED IAP FIRST; AND ENSURE GPS GUIDANCE FOLLOWS IT.