A SA226TC FLYING THROUGH ZAU AIRSPACE TURNS TO A DIFFERENT HEADING AS IT CROSSES OVER GIP; MI. THIS TURN WAS NOT IN ACCORDANCE WITH ATC INSTRUCTIONS.

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FOR THE PAST SEVERAL WKS WE HAD BEEN GIVEN A 090 DEG HDG OUT OF MDW CLB TO 11000 FT DIRECT GIP. AFTER GIP WE HAVE BEEN GIVEN DIRECT FWA. A 'CANNED' FLT PLAN CALLS FOR GIP V6 SEWTO FWA. NO XFER OR HDOF WAS GIVEN TO THE NEXT CTLR AND I 'OUT OF HABIT' MADE THE TURN GIP DIRECT FWA. AFTER SEVERAL MI; READY FOR DSCNT TO FWA I ASKED FOR LOWER. THE CTLR RESPONDED WITH I SHOULD BE TALKING TO SOMEONE ELSE. WHEN HANDED OFF -- THE NEXT CTLR (CHICAGO) WENT BALLISTIC. ASKED WHO AUTH THE TURN; ETC; NOT REALIZING MY ERROR. I APOLOGIZED FOR THE INFRACTION. HE CONTINUED WITH HIS THREATS; ETC; THEN HANDED ME OFF TO FWA APCH. FWA ADVISED ME THAT A PHONE CALL TO ZAU CAN BE DISREGARDED; I DID NOT KNOW OF A REQUIRED CALL WHEN I GOT TO FWA. THE CTLR NEVER ADVISED OF SUCH; NOR ASKED. BEING VMC THERE WAS NO HAZARD (APPARENT). LACK OF USE OF THE 'CANNED' ROUTING. NO HDOF BY THE CTLRS. CONTINUED GIP DIRECT FWA TURNS 4 DAYS A WK FOR THE PAST SEVERAL WKS ALL LED TO THIS 'INFRACTION.' I DID APOLOGIZE OVER AND OVER. I AM ALSO AWARE OF THE SIT THAT COULD HAVE AROSE FROM THIS.