SMA PLT ENROUTE DUPAGE MAY HAVE PENETRATED TCA.

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TKOF FROM MKE WITH RADAR VECTORS TO SW. DEP CTL RELEASED US AT 4500' MSL ON 260 DEG HDG APPROX 10 NM FROM MKE. ADVISED TO PROCEED DIRECT TO COURSE FOR DPA (HOME LOCATION). PROCEEDED ON COURSE (260 DEGS); TURNING TO NEW COURSE OF 210 DEGS TO INTERCEPT THE 360 DEG R OF DPA VOR (TO INDICATION). ALSO SET UP INTERCEPT ON #2 NAV FOR 300 DEG R FROM OBK VOR. THIS WOULD PUT US APPROX 9 NM FROM OUTER BOUNDARY OF O'HARE TCA. AT THIS POINT WE WOULD START DSCNT TO 2700' MSL. TAILWIND OF MORE THAN 25 KTS PUSHED GND SPD IN EXCESS OF 150 KTS. WIND FROM 030 DEGS; MEANING WE WERE APCHING INTERCEPT POINT FASTER THAN WE ANTICIPATED. WE WERE CONCENTRATING ON NAV INSTRUMENTS MORE THAN ON VISUAL LANDMARKS. 2 PAX WERE ALSO LOOKING FOR TFC BEFORE OR ABOUT WHERE INTERCEPT WOULD SHOW ON NAV INSTRUMENTS. #2 NAV WENT INOP; SHOWING A DEAD CENTER INDICATION. HOWEVER; WITH DELAY IN RECOGNIZING INOP; WITH EXCESS GND SPD; POSSIBLY TOUCHED EDGE OF TCA BEFORE DSCNT TO PLANNED ALT OF 2700' MSL. ALSO; WITH A 2 DEG LEFT ERROR FROM INBND RADIAL; WOULD PUT US ABOUT 3 NM INSIDE OUTER LIMIT AT INCORRECT ALT. I'M NOT SURE THAT THE ALT WAS VIOLATED--THERE IS A POSSIBILITY. CORRECTIVE ACTION WILL BE TO NAVIGATE WITH GND REF WHEN ON VFR FLTS IN THIS AREA AND XCHK WITH NAV INSTRUMENTS. WORK ORDER FOR REPAIR OF #2 NAV INDICATES 2 BLOWN TUBES (OMNI CONVERTER). WORK PERFORMED 5/WED/88. NO OTHER TFC WAS OBSERVED IN THIS AREA AND MODE C TRANSPONDER WAS OPERATING; SET TO 1200.