A C210 PLT EXPERIENCES A POS AND TRACK DEV WHEN USING HIS AGED KFC-200 AUTOPLT 5 MI NE OF LSE; WI.

Narrative

I WAS PERFORMING THE DME ARC (14 MI ARC) TO THE ILS RWY 18 APCH AT LA CROSSE. AS I APCHED THE LOC; I ACTIVATED THE KFC-200 AUTOPLT AND IT WENT INTO THE ARM MODE. A SHORT TIME LATER (1 MIN MAYBE) THE AUTOPLT WENT INTO THE COUPLED MODE AND PROCEEDED TO INTERCEPT THE LOC. ABOUT 30 SECONDS LATER IT SHOWED ME FLYING THROUGH THE LOC AND THEN THE AUTOPLT CORRECTED AND TOOK ME BACK ON A COURSE OF ABOUT 170 DEGS. AT THIS POINT; THE LOC NEEDLE WAS SHOWING ABOUT 1-1 1/2 DOTS TO THE L OF CTR; AND BASICALLY STAYED THERE OR WOULD OCCASIONALLY MOVE CLOSER TO THE CTR. THE LOC DID FLAG MOMENTARILY ABOUT ONCE EVERY 45-60 SECONDS FOR ABOUT 1/2 SECOND. WHEN I DID NOT INTERCEPT THE OM OR GS; I DECLARED A MISSED APCH. WHEN I TUNED IN THE LCL FIELD VOR FOR THE MISSED APCH; I THEN REALIZED I WAS 5 MI NE OF THE FIELD AND WAS OFF THE LOC COURSE. I WAS NOW VFR AND LANDED WITHOUT INCIDENT. I HAVE SINCE CHKED THE KFC-200 ON OTHER ILS APCHS AND HAVE FOUND NO PROBS. I DO NOT KNOW WHAT CAUSED THE PROB; BUT WILL BE WATCHING THE AUTOPLT VERY VIGILANTLY SINCE THIS INCIDENT AND ALSO SINCE THE UNIT IS 21 YRS OLD. IN THE FUTURE; I WILL NOT ARM THE KFC-200 SYS UNTIL THE LOC STARTS TO CTR; AND WILL BE MORE ALERT FOR ANY TYPE OF NAV FLAGGING; ESPECIALLY A LOC.