ZLIN 526 PLT HAD A MALFUNCTIONING MODE C XPONDER READOUT THAT INDICATED THAT HE HAD PENETRATED MHT CLASS C.

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RETURNING TO CON FROM ORE; I CALLED MHT APCH PRIOR TO ENTERING THE MHT CLASS C AIRSPACE AT 3500 FT. RADAR CONTACT WAS CONFIRMED AND I WAS ASKED TO REMAIN AT 3500 FT UNTIL CLRED TO DSND. THE FREQ WAS QUIET AND I THOUGHT APCH HAD FORGOTTEN ABOUT ME. JUST AS I WAS ABOUT TO REQUEST PERMISSION TO DSND APCH CALLED ME ASKING; 'DID I ASK YOU TO REMAIN AT 3500 FT?' I MISINTERPRETED THE QUESTION AS CONFIRMATION THAT THEY HAD FORGOTTEN ABOUT ME AND REPLIED THAT I WAS LOOKING FOR LOWER. APCH THEN TOLD ME THAT I HAD VIOLATED MHT TWR AIRSPACE AT 2500 FT BUT THAT THERE HAD BEEN NO CONFLICT. I RESPONDED THAT I WAS STILL LEVEL AT 3500 FT (ACTUALLY I HAD DRIFTED UP TO 3700 FT). APCH THEN SAID MY MODE C WAS 'GOING CRAZY INDICATING A CLB.' IT WAS BECOMING APPARENT TO ME THAT MY ALT ENCODER WAS MALFUNCTIONING; BUT THERE WAS NO WAY THAT APCH WOULD ACCEPT THAT EXPLANATION. BY THAT TIME I WAS ABOUT TO LEAVE THE CLASS C AIRSPACE AND I WAS CLRED TO CHANGE FREQ AND SQUAWK VFR. MY INITIAL CONCERN WAS THAT THERE WAS NO WAY TO PROVE THAT I WAS AT THE CORRECT ALT. ON LNDG AT CON; I IMMEDIATELY CONTACTED THE AVIONICS SHOP. THEY CONFIRMED THAT MY ALT ENCODER HAD FAILED AND WAS READING OVER 1000 FT LOW. FEELING THAT I HAD SOME DEFENSE IF ANY ENFORCEMENT ACTION WAS TAKEN I BEGAN TO THINK ABOUT OTHER IMPLICATIONS OF THIS MODE OF FAILURE. IF ANOTHER ACFT HAD BEEN AT OR SLIGHTLY ABOVE 2500 FT AND THEIR TCASII HAD INDICATED A CONFLICT MIGHT THEY HAVE TRIED TO RESOLVE IT BY CLBING? EVEN LOOKING OUT THE WINDOW THEY MIGHT NOT SPOT ME READILY BECAUSE THEY WOULD BE LOOKING FOR CO-ALT TFC. WHAT IF THIS FAILURE HAD OCCURRED WHEN I WAS VFR AND NOT TALKING TO ANYONE? I DON'T KNOW HOW COMMON THIS MODE OF FAILURE IS; BUT IT EMPHASIZES THE IMPORTANCE OF ATC'S PRACTICE OF RPTING CONFLICTING TFC ALT AS 'UNVERIFIED' WHEN THEY ARE RECEIVING MODE C BUT NOT COMMUNICATING WITH THAT TFC. IT ALSO SERVES AS A REMINDER OF THE IMPORTANCE OF KEEPING UP A GOOD VISUAL SCAN IN HIGH TFC AREAS EVEN WITH TCASII ONBOARD.