Narrative:

Returning to con from ore, I called mht approach 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 cleared to descend. The frequency was quiet and I thought approach had forgotten about me. Just as I was about to request permission to descend approach 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. Approach then told me that I had violated mht tower 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). Approach then said my mode C was 'going crazy indicating a climb.' it was becoming apparent to me that my altitude encoder was malfunctioning, but there was no way that approach would accept that explanation. By that time I was about to leave the class C airspace and I was cleared to change frequency and squawk VFR. My initial concern was that there was no way to prove that I was at the correct altitude. On landing at con, I immediately contacted the avionics shop. They confirmed that my altitude 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 aircraft had been at or slightly above 2500 ft and their TCASII had indicated a conflict might they have tried to resolve it by climbing? Even looking out the window they might not spot me readily because they would be looking for co-altitude traffic. 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 reporting conflicting traffic altitude as 'unverified' when they are receiving mode C but not communicating with that traffic. It also serves as a reminder of the importance of keeping up a good visual scan in high traffic areas even with TCASII onboard.

Related reports:

Google
 

Original NASA ASRS Text

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

Narrative: 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.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.