Narrative:

I was working radar in roa TRACON. An small aircraft, aircraft X, was climbing sebound toward practice area. I advised him that his mode C was inoperative. He told me 'it takes a while to warm up.' when small aircraft X was about 5 mi southeast of roa airport the mode C began to indicate 3100 intermittently. I so advised the pilot and asked his altitude. He concurred with 3100. The mode C began to function continuously as the aircraft X reached 3500. An small transport Y was VFR, inbound to roa from the southeast. He reported the airport in sight and began descending from 4500. I told him to fly straight in runway 33 and maintain 4000 for opposite direction traffic at 3500. I asked the aircraft X to verify altitude and described small transport Y traffic. Small aircraft X verified altitude. As the targets passed (beacon slashes crossed and primaries touched), I cancelled the small transport Y's altitude restriction. He saw the small aircraft X as the 2 passed abeam. X did not report Y in sight. Later, in the afternoon, the small aircraft X pilot reported for duty as an air traffic assistant at roa TRACON. He did not know I was in the room. I overheard him telling someone that he knew the aircraft's mode C to be inaccurate and always concurred with whatever altitude the controller said. He further stated that he cruised at whatever altitude caused the mode C to 'read right.' (in this case 3800 to show 3500). I asked him to verify that, when he had reported, and my radar had indicated 3500, that he was actually at 3800. He verified it. I asked if he had seen the opposite direction at 4000. He did not answer. I said something unprofessional. He responded, 'you can't prove nuthin.' I later contacted the small aircraft X owner who had trusted his aircraft to the pilot. The owner stated that the aircraft's mode C was not certified and consistently indicated 300 ft. Low. The above described is not unusual behavior for this particular pilot/air traffic area -- a cfii who teaches aviation in a local high school.

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Original NASA ASRS Text

Title: DELIBERATE ALTDEV IN ORDER TO MAKE ACFT'S MODE C TO AGREE WITH DEP CTLR'S READOUT ON ALT. NMAC.

Narrative: I WAS WORKING RADAR IN ROA TRACON. AN SMA, ACFT X, WAS CLBING SEBOUND TOWARD PRACTICE AREA. I ADVISED HIM THAT HIS MODE C WAS INOP. HE TOLD ME 'IT TAKES A WHILE TO WARM UP.' WHEN SMA X WAS ABOUT 5 MI SE OF ROA ARPT THE MODE C BEGAN TO INDICATE 3100 INTERMITTENTLY. I SO ADVISED THE PLT AND ASKED HIS ALT. HE CONCURRED WITH 3100. THE MODE C BEGAN TO FUNCTION CONTINUOUSLY AS THE ACFT X REACHED 3500. AN SMT Y WAS VFR, INBOUND TO ROA FROM THE SE. HE RPTED THE ARPT IN SIGHT AND BEGAN DSNDING FROM 4500. I TOLD HIM TO FLY STRAIGHT IN RWY 33 AND MAINTAIN 4000 FOR OPPOSITE DIRECTION TFC AT 3500. I ASKED THE ACFT X TO VERIFY ALT AND DESCRIBED SMT Y TFC. SMA X VERIFIED ALT. AS THE TARGETS PASSED (BEACON SLASHES CROSSED AND PRIMARIES TOUCHED), I CANCELLED THE SMT Y'S ALT RESTRICTION. HE SAW THE SMA X AS THE 2 PASSED ABEAM. X DID NOT RPT Y IN SIGHT. LATER, IN THE AFTERNOON, THE SMA X PLT RPTED FOR DUTY AS AN AIR TFC ASSISTANT AT ROA TRACON. HE DID NOT KNOW I WAS IN THE ROOM. I OVERHEARD HIM TELLING SOMEONE THAT HE KNEW THE ACFT'S MODE C TO BE INACCURATE AND ALWAYS CONCURRED WITH WHATEVER ALT THE CTLR SAID. HE FURTHER STATED THAT HE CRUISED AT WHATEVER ALT CAUSED THE MODE C TO 'READ RIGHT.' (IN THIS CASE 3800 TO SHOW 3500). I ASKED HIM TO VERIFY THAT, WHEN HE HAD RPTED, AND MY RADAR HAD INDICATED 3500, THAT HE WAS ACTUALLY AT 3800. HE VERIFIED IT. I ASKED IF HE HAD SEEN THE OPPOSITE DIRECTION AT 4000. HE DID NOT ANSWER. I SAID SOMETHING UNPROFESSIONAL. HE RESPONDED, 'YOU CAN'T PROVE NUTHIN.' I LATER CONTACTED THE SMA X OWNER WHO HAD TRUSTED HIS ACFT TO THE PLT. THE OWNER STATED THAT THE ACFT'S MODE C WAS NOT CERTIFIED AND CONSISTENTLY INDICATED 300 FT. LOW. THE ABOVE DESCRIBED IS NOT UNUSUAL BEHAVIOR FOR THIS PARTICULAR PLT/ATA -- A CFII WHO TEACHES AVIATION IN A LCL HIGH SCHOOL.

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.