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.