Narrative:

A C150 departed hky direct bzm. They forgot to turn their transponder on. I told them radar contact, turn on your transponder. When I got back to them a couple of mins later, I saw a VFR code. It dawned on me that this was probably a student. I slowed down a little and asked them to squawk XXXX when they got a chance. I finally got a good beacon when they were over tawba, about 5 mi from the airport. Aircraft X was only at 3000 ft and climbing very slowly toward bem. I had a BE58 landing hky that I was vectoring parallel to aircraft X, waiting for altitude separation so I could turn the baron onto the ILS/facility. I asked aircraft X if he could increase his climb rate. He couldn't. I informed the baron of the traffic and advised that I would turn him toward the localizer/facility when aircraft X was out of 4400 ft. The baron was at 3400 ft. I was very busy. Multiple approachs and departures to multiple airports. Finally! I saw aircraft X leave 4400 ft and turned the baron toward the localizer/facility. About 1 min later, aircraft X advised me they had some trouble with their mode C earlier. They were out of 4100 ft, what did their mode C say? It said 4700 ft. Aircraft X and the baron were only 2 mi apart. There was nothing to be done. I was already turning the baron inbound, which turned him away from aircraft X. I religiously verify mode C on departures, but because of all the transponder trouble and traffic volume, I missed checking this one.

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Original NASA ASRS Text

Title: ARTCC CTLR AT ZTL LOST SEPARATION BECAUSE OF FAULTY MODE C INFO.

Narrative: A C150 DEPARTED HKY DIRECT BZM. THEY FORGOT TO TURN THEIR XPONDER ON. I TOLD THEM RADAR CONTACT, TURN ON YOUR XPONDER. WHEN I GOT BACK TO THEM A COUPLE OF MINS LATER, I SAW A VFR CODE. IT DAWNED ON ME THAT THIS WAS PROBABLY A STUDENT. I SLOWED DOWN A LITTLE AND ASKED THEM TO SQUAWK XXXX WHEN THEY GOT A CHANCE. I FINALLY GOT A GOOD BEACON WHEN THEY WERE OVER TAWBA, ABOUT 5 MI FROM THE ARPT. ACFT X WAS ONLY AT 3000 FT AND CLBING VERY SLOWLY TOWARD BEM. I HAD A BE58 LNDG HKY THAT I WAS VECTORING PARALLEL TO ACFT X, WAITING FOR ALT SEPARATION SO I COULD TURN THE BARON ONTO THE ILS/FAC. I ASKED ACFT X IF HE COULD INCREASE HIS CLB RATE. HE COULDN'T. I INFORMED THE BARON OF THE TFC AND ADVISED THAT I WOULD TURN HIM TOWARD THE LOC/FAC WHEN ACFT X WAS OUT OF 4400 FT. THE BARON WAS AT 3400 FT. I WAS VERY BUSY. MULTIPLE APCHS AND DEPS TO MULTIPLE ARPTS. FINALLY! I SAW ACFT X LEAVE 4400 FT AND TURNED THE BARON TOWARD THE LOC/FAC. ABOUT 1 MIN LATER, ACFT X ADVISED ME THEY HAD SOME TROUBLE WITH THEIR MODE C EARLIER. THEY WERE OUT OF 4100 FT, WHAT DID THEIR MODE C SAY? IT SAID 4700 FT. ACFT X AND THE BARON WERE ONLY 2 MI APART. THERE WAS NOTHING TO BE DONE. I WAS ALREADY TURNING THE BARON INBOUND, WHICH TURNED HIM AWAY FROM ACFT X. I RELIGIOUSLY VERIFY MODE C ON DEPS, BUT BECAUSE OF ALL THE XPONDER TROUBLE AND TFC VOLUME, I MISSED CHKING THIS ONE.

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.