Narrative:

I was working local control position when an small aircraft X reported that he was inbound for landing from perimeter mall, which is northwest of pdk airport. I instructed the small aircraft X to enter a left downwind and issued traffic above and behind him descending from 5400 ft MSL. Small aircraft X was descending from 2400 MSL to 2000 MSL. A short time later, small aircraft Y reported inbound from the northwest. At that time, I instructed the small aircraft Y to enter a widened out 3 mi left downwind to avert the small aircraft X traffic below and in front of him, which I advised him of. The small aircraft Y neglected to follow my instructions and proceeded to enter a normal left downwind about 500 ft above the small aircraft X. Once I observed both aircraft in the left downwind, I instructed small aircraft Y to make a right 360 degree turn and follow a third aircraft behind him. As I observed the turn, I realized small aircraft Y was at a vantage point where he could sight the small aircraft X, so I told him to remain in the downwind and follow the small aircraft X off his left. The small aircraft X did have small aircraft Y in sight, so I changed his runway to runway 2R and cleared small aircraft Y to land on runway 2L. I believe there was a student flying small aircraft Y with an instructor on board.

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Original NASA ASRS Text

Title: SMA Y NON ADHERENCE TO ATC INSTRUCTION HAD NMAC WITH SMA X IN TFC PATTERN. SEE AND AVOID CONCEPT.

Narrative: I WAS WORKING LCL CTL POS WHEN AN SMA X RPTED THAT HE WAS INBOUND FOR LNDG FROM PERIMETER MALL, WHICH IS NW OF PDK ARPT. I INSTRUCTED THE SMA X TO ENTER A L DOWNWIND AND ISSUED TFC ABOVE AND BEHIND HIM DSNDING FROM 5400 FT MSL. SMA X WAS DSNDING FROM 2400 MSL TO 2000 MSL. A SHORT TIME LATER, SMA Y RPTED INBOUND FROM THE NW. AT THAT TIME, I INSTRUCTED THE SMA Y TO ENTER A WIDENED OUT 3 MI L DOWNWIND TO AVERT THE SMA X TFC BELOW AND IN FRONT OF HIM, WHICH I ADVISED HIM OF. THE SMA Y NEGLECTED TO FOLLOW MY INSTRUCTIONS AND PROCEEDED TO ENTER A NORMAL L DOWNWIND ABOUT 500 FT ABOVE THE SMA X. ONCE I OBSERVED BOTH ACFT IN THE L DOWNWIND, I INSTRUCTED SMA Y TO MAKE A R 360 DEG TURN AND FOLLOW A THIRD ACFT BEHIND HIM. AS I OBSERVED THE TURN, I REALIZED SMA Y WAS AT A VANTAGE POINT WHERE HE COULD SIGHT THE SMA X, SO I TOLD HIM TO REMAIN IN THE DOWNWIND AND FOLLOW THE SMA X OFF HIS L. THE SMA X DID HAVE SMA Y IN SIGHT, SO I CHANGED HIS RWY TO RWY 2R AND CLRED SMA Y TO LAND ON RWY 2L. I BELIEVE THERE WAS A STUDENT FLYING SMA Y WITH AN INSTRUCTOR ON BOARD.

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.