Light Sport Aircraft
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 5
ATC Issue All Types; Conflict NMAC; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Issued New Clearance
Human Factors; Procedure
I was getting relieved from work after one hour on Local Control position. Aircraft Y was in the traffic pattern doing touch and go's for most of the hour. During the position relief briefing; I noticed that Aircraft Y was flying close to Aircraft X who was on final approach. I took corrective action; asked Aircraft Y if they were still on the downwind. They replied; 'Affirmative.' Then I instructed Aircraft Y to turn left 10 degrees; to not cut off the traffic on final. Aircraft Y read back; 'Turning left 10 degrees.' I then gave a traffic call. On the next sweep of the radar; I noticed Aircraft Y had made a right turn toward Aircraft X so I instructed Aircraft Y to fly north and then continue north until advised. Aircraft X advised me that Aircraft Y just flew beneath him about 200 feet away. I finished the relief briefing; and then went on break. I gave this pilot many control instructions and he read back everything in accordance with my instructions but still did the opposite of my instructions putting both aircraft at risk. I recommend the pilot in command get remedial training on compliance.
CRQ Local Controller reported a NMAC when an aircraft flew beneath another aircraft after not complying with ATC instructions.