Small Aircraft; High Wing; 1 Eng; Fixed Gear
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 7
Communication Breakdown
ATC Issue All Types; Conflict Airborne Conflict; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Issued Advisory / Alert; Air Traffic Control Separated Traffic; Flight Crew Returned To Clearance
Procedure; Human Factors
I was working the Local Control position. There was a moderate level of VFR traffic and complexity. Aircraft Y called inbound from the southwest and was instructed to make left traffic. Aircraft X was VFR on final for runway 24. Aircraft Z was in the VFR traffic pattern; right closed traffic. When Aircraft Y reached the left downwind; she was instructed that the tower would call her base turn. She responded; acknowledging that she understood the tower would instruct her when to turn base. As the 3 aircraft approached parallel positions (Aircraft X on final; Aircraft Z extended right downwind; Aircraft Y extended left downwind) Aircraft Y turned base; directly towards Aircraft X. I immediately instructed Aircraft Y to turn back to the downwind and issued a traffic alert. Aircraft Y returned to the downwind as Aircraft X reported them in sight. Aircraft Y seemed completely unaware of what she was supposed to do even after acknowledging the clear instructions that were issued. Pilot training. Don't just read back the controller's instructions to him/her. Be sure to understand the instructions. If they are unclear; or you just don't comprehend them; ask the controller to clarify.
CRQ Tower Controller reported a pilot was given an instruction and read it back correctly. Pilot turned against instructions causing an airborne conflict.