Crq tower controller reported a pilot was given an instruction and read it back correctly. Pilot tur...

Attributes
ACN	1469863
Time
Date	201707
Local Time Of Day	1201-1800
Place
Locale Reference	CRQ.Tower
State Reference	CA
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	Small Aircraft High Wing 1 Eng Fixed Gear
Operating Under FAR Part	Part 91
Flight Phase	Final Approach
Route In Use	None
Aircraft 2
Make Model Name	Small Aircraft Low Wing 1 Eng Fixed Gear
Operating Under FAR Part	Part 91
Flight Phase	Initial Approach
Route In Use	None
Flight Plan	VFR
Person 1
Function	Local
Qualification	Air Traffic Control Fully Certified
Experience	Air Traffic Control Time Certified In Pos 1 (yrs) 7
Events
Anomaly	ATC Issue All Types Conflict Airborne Conflict Deviation - Procedural Clearance Deviation - Track / Heading All Types

Narrative:

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.

Original NASA ASRS Text

Title: CRQ Tower Controller reported a pilot was given an instruction and read it back correctly. Pilot turned against instructions causing an airborne conflict.

Narrative: 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.

Data retrieved from NASA's ASRS site 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.

Crq tower controller reported a pilot was given an instruction and read it back correctly. Pilot turned against instructions causing an airborne conflict.