Crq local controller reported a near midair collision when an aircraft flew beneath another aircraft...

Attributes
ACN	1447852
Time
Date	201705
Local Time Of Day	1201-1800
Place
Locale Reference	CRQ.Tower
State Reference	CA
Environment
Light	Daylight
Aircraft 1
Make Model Name	Light Sport Aircraft
Operating Under FAR Part	Part 91
Flight Phase	Final Approach
Flight Plan	VFR
Aircraft 2
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
Person 1
Function	Local
Qualification	Air Traffic Control Fully Certified
Experience	Air Traffic Control Time Certified In Pos 1 (yrs) 5
Events
Anomaly	ATC Issue All Types Conflict NMAC Deviation - Procedural Clearance Deviation - Track / Heading All Types
Miss Distance	Vertical 200

Narrative:

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.

Original NASA ASRS Text

Title: CRQ Local Controller reported a NMAC when an aircraft flew beneath another aircraft after not complying with ATC instructions.

Narrative: 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.

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 local controller reported a near midair collision when an aircraft flew beneath another aircraft after not complying with ATC instructions.