Sct controller described a conflict event when clearing downwind traffic landing san to follow traff...

Attributes
ACN	983253
Time
Date	201112
Local Time Of Day	1801-2400
Place
Locale Reference	SAN.Airport
State Reference	CA
Aircraft 1
Make Model Name	Citation Excel (C560XL)
Operating Under FAR Part	Part 91
Flight Phase	Final Approach
Route In Use	Visual Approach
Flight Plan	IFR
Aircraft 2
Make Model Name	B737 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	Final Approach
Route In Use	Visual Approach
Flight Plan	IFR
Person 1
Function	Approach Departure
Qualification	Air Traffic Control Fully Certified
Person 2
Function	Pilot Not Flying Captain
Qualification	Flight Crew Multiengine Flight Crew Instrument Flight Crew Air Transport Pilot (ATP)
Experience	Flight Crew Last 90 Days 100 Flight Crew Total 7000 Flight Crew Type 1200
Events
Anomaly	Conflict Airborne Conflict Deviation - Procedural Clearance Deviation - Track / Heading All Types

Narrative:

A C560XL was on the north downwind IFR being vector for a visual approach at 6;000 ft to san airport. I pointed out traffic; a B737 ahead on downwind 6 miles; at 6;000 ft; about to turn base. The C560XL reported the B737 in sight. I instructed the C560XL to turn base at or above 5;000 ft (this is to keep the aircraft in class bravo airspace); follow the B737 ahead; caution wake turbulence; cleared visual approach. The C560XL acknowledged the clearance. Also; there was a second B737 on straight in final for san airport at 3;000 ft. The first B737 jet that the C560XL was sequenced behind was on a visual approach to follow the other B737. Shortly after; I observed the C560XL turning base leg inside of the B737 [they were to follow] and now on a converging course. I issued an immediate stop to the C560XL on his descent; a left turn away from the B737; and traffic on the B737. The C560XL reported the correct B737 in sight and I re sequenced him again to follow the traffic. I am estimating there was approximately 2 miles lateral and 800 ft vertical between the aircraft at closest proximity. It appeared the C560XL had the wrong aircraft in sight and followed the other B737 on final and not the intended B737 on downwind. Recommendation; I was very specific that the aircraft the C560XL was following was on downwind at his altitude. I made no mention of the aircraft on final 3;000 ft below. I'm not sure what can be done to preclude this from happening. I also used company livery to help preclude getting the wrong aircraft in sight.

Original NASA ASRS Text

Title: SCT Controller described a conflict event when clearing downwind traffic landing SAN to follow traffic ahead on downwind; the traffic turned base inside traffic and conflicted with a straight in aircraft.

Narrative: A C560XL was on the north downwind IFR being vector for a Visual Approach at 6;000 FT to SAN airport. I pointed out traffic; a B737 ahead on downwind 6 miles; at 6;000 FT; about to turn base. The C560XL reported the B737 in sight. I instructed the C560XL to turn base at or above 5;000 FT (this is to keep the aircraft in Class Bravo airspace); follow the B737 ahead; caution wake turbulence; cleared Visual Approach. The C560XL acknowledged the clearance. Also; there was a second B737 on straight in final for SAN airport at 3;000 FT. The first B737 jet that the C560XL was sequenced behind was on a Visual Approach to follow the other B737. Shortly after; I observed the C560XL turning base leg inside of the B737 [they were to follow] and now on a converging course. I issued an immediate stop to the C560XL on his descent; a left turn away from the B737; and traffic on the B737. The C560XL reported the correct B737 in sight and I re sequenced him again to follow the traffic. I am estimating there was approximately 2 miles lateral and 800 FT vertical between the aircraft at closest proximity. It appeared the C560XL had the wrong aircraft in sight and followed the other B737 on final and not the intended B737 on downwind. Recommendation; I was very specific that the aircraft the C560XL was following was on downwind at his altitude. I made no mention of the aircraft on final 3;000 FT below. I'm not sure what can be done to preclude this from happening. I also used company livery to help preclude getting the wrong aircraft in sight.

Data retrieved from NASA's ASRS site as of April 2012 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.

Sct controller described a conflict event when clearing downwind traffic landing san to follow traffic ahead on downwind; the traffic turned base inside traffic and conflicted with a straight in aircraft.