TRACON controller described a developing conflict event when clearing an aircraft for a visual appro...

Narrative:

A MD88 was 4 miles in trail of a B737 on the downwind to ZZZ. Traffic to follow was issued as 'just left of 12 o'clock 4 miles about to turn base.' the MD88 responded 'traffic in sight; runway in sight.' the MD88 was instructed to follow the B737 and cleared for the visual approach. I then instructed the B737 to start his base and switched him to the tower. Moments later; after other traffic checked on frequency; I noticed the MD88 cutting off the traffic he was instructed to follow and turning in on the B737's traffic to follow. I instructed the MD88 to stop his descent and asked if he still had his traffic at 9 o'clock in sight; he told me he had 2 o'clock traffic in sight and then that he had the 9 o'clock traffic now as well. I turned the MD88 back to the southwest for re-sequence; coordinated with the tower regarding what was happening and re sequenced the MD88 to follow his original traffic again; and switched him to the tower. Don't clear aircraft on the downwind to follow traffic on the downwind if there is traffic in the base turn. Also; don't include terms like 'about to turn base' when describing traffic to follow when additional traffic is in the base turn already.

Original NASA ASRS Text

Title: TRACON Controller described a developing conflict event when clearing an aircraft for a visual approach only to discover the aircraft was following the wrong aircraft.

Narrative: A MD88 was 4 miles in trail of a B737 on the downwind to ZZZ. Traffic to follow was issued as 'just left of 12 o'clock 4 miles about to turn base.' The MD88 responded 'traffic in sight; runway in sight.' The MD88 was instructed to follow the B737 and cleared for the visual approach. I then instructed the B737 to start his base and switched him to the Tower. Moments later; after other traffic checked on frequency; I noticed the MD88 cutting off the traffic he was instructed to follow and turning in on the B737's traffic to follow. I instructed the MD88 to stop his descent and asked if he still had his traffic at 9 o'clock in sight; he told me he had 2 o'clock traffic in sight and then that he had the 9 o'clock traffic now as well. I turned the MD88 back to the southwest for re-sequence; coordinated with the Tower regarding what was happening and re sequenced the MD88 to follow his original traffic again; and switched him to the Tower. Don't clear aircraft on the downwind to follow traffic on the downwind if there is traffic in the base turn. Also; don't include terms like 'about to turn base' when describing traffic to follow when additional traffic is in the base turn already.

Data retrieved from NASA's ASRS site as of July 2013 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.

Attributes
ACN	1073185
Time
Date	201303
Local Time Of Day	1801-2400
Place
Locale Reference	ZZZ.Airport
State Reference	US
Aircraft 1
Make Model Name	MD-88
Operating Under FAR Part	Part 121
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
Events
Anomaly	ATC Issue All Types Conflict Airborne Conflict

TRACON controller described a developing conflict event when clearing an aircraft for a visual approach only to discover the aircraft was following the wrong aircraft.