MD-88
Air Traffic Control Fully Certified
Communication Breakdown; Situational Awareness
ATC Issue All Types; Conflict Airborne Conflict
Air Traffic Control Issued New Clearance; Air Traffic Control Separated Traffic
Human Factors; Procedure
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.
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.