Narrative:

Air carrier X was being vectored for a right base visual approach to ZZZ's runway 32. When I issued the visual approach clearance he was established on a 5 mile base leg and had approximately 8 flying miles to the airport. Upon receipt of the visual approach clearance the pilot gave me no warning of his intentions to turn away from the airport for a downwind instead of continue on his natural path inbound. In my experience with erj-170's; I am confident that I put the aircraft in a position to allow safe and efficient descent into the airport without this downwind turn. When air carrier X took this unexpected turn he became a conflict with aircraft Y; a VFR C182 being vectored inbound to follow the erj-170. Aircraft Y was southeast of ZZZ to 2;000 on a 020 heading which kept him southeast of air carrier X's projected base leg entry to runway 32. When I observed the conflict I issued traffic to both aircraft and aircraft Y reported the erj-170 in sight. Recommendation; as an atcs; when I clear an aircraft for a visual approach I expect that aircraft to continue on a flight path inbound to the prescribed runway; and I am basing the rest of my traffic sequence on that expectation. If a pilot is in a position where he feels he has to do something different than that it is important that I have that information so that I can react accordingly. Air carrier X was clearly established on the base leg for his approach and was given ample time for descent to runway 32 without a downwind turn; therefore; I was expecting aircraft Y to be in a safe position to follow him to the airport. I believe the lesson to be learned for this incident is that pilots and atcs' always need to keep each other informed of information that could effect the safety of the NAS.

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Original NASA ASRS Text

Title: TRACON Controller described a conflict event between an air carrier on base cleared for a Visual Approach and another aircraft on a parallel track to follow; the base traffic made an unexpected turn to downwind.

Narrative: Air Carrier X was being vectored for a right base visual approach to ZZZ's Runway 32. When I issued the visual approach clearance he was established on a 5 mile base leg and had approximately 8 flying miles to the airport. Upon receipt of the visual approach clearance the pilot gave me no warning of his intentions to turn away from the airport for a downwind instead of continue on his natural path inbound. In my experience with ERJ-170's; I am confident that I put the aircraft in a position to allow safe and efficient descent into the airport without this downwind turn. When Air Carrier X took this unexpected turn he became a conflict with Aircraft Y; a VFR C182 being vectored inbound to follow the ERJ-170. Aircraft Y was southeast of ZZZ to 2;000 on a 020 heading which kept him southeast of Air Carrier X's projected base leg entry to Runway 32. When I observed the conflict I issued traffic to both aircraft and Aircraft Y reported the ERJ-170 in sight. Recommendation; as an ATCS; when I clear an aircraft for a visual approach I expect that aircraft to continue on a flight path inbound to the prescribed runway; and I am basing the rest of my traffic sequence on that expectation. If a pilot is in a position where he feels he has to do something different than that it is important that I have that information so that I can react accordingly. Air Carrier X was clearly established on the base leg for his approach and was given ample time for descent to Runway 32 without a downwind turn; therefore; I was expecting Aircraft Y to be in a safe position to follow him to the airport. I believe the lesson to be learned for this incident is that pilots and ATCS' always need to keep each other informed of information that could effect the safety of the NAS.

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.