Narrative:

Aircraft X was in the tower VFR pattern. Local control coordinated with approach that aircraft X will follow traffic on the ILS approach. Aircraft Y was turned on the approach at 2;000 feet and was tracking the approach inbound. I was conducting training and we were discussing how aircraft X will fall in nice behind aircraft Y. Right before approach was going to switch aircraft Y to the tower I noticed aircraft X turning his base turn. He was only 2 miles off the approach course opposite direction as it was. The trainee on approach issued a traffic alert and gave aircraft Y a turn to the right heading 270 degrees. This put the aircraft nose to nose. I took over the frequency and gave aircraft Y a left turn 040 away from aircraft X.aircraft Y did not see aircraft X due to a cloud. There was maybe a mile between the 2 aircraft and closing. It was close. Closer than anything I have seen in [all my] years. The pilot of [aircraft X] later called the approach control and spoke to the supervisor admitting to the error. I was able to watch the radar replay and it was close. Disturbing.

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Original NASA ASRS Text

Title: A TRACON Controller reported an aircraft on downwind turned base leg on its own toward traffic established on short final.

Narrative: Aircraft X was in the Tower VFR pattern. Local Control coordinated with Approach that Aircraft X will follow traffic on the ILS Approach. Aircraft Y was turned on the approach at 2;000 feet and was tracking the approach inbound. I was conducting training and we were discussing how Aircraft X will fall in nice behind Aircraft Y. Right before Approach was going to switch Aircraft Y to the Tower I noticed Aircraft X turning his base turn. He was only 2 miles off the approach course opposite direction as it was. The trainee on Approach issued a traffic alert and gave Aircraft Y a turn to the right heading 270 degrees. This put the aircraft nose to nose. I took over the frequency and gave Aircraft Y a left turn 040 away from Aircraft X.Aircraft Y did not see Aircraft X due to a cloud. There was maybe a mile between the 2 aircraft and closing. It was close. Closer than anything I have seen in [all my] years. The pilot of [Aircraft X] later called the Approach Control and spoke to the Supervisor admitting to the error. I was able to watch the radar replay and it was close. Disturbing.

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.