Narrative:

Was windy on final approach and had good visibility. The first aircraft (an A320 or similar) was given a visual approach. While the aircraft could have turned toward the field; the pilot turned back up to join the localizer on his own. This created about a loss of about a mile; but not a loss of separation with the heavy aircraft that was about 4 behind and higher which turned into 3. I did not expect this so I maintained 1;000 feet and turned the trailing aircraft northbound to gain additional mileage; looking for around 4 miles on final. Additional traffic was to the north eastbound at 4;000 feet; but the path of the trailing aircraft I turned was behind that traffic so not a factor. When an additional mile was gained; I cleared the heavy for a visual approach. The company traffic was well ahead and there was not a separation problem but a slight over take on the speed due to an increase in compression caused by strong headwinds. I asked the trailing aircraft to advise when he saw the traffic ahead so he could maintain visual separation. He advised he had traffic in sight and instructed him to 'maintain visual separation' with traffic ahead. It took numerous attempts to get a correct read back from the pilot. He said he had responded three times but had only responded with his call sign; nothing else. I advised him that I needed more than just a call sign as a response. I could have called the tower to have them maintain visual separation if able with the traffic instead of continually trying to get a proper read back. Many times the pilot will say they have the airport in sight on their own accord and I will clear them for the visual approach. Instead of this; I believe I will run more ILS approaches even with good VFR weather to control the aircraft.

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Original NASA ASRS Text

Title: PHL Controller reported a possible loss of separation when traffic on final failed to respond appropriately to a visual approach clearance.

Narrative: Was windy on final approach and had good visibility. The first aircraft (an A320 or similar) was given a visual approach. While the aircraft could have turned toward the field; the pilot turned back up to join the localizer on his own. This created about a loss of about a mile; but not a loss of separation with the heavy aircraft that was about 4 behind and higher which turned into 3. I did not expect this so I maintained 1;000 feet and turned the trailing aircraft northbound to gain additional mileage; looking for around 4 miles on final. Additional traffic was to the north eastbound at 4;000 feet; but the path of the trailing aircraft I turned was behind that traffic so not a factor. When an additional mile was gained; I cleared the heavy for a visual approach. The company traffic was well ahead and there was not a separation problem but a slight over take on the speed due to an increase in compression caused by strong headwinds. I asked the trailing aircraft to advise when he saw the traffic ahead so he could maintain visual separation. He advised he had traffic in sight and instructed him to 'maintain visual separation' with traffic ahead. It took numerous attempts to get a correct read back from the pilot. He said he had responded three times but had only responded with his call sign; nothing else. I advised him that I needed more than just a call sign as a response. I could have called the tower to have them maintain visual separation if able with the traffic instead of continually trying to get a proper read back. Many times the pilot will say they have the airport in sight on their own accord and I will clear them for the visual approach. Instead of this; I believe I will run more ILS approaches even with good VFR weather to control the aircraft.

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.