Narrative:

Occasional moderate turbulence was reported throughout the area. An A319 was an arrival [to a class C airport] from the east over the intersection; leveling at 14;000 MSL. A falcon 2000 was a [departing from a nearby class D airport] en route to [the west]. Opposite direction to both the A319 and the falcon 2000; was an IFR general aviation aircraft established on [a victor airway] to points east; level at 11;000 ft MSL. When the A319 was ten miles west of [the class D] it was cleared direct [to the] NDB to intercept the intersections localizer; (approximately heading 275 degrees.) about that time I observed the falcon 2000 departing [the class D] and in hand off status to my sector. The hand off was accepted. At this point the A319 was level at 14;000 ft MSL with a ground speed of 350 KTS and the falcon 2000 was climbing through 6;900 ft MSL with a ground speed of 220 KTS. The spacing between the aircraft was roughly five miles and increasing. At or near the intersection; the A319 was cleared to 12;000 ft. [We were] descending through 13;700 ft MSL and without my notice; the A319 began slowing through 270 KTS; (perhaps anticipating the turbulence.) the A319 was issued 12;000 ft to remain above the IFR general aviation aircraft. Since the A319 and the falcon 2000 had different destinations speed restrictions were not issued. About that time the falcon 2000 was climbing through 11;000 ft and accelerating through 290 KTS. As the A319 neared the IFR general aviation aircraft; traffic was issued to establish visual separation and allow the A319 to continue its descent to [the class C airport]. What I did not anticipate; nor did I recognize; was that the A319 slowed down to 220 KTS ground speed while the falcon 2000 accelerated to 380 KTS; or greater; thus allowing a closure rate over 2 1/2 miles per minute. Although the falcon 2000 did not check in with my sector; I did reach out to the pilot. He responded that he had the airplane in sight; (the A319.) as the A319 descended through 11;700 ft; I advised the falcon 2000 to maintain visual separation. However; by this time; I believe that minimum IFR separation was already compromised. One needs to maintain continued vigilance scanning not only ahead of; and laterally for conflict ions; but also behind the airplane. It's easy to forget scanning ground speeds when one is not sequencing aircraft to a point or managing compression on final. However; where high performance aircraft are concerned; especially when closure rates can be excessive; one needs to be particularly careful to include the ground speed with the scan. One cannot take for granted aircraft performance; particularly during turbulent flight conditions. In this case; both were turbojet aircraft and similar performance was assumed on my part. Yet; one aircraft slowed considerably; while the other accelerated. If a speed restriction is necessary; issue it. The pilot will inform if the speed cannot be maintained. Since the falcon 2000 did not check in it's possible that my attention to his position relative to the A319 and the closure rate was overlooked. However; even when a pilot does not check in; one can still reach out to the airplane issuing a corrective measure should the pilot be 'sitting' on the frequency. One does not need to be in communication with both aircraft to establish and maintain ensured separation. I could have issued a vector to the A319 that would have instantly resolved the conflict. One should not be over focused on one situation; (establishing visual separation between the A319 and IFR general aviation aircraft) because chances are likely that another situation is developing; (the overtake between the A319 and the falcon 2000.)

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Original NASA ASRS Text

Title: TRACON Controller described a loss of separation between two different Air Carrier aircraft when each altered their airspeed without due recognition by the controller; the reporter providing some excellent insight to the event.

Narrative: Occasional moderate turbulence was reported throughout the area. An A319 was an arrival [to a Class C airport] from the east over the intersection; leveling at 14;000 MSL. A Falcon 2000 was a [departing from a nearby Class D airport] en route to [the west]. Opposite direction to both the A319 and the Falcon 2000; was an IFR general aviation aircraft established on [a victor airway] to points east; level at 11;000 FT MSL. When the A319 was ten miles west of [the Class D] it was cleared direct [to the] NDB to intercept the intersections localizer; (approximately heading 275 degrees.) About that time I observed the Falcon 2000 departing [the Class D] and in hand off status to my sector. The hand off was accepted. At this point the A319 was level at 14;000 FT MSL with a ground speed of 350 KTS and the Falcon 2000 was climbing through 6;900 FT MSL with a ground speed of 220 KTS. The spacing between the aircraft was roughly five miles and increasing. At or near the intersection; The A319 was cleared to 12;000 FT. [We were] descending through 13;700 FT MSL and without my notice; the A319 began slowing through 270 KTS; (perhaps anticipating the turbulence.) The A319 was issued 12;000 FT to remain above the IFR general aviation aircraft. Since the A319 and the Falcon 2000 had different destinations speed restrictions were not issued. About that time the Falcon 2000 was climbing through 11;000 FT and accelerating through 290 KTS. As the A319 neared the IFR general aviation aircraft; traffic was issued to establish visual separation and allow the A319 to continue its descent to [the Class C airport]. What I did not anticipate; nor did I recognize; was that the A319 slowed down to 220 KTS ground speed while the Falcon 2000 accelerated to 380 KTS; or greater; thus allowing a closure rate over 2 1/2 miles per minute. Although the Falcon 2000 did not check in with my sector; I did reach out to the pilot. He responded that he had the airplane in sight; (the A319.) As the A319 descended through 11;700 FT; I advised the Falcon 2000 to maintain visual separation. However; by this time; I believe that minimum IFR separation was already compromised. One needs to maintain continued vigilance scanning not only ahead of; and laterally for conflict ions; but also behind the airplane. It's easy to forget scanning ground speeds when one is not sequencing aircraft to a point or managing compression on final. However; where high performance aircraft are concerned; especially when closure rates can be excessive; one needs to be particularly careful to include the ground speed with the scan. One cannot take for granted aircraft performance; particularly during turbulent flight conditions. In this case; both were turbojet aircraft and similar performance was assumed on my part. Yet; one aircraft slowed considerably; while the other accelerated. If a speed restriction is necessary; issue it. The pilot will inform if the speed cannot be maintained. Since the Falcon 2000 did not check in it's possible that my attention to his position relative to the A319 and the closure rate was overlooked. However; even when a pilot does not check in; one can still reach out to the airplane issuing a corrective measure should the pilot be 'sitting' on the frequency. One does not need to be in communication with both aircraft to establish and maintain ensured separation. I could have issued a vector to the A319 that would have instantly resolved the conflict. One should not be over focused on one situation; (establishing visual separation between the A319 and IFR general aviation aircraft) because chances are likely that another situation is developing; (the overtake between the A319 and the Falcon 2000.)

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.