Narrative:

While working the carlton sector (at or below FL230), a moderately busy traffic situation developed. During this period air carrier X FL240 checked on the frequency. Shortly after, air carrier Y FL220, reported on the frequency. Approximately 10 NM existed between these 2 aircraft, in trail, at this time with lead aircraft air carrier X showing ground speed 540 KTS. Air carrier Y in trail showed ground speed 470 KTS. No speed adjustment appeared necessary. Both aircraft were issued descent to 12000 ft. Subsequently the ground speeds changed significantly due to wind variations and/or pilot operational procedure. When both aircraft were below FL200 descending, the lead aircraft had reduced ground speed to 380 KTS and following aircraft was at ground speed 420 KTS, a 40 KT overtake. Several instructions were issued to prevent a loss of separation. Following air carrier Y reduced to 250 KTS IAS, following air carrier Y issued left turn to heading 260, and following air carrier Y leveled off at present altitude (FL195). No action had been taken sooner due to normal procedure to observe present ground speeds and in trail (longitudinal) separation. When these 2 observations indicate constant or increasing separation, vertical separation is no longer necessary, thus, both aircraft were given descent clrncs to 12000 ft. Callback conversation with reporter revealed the following information. The reporter stated the incident was investigated as a system error because conflict alert activated and 1 hit from the radar plot indicated 4.8 mi separation. The reporter further stated facility management investigator showed the radar pilot as in error and the incident has been downgraded to a non- event. Radar separation was maintained.

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Original NASA ASRS Text

Title: ACR X POTENTIAL LTSS FROM ACR Y. POTENTIAL SYS ERROR. SEE CALLBACK.

Narrative: WHILE WORKING THE CARLTON SECTOR (AT OR BELOW FL230), A MODERATELY BUSY TFC SITUATION DEVELOPED. DURING THIS PERIOD ACR X FL240 CHKED ON THE FREQ. SHORTLY AFTER, ACR Y FL220, RPTED ON THE FREQ. APPROX 10 NM EXISTED BTWN THESE 2 ACFT, IN TRAIL, AT THIS TIME WITH LEAD ACFT ACR X SHOWING GND SPD 540 KTS. ACR Y IN TRAIL SHOWED GND SPD 470 KTS. NO SPD ADJUSTMENT APPEARED NECESSARY. BOTH ACFT WERE ISSUED DSCNT TO 12000 FT. SUBSEQUENTLY THE GND SPDS CHANGED SIGNIFICANTLY DUE TO WIND VARIATIONS AND/OR PLT OPERATIONAL PROC. WHEN BOTH ACFT WERE BELOW FL200 DSNDING, THE LEAD ACFT HAD REDUCED GND SPD TO 380 KTS AND FOLLOWING ACFT WAS AT GND SPD 420 KTS, A 40 KT OVERTAKE. SEVERAL INSTRUCTIONS WERE ISSUED TO PREVENT A LOSS OF SEPARATION. FOLLOWING ACR Y REDUCED TO 250 KTS IAS, FOLLOWING ACR Y ISSUED L TURN TO HDG 260, AND FOLLOWING ACR Y LEVELED OFF AT PRESENT ALT (FL195). NO ACTION HAD BEEN TAKEN SOONER DUE TO NORMAL PROC TO OBSERVE PRESENT GND SPDS AND IN TRAIL (LONGITUDINAL) SEPARATION. WHEN THESE 2 OBSERVATIONS INDICATE CONSTANT OR INCREASING SEPARATION, VERT SEPARATION IS NO LONGER NECESSARY, THUS, BOTH ACFT WERE GIVEN DSCNT CLRNCS TO 12000 FT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO. THE RPTR STATED THE INCIDENT WAS INVESTIGATED AS A SYS ERROR BECAUSE CONFLICT ALERT ACTIVATED AND 1 HIT FROM THE RADAR PLOT INDICATED 4.8 MI SEPARATION. THE RPTR FURTHER STATED FACILITY MGMNT INVESTIGATOR SHOWED THE RADAR PLT AS IN ERROR AND THE INCIDENT HAS BEEN DOWNGRADED TO A NON- EVENT. RADAR SEPARATION WAS MAINTAINED.

Data retrieved from NASA's ASRS site as of July 2007 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.