Controller reports of incident where the trailing aircraft doesn't slow and overtakes preceding airc...

Narrative:

Working the arrival portion of east combined with dre and sate. Aircraft Y checked on the RNAV arrival descending via and aircraft X checked on about 7-8 miles behind also descending via. As aircraft Y began a slow descent they slowed while aircraft X did not. As the distance rapidly decreased I stopped aircraft X at 11;000 to keep them from hitting aircraft Y. The conflict alert (ca) went off (way too late like it always does now that we have been forced to change to fusion and the stars software wasn't updated). Aircraft X continued at 11;000 and actually passed above aircraft Y well over 100 knots faster. Aircraft X ended up first in the sequence due to their speed which may have been their plan all along. There is nothing in the RNAV arrivals that made this illegal. However this shows it is not a safe procedure. Almost every pilot flies it different and there is no automatic safety net provided where all the aircraft should be at the same speed descending via a profile to the same altitude. They can come into our airspace at flight levels going 400 plus knots or come in over the boundary already at 100 with 70 miles left to fly doing 210 knots and both are perfectly legal. As you can see by this example this can really burn you. If you are distracted with other duties it can sneak up on you in a heartbeat. I recommend that we add speeds and altitudes on the arrival tying the pilots down to more of a 'tunnel' instead of leaving it so wide open.

Original NASA ASRS Text

Title: Controller reports of incident where the trailing aircraft doesn't slow and overtakes preceding aircraft which are separated by 1;000 feet. Reporter states that Conflict Alert goes off but not in a timely manner which may be due to use of fusion radar and a software problem.

Narrative: Working the arrival portion of east combined with DRE and SATE. Aircraft Y checked on the RNAV arrival descending via and Aircraft X checked on about 7-8 miles behind also descending via. As Aircraft Y began a slow descent they slowed while Aircraft X did not. As the distance rapidly decreased I stopped Aircraft X at 11;000 to keep them from hitting Aircraft Y. The Conflict Alert (CA) went off (way too late like it always does now that we have been forced to change to fusion and the STARS software wasn't updated). Aircraft X continued at 11;000 and actually passed above Aircraft Y well over 100 knots faster. Aircraft X ended up first in the sequence due to their speed which may have been their plan all along. There is nothing in the RNAV arrivals that made this illegal. However this shows it is not a safe procedure. Almost every pilot flies it different and there is no automatic safety net provided where all the aircraft should be at the same speed descending via a profile to the same altitude. They can come into our airspace at flight levels going 400 plus knots or come in over the boundary already at 100 with 70 miles left to fly doing 210 knots and both are perfectly legal. As you can see by this example this can really burn you. If you are distracted with other duties it can sneak up on you in a heartbeat. I recommend that we add speeds and altitudes on the arrival tying the pilots down to more of a 'tunnel' instead of leaving it so wide open.

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.

Attributes
ACN	1172033
Time
Date	201405
Local Time Of Day	1201-1800
Place
Locale Reference	ZZZ.Airport
State Reference	US
Environment
Light	Daylight
Aircraft 1
Make Model Name	Light Transport Low Wing 2 Turbojet Eng
Flight Phase	Descent
Flight Plan	IFR
Aircraft 2
Make Model Name	Medium Large Transport
Operating Under FAR Part	Part 121
Flight Phase	Descent
Flight Plan	IFR
Person 1
Function	Approach
Qualification	Air Traffic Control Fully Certified
Experience	Air Traffic Control Time Certified In Pos 1 (yrs) 14
Events
Anomaly	ATC Issue All Types Conflict Airborne Conflict Deviation - Procedural Published Material / Policy Deviation - Speed All Types

Controller reports of incident where the trailing aircraft doesn't slow and overtakes preceding aircraft which are separated by 1;000 feet. Reporter states that conflict alert goes off but not in a timely manner which may be due to use of fusion radar and a software problem.