Air carrier X overtakes air carrier Y on approach, had less than standard separation. System error.

Attributes
ACN	245378
Time
Date	199306
Day	Wed
Local Time Of Day	0601 To 1200
Place
Locale Reference	atc facility : dfw
State Reference	TX
Altitude	msl bound lower : 2300 msl bound upper : 2300
Environment
Flight Conditions	Mixed
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : dfw
Operator	common carrier : air carrier
Make Model Name	Light Transport, Low Wing, 2 Turboprop Eng
Flight Phase	descent : approach
Route In Use	enroute : on vectors
Flight Plan	IFR
Aircraft 2
Operator	common carrier : air carrier
Make Model Name	Medium Large Transport, Low Wing, 2 Turbojet Eng
Flight Phase	descent : approach
Flight Plan	IFR
Person 1
Affiliation	government : faa
Function	controller : approach
Qualification	controller : radar
Experience	controller military : 8 controller radar : 18
ASRS Report	245378
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	conflict : airborne less severe non adherence : published procedure non adherence : required legal separation other anomaly
Independent Detector	other controllera
Resolutory Action	controller : issued new clearance other
Consequence	Other
Miss Distance	horizontal : 15000 vertical : 0
Supplementary
Primary Problem	ATC Human Performance
Air Traffic Incident	Operational Error

Narrative:

I had air carrier X turned towards the final. I had matched the speeds with air carrier Y and had 4 mi of separation. I turned the air carrier X to a dogleg intercept and descended to 3300 from 4000 ft. The pilot read back 23, but it did not really matter. I did not correct it. The intercept and distance to follow air carrier Y looked good. I cleared air carrier X for the ILS at 33. Air carrier Y slowed considerably and air carrier X now had a 50 KT overtake. My 4 mi rapidly turned to 3. I slowed air carrier X from 170 to 150. Now it's less than 3 mi and no apparent speed change. I broke air carrier X right to 180 and climb to 3000. At that moment, the conflict alert went off. Air carrier X said he had air carrier Y in sight. I asked could he follow him for a visual. Air carrier Y pilot replied if he could s-turn. I cleared him for a visual to follow the air carrier Y. Lesson: use earlier speed control because the air carrier X aircraft types are relatively new here. It seems sometimes in the terminal pattern they do not slow very quickly in comparison to most other air carrier airplanes.

Original NASA ASRS Text

Title: ACR X OVERTAKES ACR Y ON APCH, HAD LTSS. SYS ERROR.

Narrative: I HAD ACR X TURNED TOWARDS THE FINAL. I HAD MATCHED THE SPDS WITH ACR Y AND HAD 4 MI OF SEPARATION. I TURNED THE ACR X TO A DOGLEG INTERCEPT AND DSNDED TO 3300 FROM 4000 FT. THE PLT READ BACK 23, BUT IT DID NOT REALLY MATTER. I DID NOT CORRECT IT. THE INTERCEPT AND DISTANCE TO FOLLOW ACR Y LOOKED GOOD. I CLRED ACR X FOR THE ILS AT 33. ACR Y SLOWED CONSIDERABLY AND ACR X NOW HAD A 50 KT OVERTAKE. MY 4 MI RAPIDLY TURNED TO 3. I SLOWED ACR X FROM 170 TO 150. NOW IT'S LESS THAN 3 MI AND NO APPARENT SPD CHANGE. I BROKE ACR X R TO 180 AND CLB TO 3000. AT THAT MOMENT, THE CONFLICT ALERT WENT OFF. ACR X SAID HE HAD ACR Y IN SIGHT. I ASKED COULD HE FOLLOW HIM FOR A VISUAL. ACR Y PLT REPLIED IF HE COULD S-TURN. I CLRED HIM FOR A VISUAL TO FOLLOW THE ACR Y. LESSON: USE EARLIER SPD CTL BECAUSE THE ACR X ACFT TYPES ARE RELATIVELY NEW HERE. IT SEEMS SOMETIMES IN THE TERMINAL PATTERN THEY DO NOT SLOW VERY QUICKLY IN COMPARISON TO MOST OTHER ACR AIRPLANES.

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.