Air carrier Y had less than standard separation from air carrier Z. System error.

Attributes
ACN	229667
Time
Date	199212
Day	Tue
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : den
State Reference	CO
Altitude	msl bound lower : 18400 msl bound upper : 19100
Environment
Flight Conditions	VMC
Light	Dusk
Aircraft 1
Controlling Facilities	tower : jfk
Operator	common carrier : air carrier
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Flight Phase	descent other
Route In Use	enroute : on vectors enroute airway : zdv
Flight Plan	IFR
Aircraft 2
Operator	common carrier : air carrier
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Flight Phase	descent other
Route In Use	enroute : on vectors
Flight Plan	IFR
Person 1
Affiliation	government : faa
Function	controller : radar
Qualification	controller : radar
Experience	controller radar : 1
ASRS Report	229667
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
Independent Detector	other controllera
Resolutory Action	controller : issued new clearance other
Consequence	faa : investigated
Miss Distance	horizontal : 24000 vertical : 600
Supplementary
Primary Problem	ATC Human Performance
Air Traffic Incident	Operational Error

Narrative:

Strong winds from southwest. Working sector 13 radar position ZDV. Evening rush of arrival aircraft going to den stapleton airport. Blended arrs from west, northwest, and north on J170. Sequence of related aircraft were (in order) air carrier X, air carrier Y, air carrier Z. Air carrier X cleared to cross drake intersection at 17000 ft MSL. Air carrier Y stepped down reference air carrier X and air carrier Z. Air carrier X cleared to cross drake at 17000 MSL. To widen the spacing between aircraft, air carrier Z assigned heading of 200 and air carrier Y assigned heading of 220. All aircraft had been assigned 250 KTS prior to blending. Air carrier Y (when clear of air carrier X) was assigned 100 heading to intercept the den 313 degree radial and proceed inbound on arrival routing. Air carrier Y assigned FL190 but not entered into data block. Noticed loss of separation and instructed air carrier Y to maintain (again) FL190 because the data block reflected assigned altitude FL200. This caused confusion as to which aircraft had actually read back the correct assigned altitude. Instructed air carrier Z to turn right to heading of 220. Conflict alert had not gone off, yet aircraft were approximately 3 NM lateral and 600 ft vertical. Conflict did go off at 4 NM and 700 ft. Being a relatively inexperienced controller, I should have not been trying to sequence so tightly allowing myself some leeway on slow turns by pilots and different rates of descent.

Original NASA ASRS Text

Title: ACR Y HAD LTSS FROM ACR Z. SYS ERROR.

Narrative: STRONG WINDS FROM SW. WORKING SECTOR 13 RADAR POS ZDV. EVENING RUSH OF ARR ACFT GOING TO DEN STAPLETON ARPT. BLENDED ARRS FROM W, NW, AND N ON J170. SEQUENCE OF RELATED ACFT WERE (IN ORDER) ACR X, ACR Y, ACR Z. ACR X CLRED TO CROSS DRAKE INTXN AT 17000 FT MSL. ACR Y STEPPED DOWN REF ACR X AND ACR Z. ACR X CLRED TO CROSS DRAKE AT 17000 MSL. TO WIDEN THE SPACING BTWN ACFT, ACR Z ASSIGNED HDG OF 200 AND ACR Y ASSIGNED HDG OF 220. ALL ACFT HAD BEEN ASSIGNED 250 KTS PRIOR TO BLENDING. ACR Y (WHEN CLR OF ACR X) WAS ASSIGNED 100 HDG TO INTERCEPT THE DEN 313 DEG RADIAL AND PROCEED INBOUND ON ARR ROUTING. ACR Y ASSIGNED FL190 BUT NOT ENTERED INTO DATA BLOCK. NOTICED LOSS OF SEPARATION AND INSTRUCTED ACR Y TO MAINTAIN (AGAIN) FL190 BECAUSE THE DATA BLOCK REFLECTED ASSIGNED ALT FL200. THIS CAUSED CONFUSION AS TO WHICH ACFT HAD ACTUALLY READ BACK THE CORRECT ASSIGNED ALT. INSTRUCTED ACR Z TO TURN R TO HDG OF 220. CONFLICT ALERT HAD NOT GONE OFF, YET ACFT WERE APPROX 3 NM LATERAL AND 600 FT VERT. CONFLICT DID GO OFF AT 4 NM AND 700 FT. BEING A RELATIVELY INEXPERIENCED CTLR, I SHOULD HAVE NOT BEEN TRYING TO SEQUENCE SO TIGHTLY ALLOWING MYSELF SOME LEEWAY ON SLOW TURNS BY PLTS AND DIFFERENT RATES OF DSCNT.

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.