ZDV ARTCC misjudges closure between traffic leaving his sector and traffic just entering his sector resulting in a loss of separation.

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Attributes
ACN	547332
Time
Date	200205
Day	Fri
Local Time Of Day	1201 To 1800
Place
State Reference	CO
Altitude	msl bound lower : 31000 msl bound upper : 32700
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zdv.artcc
Operator	common carrier : air carrier
Make Model Name	MD-80 Series (DC-9-80) Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	cruise : level
Flight Plan	IFR
Aircraft 2
Controlling Facilities	artcc : zdv.artcc
Operator	common carrier : air carrier
Make Model Name	B737-300
Operating Under FAR Part	Part 121
Flight Phase	cruise : enroute altitude change
Flight Plan	IFR
Person 1
Affiliation	government : faa
Function	controller : radar
Qualification	controller : radar
Experience	controller military : 9 controller non radar : 9 controller radar : 9
ASRS Report	547332
Person 2
Affiliation	government : faa
Function	controller : radar
Events
Anomaly	conflict : airborne less severe non adherence : required legal separation non adherence : published procedure
Independent Detector	atc equipment : conflict alert atc equipment other atc equipment : radar other controllera other controllerb other flight crewa other flight crewb
Resolutory Action	flight crew : took evasive action none taken : insufficient time
Consequence	faa : investigated
Miss Distance	horizontal : 24600 vertical : 1700
Supplementary
Problem Areas	ATC Human Performance
Primary Problem	ATC Human Performance
Air Traffic Incident	Operational Error

Narrative:

Aircraft #1 in level flight at FL310, about to exit airspace. Aircraft #2 in level flight at FL330, outside airspace. The plan was to descend aircraft #2 to FL290 for additional traffic and LOA restrs. The traffic at FL310 was noted prior to exiting the airspace, however, there was a 100 KT converging speed difference. The lateral separation was being lost at a greater rate than accounted for and was not re-evaluated. I then descended aircraft #2 to FL290. The controller in adjacent airspace called and questioned the separation. Aircraft #2 was climbed to FL330, and I requested aircraft #1 (which the adjacent controller was talking to), via the adjacent controller, to descend. The other controller figured there wasn't going to be a midair conflict and after observing the #2 aircraft descending, decided to turn aircraft #1. Separation was lost when aircraft #2 entered the back side of aircraft #1 protected airspace (5 mi).

Original NASA ASRS Text

Title: ZDV ARTCC MISJUDGES CLOSURE BTWN TFC LEAVING HIS SECTOR AND TFC JUST ENTERING HIS SECTOR RESULTING IN A LOSS OF SEPARATION.

Narrative: ACFT #1 IN LEVEL FLT AT FL310, ABOUT TO EXIT AIRSPACE. ACFT #2 IN LEVEL FLT AT FL330, OUTSIDE AIRSPACE. THE PLAN WAS TO DSND ACFT #2 TO FL290 FOR ADDITIONAL TFC AND LOA RESTRS. THE TFC AT FL310 WAS NOTED PRIOR TO EXITING THE AIRSPACE, HOWEVER, THERE WAS A 100 KT CONVERGING SPD DIFFERENCE. THE LATERAL SEPARATION WAS BEING LOST AT A GREATER RATE THAN ACCOUNTED FOR AND WAS NOT RE-EVALUATED. I THEN DSNDED ACFT #2 TO FL290. THE CTLR IN ADJACENT AIRSPACE CALLED AND QUESTIONED THE SEPARATION. ACFT #2 WAS CLBED TO FL330, AND I REQUESTED ACFT #1 (WHICH THE ADJACENT CTLR WAS TALKING TO), VIA THE ADJACENT CTLR, TO DSND. THE OTHER CTLR FIGURED THERE WASN'T GOING TO BE A MIDAIR CONFLICT AND AFTER OBSERVING THE #2 ACFT DSNDING, DECIDED TO TURN ACFT #1. SEPARATION WAS LOST WHEN ACFT #2 ENTERED THE BACK SIDE OF ACFT #1 PROTECTED AIRSPACE (5 MI).

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.