Air carrier X descent to occupied altitude had less than standard separation from small aircraft Y. ...

Attributes
ACN	257434
Time
Date	199311
Day	Wed
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : czq
State Reference	CA
Altitude	msl bound lower : 6000 msl bound upper : 6000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Operator	general aviation : personal
Make Model Name	Small Aircraft, Low Wing, 1 Eng, Retractable Gear
Flight Phase	cruise other
Route In Use	enroute : direct enroute airway : fat
Flight Plan	IFR
Aircraft 2
Operator	common carrier : air carrier
Make Model Name	Medium Transport, Low Wing, 2 Turboprop Eng
Navigation In Use	Other
Flight Phase	descent other
Route In Use	enroute : direct
Flight Plan	IFR
Person 1
Affiliation	government : faa
Function	controller : approach
Qualification	controller : radar
Experience	controller military : 2 controller non radar : 7 controller radar : 11 controller supervisory : 6
ASRS Report	257434
Person 2
Affiliation	Other
Function	flight crew : single pilot
Qualification	pilot : instrument
Events
Anomaly	conflict : airborne less severe non adherence : required legal separation non adherence : published procedure
Independent Detector	other controllera
Resolutory Action	none taken : detected after the fact
Consequence	faa : investigated
Miss Distance	horizontal : 3000 vertical : 0
Supplementary
Primary Problem	ATC Human Performance
Air Traffic Incident	Operational Error

Narrative:

During a period of moderate traffic, I was in contact with small aircraft X en route V23, northbound at 6000 ft MSL. Air carrier Y was inbound to fresno (fat) from the northwest at 11000 ft MSL direct clovis VORTAC. Descent clearance was issued to air carrier Y to maintain 6000 ft. When the conflict was realized, I felt there was no need to stop descent for air carrier Y at 7000 ft MSL based on the 2 aircraft's relative position in distance and the vertical spacing (more than 4000 ft). I anticipated no problem, believing atx X could not possibly descend rapidly enough to be a factor for small aircraft X. The aircraft did converge in near proximity at the VORTAC and a TA was issued. Rather than assuring positive separation through use of assigned altitude, I anticipated separation based on assumed aircraft performance and radar position. This gave no alternative options except to continuously monitor both aircraft for potential conflict and early resolution, setting up this situation when other duties divert attention.

Original NASA ASRS Text

Title: ACR X DSCNT TO OCCUPIED ALT HAD LTSS FROM SMA Y. SYS ERROR.

Narrative: DURING A PERIOD OF MODERATE TFC, I WAS IN CONTACT WITH SMA X ENRTE V23, NBOUND AT 6000 FT MSL. ACR Y WAS INBOUND TO FRESNO (FAT) FROM THE NW AT 11000 FT MSL DIRECT CLOVIS VORTAC. DSCNT CLRNC WAS ISSUED TO ACR Y TO MAINTAIN 6000 FT. WHEN THE CONFLICT WAS REALIZED, I FELT THERE WAS NO NEED TO STOP DSCNT FOR ACR Y AT 7000 FT MSL BASED ON THE 2 ACFT'S RELATIVE POS IN DISTANCE AND THE VERT SPACING (MORE THAN 4000 FT). I ANTICIPATED NO PROB, BELIEVING ATX X COULD NOT POSSIBLY DSND RAPIDLY ENOUGH TO BE A FACTOR FOR SMA X. THE ACFT DID CONVERGE IN NEAR PROX AT THE VORTAC AND A TA WAS ISSUED. RATHER THAN ASSURING POSITIVE SEPARATION THROUGH USE OF ASSIGNED ALT, I ANTICIPATED SEPARATION BASED ON ASSUMED ACFT PERFORMANCE AND RADAR POS. THIS GAVE NO ALTERNATIVE OPTIONS EXCEPT TO CONTINUOUSLY MONITOR BOTH ACFT FOR POTENTIAL CONFLICT AND EARLY RESOLUTION, SETTING UP THIS SIT WHEN OTHER DUTIES DIVERT ATTN.

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.

Air carrier X descent to occupied altitude had less than standard separation from small aircraft Y. System error.