Small aircraft X had less than standard separation from atx Y. System error.

Attributes
ACN	167852
Time
Date	199101
Day	Tue
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : avl
State Reference	NC
Altitude	msl bound lower : 5000 msl bound upper : 5600
Environment
Flight Conditions	IMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : avl tracon : dca
Operator	general aviation : personal
Make Model Name	Small Aircraft, High Wing, 1 Eng, Fixed Gear
Flight Phase	descent other
Flight Plan	IFR
Aircraft 2
Operator	common carrier : air taxi
Make Model Name	Light Transport, Low Wing, 2 Turbojet Eng
Flight Phase	descent other
Route In Use	enroute : on vectors
Flight Plan	IFR
Person 1
Affiliation	government : faa
Function	controller : approach
Qualification	controller : developmental
ASRS Report	167852
Person 2
Affiliation	government : faa
Function	controller : approach
Qualification	controller : radar
Experience	controller radar : 15
ASRS Report	168039
Events
Anomaly	conflict : airborne less severe non adherence : published procedure non adherence : required legal separation
Independent Detector	other controllera
Resolutory Action	none taken : detected after the fact
Consequence	Other
Miss Distance	horizontal : 12000 vertical : 600
Supplementary
Primary Problem	ATC Human Performance
Air Traffic Incident	Operational Error

Narrative:

Small aircraft X at 6000' was descending to 4600' inbound to airport and atx Y at 7000' to pass overhead. It appeared small aircraft X had left 5000' for 4600' via mode C readout. At that time I descended, atx Y passing overhead or just behind small aircraft X from 7000 to 5600'. A few moments later the mode C of small aircraft X indicated 5000', at which time I instructed small aircraft X to maintain 4600'. At this time atx Y aircraft was leaving 5800' descending to 5600'. There was no time for further corrective actions. Atx Y performance characteristics much higher than small aircraft X caused the problem. Also, with close proximity of targets sometimes mode C readouts will fluctuate. Supplemental information from acn 168039: caused by inattn on the part of the developmental on the radar position.

Original NASA ASRS Text

Title: SMA X HAD LESS THAN STANDARD SEPARATION FROM ATX Y. SYSTEM ERROR.

Narrative: SMA X AT 6000' WAS DSNDING TO 4600' INBND TO ARPT AND ATX Y AT 7000' TO PASS OVERHEAD. IT APPEARED SMA X HAD LEFT 5000' FOR 4600' VIA MODE C READOUT. AT THAT TIME I DSNDED, ATX Y PASSING OVERHEAD OR JUST BEHIND SMA X FROM 7000 TO 5600'. A FEW MOMENTS LATER THE MODE C OF SMA X INDICATED 5000', AT WHICH TIME I INSTRUCTED SMA X TO MAINTAIN 4600'. AT THIS TIME ATX Y ACFT WAS LEAVING 5800' DSNDING TO 5600'. THERE WAS NO TIME FOR FURTHER CORRECTIVE ACTIONS. ATX Y PERFORMANCE CHARACTERISTICS MUCH HIGHER THAN SMA X CAUSED THE PROB. ALSO, WITH CLOSE PROX OF TARGETS SOMETIMES MODE C READOUTS WILL FLUCTUATE. SUPPLEMENTAL INFO FROM ACN 168039: CAUSED BY INATTN ON THE PART OF THE DEVELOPMENTAL ON THE RADAR POS.

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.