Radar failed as 2 VFR aircraft were on converging courses.

Attributes
ACN	120441
Time
Date	198908
Day	Thu
Local Time Of Day	1801 To 2400
Place
Locale Reference	atc facility : wys
State Reference	MT
Altitude	msl bound lower : 10000 msl bound upper : 10000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zlc
Operator	general aviation : corporate
Make Model Name	Small Transport, Low Wing, 2 Recip Eng
Flight Phase	cruise other
Flight Plan	VFR
Aircraft 2
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Flight Phase	cruise other
Flight Plan	None
Person 1
Affiliation	government : faa
Function	controller : radar
Qualification	controller : radar
Experience	controller non radar : 3 controller radar : 3
ASRS Report	120441
Person 2
Affiliation	Other
Function	flight crew : captain oversight : pic
Qualification	pilot : commercial pilot : instrument
Events
Anomaly	other anomaly other
Independent Detector	other controllera
Resolutory Action	none taken : anomaly accepted
Consequence	Other
Miss Distance	horizontal : 6000 vertical : 800
Supplementary
Primary Problem	ATC Human Performance
Air Traffic Incident	other

Narrative:

I was working ZLC sector 5 when the ashton radar site failed for approximately 2 mins before it was restored. I was providing VFR advisories for a cga X over west yellowstone at the time. The aircraft was reidented west/O incident. Approximately 10 mins later I was calling out VFR traffic for the cga X when the radar failed again. At this time the aircraft were 6 mi apart and the unknown VFR target had no mode C. When the radar came up approximately 1 min later the targets appeared to be superimposed on each other. After the targets passed the cga X pilot told me the traffic went about a mile behind him and 800' below him.

Original NASA ASRS Text

Title: RADAR FAILED AS 2 VFR ACFT WERE ON CONVERGING COURSES.

Narrative: I WAS WORKING ZLC SECTOR 5 WHEN THE ASHTON RADAR SITE FAILED FOR APPROX 2 MINS BEFORE IT WAS RESTORED. I WAS PROVIDING VFR ADVISORIES FOR A CGA X OVER W YELLOWSTONE AT THE TIME. THE ACFT WAS REIDENTED W/O INCIDENT. APPROX 10 MINS LATER I WAS CALLING OUT VFR TFC FOR THE CGA X WHEN THE RADAR FAILED AGAIN. AT THIS TIME THE ACFT WERE 6 MI APART AND THE UNKNOWN VFR TARGET HAD NO MODE C. WHEN THE RADAR CAME UP APPROX 1 MIN LATER THE TARGETS APPEARED TO BE SUPERIMPOSED ON EACH OTHER. AFTER THE TARGETS PASSED THE CGA X PLT TOLD ME THE TFC WENT ABOUT A MILE BEHIND HIM AND 800' BELOW HIM.

Data retrieved from NASA's ASRS site as of August 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.