Narrative:

Light transport X was at FL330 inbound to jnu from the west. Large transport Y was at jnu requested departure via barlo 4 yak J501 anc FL350. He said he would accept a VFR climb to any altitude. I cleared large transport Y to bergs DME fix, no delay expected, climb to 17000'. I cleared light transport X to FL190. After departure I reclred large transport Y to 12000' and told him to expect his required altitude with a VFR climb through 17000' when I had light transport X out of 16000'. I cleared light transport X to 13000'. When large transport Y reported leaving 14500', I cleared large transport Y to anc via flight plan to maintain VFR through 16000', then climb to FL350. I cleared light transport X for approach. I then reclred large transport Y to maintain VFR through 16000', then climb to FL310 because of traffic over yak at FL350. I then saw large transport Y, limited data block, about to merge with light transport X, but didn't have time to issue traffic before they merged. Large transport Y was about to request light transport X's position, but had to break off his transmission to take evasive action. Light transport X then reported he had taken evasive action to avoid large transport Y. Large transport Y was not yet radar idented. I was about to issue a non radar traffic advisory when the above events occurred. Light transport X had just been switched to tower, but his radar service not yet terminated. His target was coasting off and on since leaving 16000'. Had I known the aircraft were already so close I would have waited for DME passage instead of a VFR climb. I should have issued a traffic advisory to both aircraft, perhaps before issuing a VFR climb. However, both aircraft were aware of each other, and since large transport Y accepted a VFR climb, I was legally authority/authorized to discontinue IFR vertical sep. Light transport X said both aircraft were on the localizer at 13000', head on and both made evasive left turns. Large transport Y apologized to light transport X and told me he would have appreciated a traffic advisory. I concurred. My recommendation is that when using VFR to separate aircraft, that traffic advisory be issued prior to issuing VFR climb or descent to see if aircraft will still accept the VFR assignment. Callback conversation with reporter revealed the following: reporter controller was surprised by how close the climbing air carrier large transport got to the descending cpr light transport before vertical sep was no longer a problem. In this area of the center's airspace, radar coverage is so poor, because of terrain near the radar site, that climbing aircraft cannot be idented until they leave 15000'. The inbound light transport had entered this area and radar service had been terminated because the controller was losing coverage. A VFR climb was given to the air carrier because the controller thought the 2 aircraft were still very far apart and would never be a factor for each other. The facility processed this as a near midair collision and not an operational error. Reporter controller's experience level was ascertained during the callback.

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Original NASA ASRS Text

Title: ACR-LGT AND CPR-LTT BOTH TAKE EVASIVE ACTION TO AVOID COLLISION.

Narrative: LTT X WAS AT FL330 INBND TO JNU FROM THE W. LGT Y WAS AT JNU REQUESTED DEP VIA BARLO 4 YAK J501 ANC FL350. HE SAID HE WOULD ACCEPT A VFR CLB TO ANY ALT. I CLRED LGT Y TO BERGS DME FIX, NO DELAY EXPECTED, CLB TO 17000'. I CLRED LTT X TO FL190. AFTER DEP I RECLRED LGT Y TO 12000' AND TOLD HIM TO EXPECT HIS REQUIRED ALT WITH A VFR CLB THROUGH 17000' WHEN I HAD LTT X OUT OF 16000'. I CLRED LTT X TO 13000'. WHEN LGT Y RPTED LEAVING 14500', I CLRED LGT Y TO ANC VIA FLT PLAN TO MAINTAIN VFR THROUGH 16000', THEN CLB TO FL350. I CLRED LTT X FOR APCH. I THEN RECLRED LGT Y TO MAINTAIN VFR THROUGH 16000', THEN CLB TO FL310 BECAUSE OF TFC OVER YAK AT FL350. I THEN SAW LGT Y, LIMITED DATA BLOCK, ABOUT TO MERGE WITH LTT X, BUT DIDN'T HAVE TIME TO ISSUE TFC BEFORE THEY MERGED. LGT Y WAS ABOUT TO REQUEST LTT X'S POS, BUT HAD TO BREAK OFF HIS XMISSION TO TAKE EVASIVE ACTION. LTT X THEN RPTED HE HAD TAKEN EVASIVE ACTION TO AVOID LGT Y. LGT Y WAS NOT YET RADAR IDENTED. I WAS ABOUT TO ISSUE A NON RADAR TFC ADVISORY WHEN THE ABOVE EVENTS OCCURRED. LTT X HAD JUST BEEN SWITCHED TO TWR, BUT HIS RADAR SVC NOT YET TERMINATED. HIS TARGET WAS COASTING OFF AND ON SINCE LEAVING 16000'. HAD I KNOWN THE ACFT WERE ALREADY SO CLOSE I WOULD HAVE WAITED FOR DME PASSAGE INSTEAD OF A VFR CLB. I SHOULD HAVE ISSUED A TFC ADVISORY TO BOTH ACFT, PERHAPS BEFORE ISSUING A VFR CLB. HOWEVER, BOTH ACFT WERE AWARE OF EACH OTHER, AND SINCE LGT Y ACCEPTED A VFR CLB, I WAS LEGALLY AUTH TO DISCONTINUE IFR VERT SEP. LTT X SAID BOTH ACFT WERE ON THE LOC AT 13000', HEAD ON AND BOTH MADE EVASIVE LEFT TURNS. LGT Y APOLOGIZED TO LTT X AND TOLD ME HE WOULD HAVE APPRECIATED A TFC ADVISORY. I CONCURRED. MY RECOMMENDATION IS THAT WHEN USING VFR TO SEPARATE ACFT, THAT TFC ADVISORY BE ISSUED PRIOR TO ISSUING VFR CLB OR DSCNT TO SEE IF ACFT WILL STILL ACCEPT THE VFR ASSIGNMENT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: RPTR CTLR WAS SURPRISED BY HOW CLOSE THE CLBING ACR LGT GOT TO THE DSNDING CPR LTT BEFORE VERT SEP WAS NO LONGER A PROB. IN THIS AREA OF THE CENTER'S AIRSPACE, RADAR COVERAGE IS SO POOR, BECAUSE OF TERRAIN NEAR THE RADAR SITE, THAT CLBING ACFT CANNOT BE IDENTED UNTIL THEY LEAVE 15000'. THE INBND LTT HAD ENTERED THIS AREA AND RADAR SVC HAD BEEN TERMINATED BECAUSE THE CTLR WAS LOSING COVERAGE. A VFR CLB WAS GIVEN TO THE ACR BECAUSE THE CTLR THOUGHT THE 2 ACFT WERE STILL VERY FAR APART AND WOULD NEVER BE A FACTOR FOR EACH OTHER. THE FAC PROCESSED THIS AS A NMAC AND NOT AN OPERROR. RPTR CTLR'S EXPERIENCE LEVEL WAS ASCERTAINED DURING THE CALLBACK.

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.