Narrative:

This near collision occurred immediately prior to a position relief briefing. I began monitoring the controller, who was to be relieved, working the position. Within a min, I indicated to the controller that I was ready to begin the relief briefing. It was then that the 'conflict alert' alarmed. I saw mlt X climbing into an en route aircraft. The other aircraft was small aircraft Y. Both aircraft were on instrument flight plans, and both aircraft were complying with their clrncs. Small aircraft Y was flying nwesterly along victor 171 at 8000 ft MSL. Mlt X was flying seasterly toward the louisville VORTAC and was climbing to an intermediate assigned altitude of 10000 ft MSL. The near collision occurred when mlt X was in front of small aircraft Y on an opposite direction course, and climbed through the same altitude of small aircraft Y. I became aware of the loss of approved separation when the 2 aircraft were between 1 and 2 mi apart. At that time, mlt X altitude readout, mode C, indicated 7000 MSL, and small aircraft Y altitude indicated 7900. After reading the flight progress strip of mlt X, I saw an inscription of the flight progress strip indicating mlt X was assigned 10000 ft MSL. As mlt X and the small aircraft Y drew nearer, the altitude readout of mlt X indicated climbing through 7200 ft MSL. At this time, when the 2 aircraft were face to face and about 1 mi apart, I pointed to the 2 aircraft and said, is mlt X climbing to 10000? When the controller did not respond I said again, is mlt X climbing to 10000? The controller did not respond and the 2 aircraft's radar targets emerged with no distinguishable horizontal separation and an estimated vertical separation of less than 500 ft. When the merging ended, mlt X altitude readout indicated climbing through 8000 ft MSL. The controller working the position told me, after the near collision, the aircraft was operating under VFR and that he did not desire to be relieved from the position.

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Original NASA ASRS Text

Title: MLT X CLB THROUGH OCCUPIED ALT HAD NMAC WITH SMA Y. POSSIBLE LTSS SYS ERROR. SEE AND AVOID CONCEPT.

Narrative: THIS NEAR COLLISION OCCURRED IMMEDIATELY PRIOR TO A POS RELIEF BRIEFING. I BEGAN MONITORING THE CTLR, WHO WAS TO BE RELIEVED, WORKING THE POS. WITHIN A MIN, I INDICATED TO THE CTLR THAT I WAS READY TO BEGIN THE RELIEF BRIEFING. IT WAS THEN THAT THE 'CONFLICT ALERT' ALARMED. I SAW MLT X CLBING INTO AN ENRTE ACFT. THE OTHER ACFT WAS SMA Y. BOTH ACFT WERE ON INST FLT PLANS, AND BOTH ACFT WERE COMPLYING WITH THEIR CLRNCS. SMA Y WAS FLYING NWESTERLY ALONG VICTOR 171 AT 8000 FT MSL. MLT X WAS FLYING SEASTERLY TOWARD THE LOUISVILLE VORTAC AND WAS CLBING TO AN INTERMEDIATE ASSIGNED ALT OF 10000 FT MSL. THE NEAR COLLISION OCCURRED WHEN MLT X WAS IN FRONT OF SMA Y ON AN OPPOSITE DIRECTION COURSE, AND CLBED THROUGH THE SAME ALT OF SMA Y. I BECAME AWARE OF THE LOSS OF APPROVED SEPARATION WHEN THE 2 ACFT WERE BTWN 1 AND 2 MI APART. AT THAT TIME, MLT X ALT READOUT, MODE C, INDICATED 7000 MSL, AND SMA Y ALT INDICATED 7900. AFTER READING THE FLT PROGRESS STRIP OF MLT X, I SAW AN INSCRIPTION OF THE FLT PROGRESS STRIP INDICATING MLT X WAS ASSIGNED 10000 FT MSL. AS MLT X AND THE SMA Y DREW NEARER, THE ALT READOUT OF MLT X INDICATED CLBING THROUGH 7200 FT MSL. AT THIS TIME, WHEN THE 2 ACFT WERE FACE TO FACE AND ABOUT 1 MI APART, I POINTED TO THE 2 ACFT AND SAID, IS MLT X CLBING TO 10000? WHEN THE CTLR DID NOT RESPOND I SAID AGAIN, IS MLT X CLBING TO 10000? THE CTLR DID NOT RESPOND AND THE 2 ACFT'S RADAR TARGETS EMERGED WITH NO DISTINGUISHABLE HORIZ SEPARATION AND AN ESTIMATED VERT SEPARATION OF LESS THAN 500 FT. WHEN THE MERGING ENDED, MLT X ALT READOUT INDICATED CLBING THROUGH 8000 FT MSL. THE CTLR WORKING THE POS TOLD ME, AFTER THE NEAR COLLISION, THE ACFT WAS OPERATING UNDER VFR AND THAT HE DID NOT DESIRE TO BE RELIEVED FROM THE 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.