Narrative:

I was working the r-pos on sector 19/20 in the slc ARTCC. A departing military mlt (military X) called me after I had taken a handoff from gtf approach control. He checked out of an altitude, which I verified, and said he was climbing to FL190. He had a request but I had other duties to attend to, so I told him to stand by. When I came back to him, he requested a change in routing which I gave to him. There was also another aircraft flying in the opp military MTR (military Y) landing at malmstrom AFB. This aircraft was descended to 11000', but then requested lower. Since 11000' was the lowest I could take him due to the minimum altitude charts in my sector, he said he could descend VFR. I asked if he wanted to maintain VFR on top. The pilot said affirmative, and was given a VFR on top clearance. This only lasted a couple of mins before the pilot requested a hard altitude of 11000' again. After looking at his traffic, military X showing out of 13500' climbing to FL190, his mode C had been verified on climb out from gtf, I gave clearance to military Y to maintain 11000'. The aircraft passed with less than 5 mi and I did not issue traffic because of the difference in altitudes. Approximately 8 mins later I asked military X to say altitude. His mode C was showing 195, then 199. He replied but was overridden by someone else on the same frequency. Now mode C was indicating 203. I asked again for his altitude and he responded that he was leaving 15600' for FL190. I asked for his altitude again, because I didn't think 15600' was correct. Again he said his altitude which was now showing FL208. I told him to stop altitude squawk and informed him that his altitude differed by 5000'. With the difference in mode C readout, I don't know how close these 2 aircraft were when they passed.

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Original NASA ASRS Text

Title: CTLR USED ALT SEPARATION BETWEEN IFR ACFT, BUT LATER DISCOVERED THAT ONE OF THE ACFT HAD ERRONEOUS MODE C READOUT. LESS THAN STANDARD SEPARATION LIKELY.

Narrative: I WAS WORKING THE R-POS ON SECTOR 19/20 IN THE SLC ARTCC. A DEPARTING MIL MLT (MIL X) CALLED ME AFTER I HAD TAKEN A HDOF FROM GTF APCH CTL. HE CHKED OUT OF AN ALT, WHICH I VERIFIED, AND SAID HE WAS CLBING TO FL190. HE HAD A REQUEST BUT I HAD OTHER DUTIES TO ATTEND TO, SO I TOLD HIM TO STAND BY. WHEN I CAME BACK TO HIM, HE REQUESTED A CHANGE IN ROUTING WHICH I GAVE TO HIM. THERE WAS ALSO ANOTHER ACFT FLYING IN THE OPP MIL MTR (MIL Y) LNDG AT MALMSTROM AFB. THIS ACFT WAS DSNDED TO 11000', BUT THEN REQUESTED LOWER. SINCE 11000' WAS THE LOWEST I COULD TAKE HIM DUE TO THE MINIMUM ALT CHARTS IN MY SECTOR, HE SAID HE COULD DSND VFR. I ASKED IF HE WANTED TO MAINTAIN VFR ON TOP. THE PLT SAID AFFIRMATIVE, AND WAS GIVEN A VFR ON TOP CLRNC. THIS ONLY LASTED A COUPLE OF MINS BEFORE THE PLT REQUESTED A HARD ALT OF 11000' AGAIN. AFTER LOOKING AT HIS TFC, MIL X SHOWING OUT OF 13500' CLBING TO FL190, HIS MODE C HAD BEEN VERIFIED ON CLBOUT FROM GTF, I GAVE CLRNC TO MIL Y TO MAINTAIN 11000'. THE ACFT PASSED WITH LESS THAN 5 MI AND I DID NOT ISSUE TFC BECAUSE OF THE DIFFERENCE IN ALTS. APPROX 8 MINS LATER I ASKED MIL X TO SAY ALT. HIS MODE C WAS SHOWING 195, THEN 199. HE REPLIED BUT WAS OVERRIDDEN BY SOMEONE ELSE ON THE SAME FREQ. NOW MODE C WAS INDICATING 203. I ASKED AGAIN FOR HIS ALT AND HE RESPONDED THAT HE WAS LEAVING 15600' FOR FL190. I ASKED FOR HIS ALT AGAIN, BECAUSE I DIDN'T THINK 15600' WAS CORRECT. AGAIN HE SAID HIS ALT WHICH WAS NOW SHOWING FL208. I TOLD HIM TO STOP ALT SQUAWK AND INFORMED HIM THAT HIS ALT DIFFERED BY 5000'. WITH THE DIFFERENCE IN MODE C READOUT, I DON'T KNOW HOW CLOSE THESE 2 ACFT WERE WHEN THEY PASSED.

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.