Narrative:

Center passed 14,000 for a final altitude for air carrier X. I missed the altitude due to coordination in the tower. I was working approach and ground control combined. I read back 17,000 as a final, that was the requested altitude on the strip. Center gave their operating initials. Since center gave their operating initials, I assumed 17,000 to be correct and issued that altitude to air carrier X. The air carrier X departed and when it was clear of inbound traffic, I sent him over to center out of 13,000. Center called over the landline for air carrier X to maintain 14,000. I tried to call air carrier X. Air carrier X was between frequencys and came over to center at 14,900. Air carrier X passed within 3.16 NM and 100' of small transport Y level at 15,000. It was a listening mistake on my part. It was a listening mistake on center's part when they acknowledged my readback but did not correct the altitude. I should have given ground control to local control when I got 2 IFR inbnds and had 2 IFR outbnds. There was also vehicles involved on ground control. That would have reduced my workload and I may have not missed the correct altitude. No evasive action by either aircraft was necessary. Supplemental information from acn 117859: during climbout at mso, our flight encountered a near miss. We were cleared to 17,000' on a southeasterly course and altitude of 14,600'. ZLC gave us traffic at 12 O'clock 2 mi. We asked for his altitude, it was 15,000'. We began an immediate descent to 14,000'. The traffic was southbound about 1 mi when we obtained visual contact at 14,000'. We called ZLC to confirm our altitude. We proceeded outbnd on V-187 at 14,000. Later we were given a climb to 17,000', our cruise altitude to great falls, mt.

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Original NASA ASRS Text

Title: ACR WAS NOT ISSUED A CENTER REVISED ALT AND CONFLICTED WITH AN SMT RESULTING IN LESS THAN STANDARD SEPARATION.

Narrative: CENTER PASSED 14,000 FOR A FINAL ALT FOR ACR X. I MISSED THE ALT DUE TO COORD IN THE TWR. I WAS WORKING APCH AND GND CTL COMBINED. I READ BACK 17,000 AS A FINAL, THAT WAS THE REQUESTED ALT ON THE STRIP. CENTER GAVE THEIR OPERATING INITIALS. SINCE CENTER GAVE THEIR OPERATING INITIALS, I ASSUMED 17,000 TO BE CORRECT AND ISSUED THAT ALT TO ACR X. THE ACR X DEPARTED AND WHEN IT WAS CLEAR OF INBND TFC, I SENT HIM OVER TO CENTER OUT OF 13,000. CENTER CALLED OVER THE LANDLINE FOR ACR X TO MAINTAIN 14,000. I TRIED TO CALL ACR X. ACR X WAS BETWEEN FREQS AND CAME OVER TO CENTER AT 14,900. ACR X PASSED WITHIN 3.16 NM AND 100' OF SMT Y LEVEL AT 15,000. IT WAS A LISTENING MISTAKE ON MY PART. IT WAS A LISTENING MISTAKE ON CENTER'S PART WHEN THEY ACKNOWLEDGED MY READBACK BUT DID NOT CORRECT THE ALT. I SHOULD HAVE GIVEN GND CTL TO LCL CTL WHEN I GOT 2 IFR INBNDS AND HAD 2 IFR OUTBNDS. THERE WAS ALSO VEHICLES INVOLVED ON GND CTL. THAT WOULD HAVE REDUCED MY WORKLOAD AND I MAY HAVE NOT MISSED THE CORRECT ALT. NO EVASIVE ACTION BY EITHER ACFT WAS NECESSARY. SUPPLEMENTAL INFORMATION FROM ACN 117859: DURING CLIMBOUT AT MSO, OUR FLT ENCOUNTERED A NEAR MISS. WE WERE CLRED TO 17,000' ON A SOUTHEASTERLY COURSE AND ALT OF 14,600'. ZLC GAVE US TFC AT 12 O'CLOCK 2 MI. WE ASKED FOR HIS ALT, IT WAS 15,000'. WE BEGAN AN IMMEDIATE DSCNT TO 14,000'. THE TFC WAS SBND ABOUT 1 MI WHEN WE OBTAINED VISUAL CONTACT AT 14,000'. WE CALLED ZLC TO CONFIRM OUR ALT. WE PROCEEDED OUTBND ON V-187 AT 14,000. LATER WE WERE GIVEN A CLIMB TO 17,000', OUR CRUISE ALT TO GREAT FALLS, MT.

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.