Narrative:

I was working ZDV sector 36 radar by myself. The WX was marginal IFR and I began to plan IFR approachs into the airports in my airspace. I was working 2 company aircraft; aircraft X and aircraft Y. Aircraft Y was inbound fmn and aircraft X was inbound dro. I cleared aircraft Y over the initial for the ILS into fmn and was waiting to step aircraft Y down reference mia's of 10000 ft and 9000 ft. As I issued a crossing restr at the initial of 9000 ft; aircraft Y said he could get a visual; so I changed the clearance to descend and maintain 9000 ft. Shortly after; I issued aircraft X a pilot's discretion descent to 9000 ft because the pilot had the WX at dro and was expecting a visual approach. The correct altitude should have been 10000 ft. I think I confused the identities of these 2 aircraft and their destinations. I have never cleared any dro inbound below 10000 ft (mia) before this. I was then busy setting up IFR approachs into pag; bdg; and pso -- rare occurrences even in winter. When I scanned back to aircraft X; I realized I had descended him below my mia. At that moment; aircraft X said he could not get dro in sight and requested an ILS. I immediately climbed aircraft X to 10000 ft and at 10000 ft began vectoring for a straight-in ILS. I will not allow these similar call signs to cause confusion of aircraft identify/destination in the future.

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Original NASA ASRS Text

Title: ZDV CTLR EXPERIENCED OPERROR WHEN DESCENDING IFR ACFT BELOW MINIMUM ALT; LISTING SIMILAR CALL SIGNS AS CONTRIBUTORY.

Narrative: I WAS WORKING ZDV SECTOR 36 RADAR BY MYSELF. THE WX WAS MARGINAL IFR AND I BEGAN TO PLAN IFR APCHS INTO THE ARPTS IN MY AIRSPACE. I WAS WORKING 2 COMPANY ACFT; ACFT X AND ACFT Y. ACFT Y WAS INBOUND FMN AND ACFT X WAS INBOUND DRO. I CLRED ACFT Y OVER THE INITIAL FOR THE ILS INTO FMN AND WAS WAITING TO STEP ACFT Y DOWN REF MIA'S OF 10000 FT AND 9000 FT. AS I ISSUED A XING RESTR AT THE INITIAL OF 9000 FT; ACFT Y SAID HE COULD GET A VISUAL; SO I CHANGED THE CLRNC TO DSND AND MAINTAIN 9000 FT. SHORTLY AFTER; I ISSUED ACFT X A PLT'S DISCRETION DSCNT TO 9000 FT BECAUSE THE PLT HAD THE WX AT DRO AND WAS EXPECTING A VISUAL APCH. THE CORRECT ALT SHOULD HAVE BEEN 10000 FT. I THINK I CONFUSED THE IDENTITIES OF THESE 2 ACFT AND THEIR DESTS. I HAVE NEVER CLRED ANY DRO INBOUND BELOW 10000 FT (MIA) BEFORE THIS. I WAS THEN BUSY SETTING UP IFR APCHS INTO PAG; BDG; AND PSO -- RARE OCCURRENCES EVEN IN WINTER. WHEN I SCANNED BACK TO ACFT X; I REALIZED I HAD DSNDED HIM BELOW MY MIA. AT THAT MOMENT; ACFT X SAID HE COULD NOT GET DRO IN SIGHT AND REQUESTED AN ILS. I IMMEDIATELY CLBED ACFT X TO 10000 FT AND AT 10000 FT BEGAN VECTORING FOR A STRAIGHT-IN ILS. I WILL NOT ALLOW THESE SIMILAR CALL SIGNS TO CAUSE CONFUSION OF ACFT IDENT/DEST IN THE FUTURE.

Data retrieved from NASA's ASRS site as of May 2009 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.