Narrative:

Small aircraft a was eastbound over brave at 11000' MSL. Medium large transport B and light transport C were converging at brave, inbound to mke. Both medium large transport B and light transport C were descended to 12000' and I vectored light transport C behind medium large transport B. I did not issue 'similar call signs', because both pilots seemed to be aware of the other. I issued several clrncs to each aircraft with no confusion. Traffic was issued to all 3 aircraft. When medium large transport B was 5 mi beyond small aircraft a, with light transport C about 2 mi in front of small aircraft a, I issued descent to 10000' to medium large transport B. Light transport C took the clearance, reading back, '1-0 thousand, C.' I heard the altitude, but missed the incorrect (partial) call sign. Light transport C then called traffic 'in sight'. Conflict alert then activated, as light transport C left 12000'. I immediately reissued 12000' and asked light transport C to verify altitude. Pilot said he was cleared to 10000', and returned to 12000'. Sep was reduced to 500' and 1 mi. Possible contributing factors: controller's issuance of clearance may have had company call sign 'clipped' by transmitter, but no way to verify this. Pilot read back wrong clearance. Controller possibly lulled by previous lack of confusion. Should have alerted pilots of similar call signs. Controller not expecting pilot to accept clearance through known traffic (ie, listening more for altitude than call sign readback?). Solution: don't let your guard down for even one second.

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Original NASA ASRS Text

Title: LTT C FLC TOOK DESCENT CLRNC FOR ANOTHER ACFT WHICH RESULTED IN LESS THAN STANDARD SEPARATION WITH AN ACFT AT 11000'.

Narrative: SMA A WAS EBND OVER BRAVE AT 11000' MSL. MLG B AND LTT C WERE CONVERGING AT BRAVE, INBND TO MKE. BOTH MLG B AND LTT C WERE DSNDED TO 12000' AND I VECTORED LTT C BEHIND MLG B. I DID NOT ISSUE 'SIMILAR CALL SIGNS', BECAUSE BOTH PLTS SEEMED TO BE AWARE OF THE OTHER. I ISSUED SEVERAL CLRNCS TO EACH ACFT WITH NO CONFUSION. TFC WAS ISSUED TO ALL 3 ACFT. WHEN MLG B WAS 5 MI BEYOND SMA A, WITH LTT C ABOUT 2 MI IN FRONT OF SMA A, I ISSUED DSNT TO 10000' TO MLG B. LTT C TOOK THE CLRNC, READING BACK, '1-0 THOUSAND, C.' I HEARD THE ALT, BUT MISSED THE INCORRECT (PARTIAL) CALL SIGN. LTT C THEN CALLED TFC 'IN SIGHT'. CONFLICT ALERT THEN ACTIVATED, AS LTT C LEFT 12000'. I IMMEDIATELY REISSUED 12000' AND ASKED LTT C TO VERIFY ALT. PLT SAID HE WAS CLRED TO 10000', AND RETURNED TO 12000'. SEP WAS REDUCED TO 500' AND 1 MI. POSSIBLE CONTRIBUTING FACTORS: CTLR'S ISSUANCE OF CLRNC MAY HAVE HAD COMPANY CALL SIGN 'CLIPPED' BY XMITTER, BUT NO WAY TO VERIFY THIS. PLT READ BACK WRONG CLRNC. CTLR POSSIBLY LULLED BY PREVIOUS LACK OF CONFUSION. SHOULD HAVE ALERTED PLTS OF SIMILAR CALL SIGNS. CTLR NOT EXPECTING PLT TO ACCEPT CLRNC THROUGH KNOWN TFC (IE, LISTENING MORE FOR ALT THAN CALL SIGN READBACK?). SOLUTION: DON'T LET YOUR GUARD DOWN FOR EVEN ONE SEC.

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.