Narrative:

Aircraft #1 was issued FL220 and advised of cheyenne traffic crossing and that further descent would be issued when clear. Aircraft #1 acknowledged traffic; but not in sight. Subsequent descent clearance was issued to aircraft #3 (same company call sign as aircraft #1; different flight number). In taped review; both aircraft #1 and #3 read back a descent clearance; but only aircraft #3 could be clearly heard. At the time of the event; 4 frequencys were being used and 2 additional frequencys were being monitored (121.5/243.0). The simultaneous readback was only revealed after aircraft #1 began descent through protected airspace of aircraft #2. As more than 15 aircraft were on frequency at the time; it is often difficult to identify (positively) who has partially read back; or simply acknowledged; an ATC clearance.

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Original NASA ASRS Text

Title: CONFLICT AT FL210 AS 2 SAME COMPANY ACFT DSNDED AT ONCE AND THE READBACK OF ONE ACFT WAS BLOCKED.

Narrative: ACFT #1 WAS ISSUED FL220 AND ADVISED OF CHEYENNE TFC XING AND THAT FURTHER DSCNT WOULD BE ISSUED WHEN CLR. ACFT #1 ACKNOWLEDGED TFC; BUT NOT IN SIGHT. SUBSEQUENT DSCNT CLRNC WAS ISSUED TO ACFT #3 (SAME COMPANY CALL SIGN AS ACFT #1; DIFFERENT FLT NUMBER). IN TAPED REVIEW; BOTH ACFT #1 AND #3 READ BACK A DSCNT CLRNC; BUT ONLY ACFT #3 COULD BE CLRLY HEARD. AT THE TIME OF THE EVENT; 4 FREQS WERE BEING USED AND 2 ADDITIONAL FREQS WERE BEING MONITORED (121.5/243.0). THE SIMULTANEOUS READBACK WAS ONLY REVEALED AFTER ACFT #1 BEGAN DSCNT THROUGH PROTECTED AIRSPACE OF ACFT #2. AS MORE THAN 15 ACFT WERE ON FREQ AT THE TIME; IT IS OFTEN DIFFICULT TO IDENT (POSITIVELY) WHO HAS PARTIALLY READ BACK; OR SIMPLY ACKNOWLEDGED; AN ATC CLRNC.

Data retrieved from NASA's ASRS site as of January 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.