Narrative:

Small transport X was on frequency descending to FL180. Cpr Y, checked on descending to FL280. I observed small transport X leaving FL270, and issued a descent clearance to FL270 to cpr Y. Since small transport X was descending at a slower rate than the traffic above, I amended his descent clearance to include a min rate of descent of at least 1000 FPM until he departed FL260. This clearance was accepted, with a question, by cpr Y, who responded as use of an incomplete (and inconsistent) call sign. The acknowledgement of the clarified instruction was accomplished without a callsign. I noticed cpr Y descending through FL270 and asked him to verify level. When he responded that he was going through FL260, I instructed him to 'stop descent'; this was to preserve any vertical separation that I still had. I then reassigned FL260 to cpr Y and insured that small transport X was under that altitude. I feel that the following factors were contributory to the incident: use of incomplete call signs by the pilot and myself. No call sign used on final amended clearance and acknowledgement. Call sign given out of order during the middle of a readback question. Staffing at the facility was short enough to prevent a radar associate being used.

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Original NASA ASRS Text

Title: CPR Y HAD LESS THAN STANDARD SEPARATION DURING DSCNT FROM SMT X. SYS ERROR. CPR Y DSCNT THROUGH ASSIGNED ALT. PLTDEV.

Narrative: SMT X WAS ON FREQ DSNDING TO FL180. CPR Y, CHKED ON DSNDING TO FL280. I OBSERVED SMT X LEAVING FL270, AND ISSUED A DSCNT CLRNC TO FL270 TO CPR Y. SINCE SMT X WAS DSNDING AT A SLOWER RATE THAN THE TFC ABOVE, I AMENDED HIS DSCNT CLRNC TO INCLUDE A MIN RATE OF DSCNT OF AT LEAST 1000 FPM UNTIL HE DEPARTED FL260. THIS CLRNC WAS ACCEPTED, WITH A QUESTION, BY CPR Y, WHO RESPONDED AS USE OF AN INCOMPLETE (AND INCONSISTENT) CALL SIGN. THE ACKNOWLEDGEMENT OF THE CLARIFIED INSTRUCTION WAS ACCOMPLISHED WITHOUT A CALLSIGN. I NOTICED CPR Y DSNDING THROUGH FL270 AND ASKED HIM TO VERIFY LEVEL. WHEN HE RESPONDED THAT HE WAS GOING THROUGH FL260, I INSTRUCTED HIM TO 'STOP DSCNT'; THIS WAS TO PRESERVE ANY VERT SEPARATION THAT I STILL HAD. I THEN REASSIGNED FL260 TO CPR Y AND INSURED THAT SMT X WAS UNDER THAT ALT. I FEEL THAT THE FOLLOWING FACTORS WERE CONTRIBUTORY TO THE INCIDENT: USE OF INCOMPLETE CALL SIGNS BY THE PLT AND MYSELF. NO CALL SIGN USED ON FINAL AMENDED CLRNC AND ACKNOWLEDGEMENT. CALL SIGN GIVEN OUT OF ORDER DURING THE MIDDLE OF A READBACK QUESTION. STAFFING AT THE FACILITY WAS SHORT ENOUGH TO PREVENT A RADAR ASSOCIATE BEING USED.

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.