Narrative:

Aircraft #1 filed for 5000 ft is currently at 4000 ft approximately 25 mi east of bna eastbound. Aircraft #2 is at 5000 ft approximately 30 mi ese of bna northbound. Aircraft #1 has been handed off to ZME and coordinated that he will be at 4000 ft because of the overflt at 5000 ft. I issued to aircraft #1 'maintain 4000 ft, traffic 2 O'clock position, 5 mi, northbound at 5000 ft, contact ZME on 132.1.' the pilot reads back in a slightly distinguishable, but garbled mode 'up to 5 ZME on 132.1.' I did not expect and did not catch such a readback. I noticed a moment later, the 2 aircraft approaching each other at 5000 ft, and issued a 'TA' to aircraft #2. Targets merged on radar with no vertical separation showing.

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Original NASA ASRS Text

Title: RADAR CTLR AT BNA FAILS TO HEAR INCORRECT READ BACK, SEPARATION LOST.

Narrative: ACFT #1 FILED FOR 5000 FT IS CURRENTLY AT 4000 FT APPROX 25 MI E OF BNA EBOUND. ACFT #2 IS AT 5000 FT APPROX 30 MI ESE OF BNA NBOUND. ACFT #1 HAS BEEN HANDED OFF TO ZME AND COORDINATED THAT HE WILL BE AT 4000 FT BECAUSE OF THE OVERFLT AT 5000 FT. I ISSUED TO ACFT #1 'MAINTAIN 4000 FT, TFC 2 O'CLOCK POS, 5 MI, NBOUND AT 5000 FT, CONTACT ZME ON 132.1.' THE PLT READS BACK IN A SLIGHTLY DISTINGUISHABLE, BUT GARBLED MODE 'UP TO 5 ZME ON 132.1.' I DID NOT EXPECT AND DID NOT CATCH SUCH A READBACK. I NOTICED A MOMENT LATER, THE 2 ACFT APCHING EACH OTHER AT 5000 FT, AND ISSUED A 'TA' TO ACFT #2. TARGETS MERGED ON RADAR WITH NO VERT SEPARATION SHOWING.

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.