Narrative:

Air carrier X was cleared to climb to 14;000 ft. Air carrier Y was descended to 15;000 ft. Air carrier X was issued traffic 12 O'clock position; 2 miles; same direction; same route at 15;000 ft. Air carrier Y reported traffic in sight. Air carrier X was issued 'maintain visual separation with company E-135 who will be descending through your altitude.' air carrier X acknowledged. At this point; I decided not to descend air carrier Y as I was uncomfortable with the pilot readback and the proximity of the 2 aircraft. I advised air carrier X to 'disregard; I am going to leave company above you.' air carrier X then said; 'I have traffic at my altitude; 15;000 ft about 1 or 2 miles. I think we need a turn to the left.' at this time; air carrier X's mode C indicated level at 14;000 ft; then the next hit was 14;800 ft. I advised air carrier X he was assigned 14;000 ft; and turned him to a 90 degree heading (approximately 110 degree turn to left); then turned air carrier Y to a 280 degree heading (approximately 90 degree right turn). Air carrier X stated he read back 15;000 ft; which upon review of FAA audio tapes was incorrect. Air carrier X was originally issued 14;000 ft and read back 14;000 ft. Neither aircraft reported a TCAS alert or advisory. Closest proximity about 1 mile in-trail and zero vertical.

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Original NASA ASRS Text

Title: ZBW controller described conflict event when visual separation descent clearance was rescinded but air carrier descended and separation was lost.

Narrative: Air Carrier X was cleared to climb to 14;000 FT. Air Carrier Y was descended to 15;000 FT. Air Carrier X was issued traffic 12 O'clock position; 2 miles; same direction; same route at 15;000 FT. Air Carrier Y reported traffic in sight. Air Carrier X was issued 'maintain visual separation with company E-135 who will be descending through your altitude.' Air Carrier X acknowledged. At this point; I decided not to descend Air Carrier Y as I was uncomfortable with the Pilot readback and the proximity of the 2 aircraft. I advised Air Carrier X to 'disregard; I am going to leave Company above you.' Air Carrier X then said; 'I have traffic at my altitude; 15;000 FT about 1 or 2 miles. I think we need a turn to the left.' At this time; Air Carrier X's Mode C indicated level at 14;000 FT; then the next hit was 14;800 FT. I advised Air Carrier X he was assigned 14;000 FT; and turned him to a 90 degree heading (approximately 110 degree turn to left); then turned Air Carrier Y to a 280 degree heading (approximately 90 degree right turn). Air Carrier X stated he read back 15;000 FT; which upon review of FAA audio tapes was incorrect. Air Carrier X was originally issued 14;000 FT and read back 14;000 FT. Neither aircraft reported a TCAS alert or advisory. Closest proximity about 1 mile in-trail and zero vertical.

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.