Narrative:

Air carrier X and air carrier Y were on converging courses; opposite direction; and separated by 1;000 ft. As per the 7110.65; my trainee issued the traffic to air carrier X. The phraseology was in accordance with the 7110.65. The aircraft acknowledged and both my trainee and myself heard 'roger traffic at 360.' moments later; I noticed air carrier X descending. I immediately called to verify the aircraft's altitude. The pilot did not respond and there was no blocked frequency. Again; I tried to verify the altitude and was again ignored by the pilot. On the third call; the pilot responded and said we are descending for traffic to FL360. At this time I climbed the aircraft back to FL370 but separation was lost. Air carrier Y also said at this time that he had responded to a TCAS RA and was returning to his assigned altitude of FL360. After the incident; we listened to the tapes and clearly heard the pilot read back air carrier X descending to FL360 for traffic. Both myself and the trainee missed the readback and thus we are partially to blame for the resulting error. However; the pilot must also bear some responsibility for taking a correctly issued traffic call and misconstruing it to mean a descent clearance to the altitude the traffic was at. Further; the fact the pilot was remiss in monitoring the frequency; i.e.; I had to call 3 times to get a response; causing a delay in corrective action which resulted in an error.

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Original NASA ASRS Text

Title: ZMA Controller providing OJT experienced operational error at FL370/360 when issued traffic altitude information was acknowledged as descent clearance; reporter/developmental both failed to note incorrect pilot response.

Narrative: Air Carrier X and Air Carrier Y were on converging courses; opposite direction; and separated by 1;000 FT. As per the 7110.65; my Trainee issued the traffic to Air Carrier X. The phraseology was in accordance with the 7110.65. The aircraft acknowledged and both my Trainee and myself heard 'Roger traffic at 360.' Moments later; I noticed Air Carrier X descending. I immediately called to verify the aircraft's altitude. The pilot did not respond and there was no blocked frequency. Again; I tried to verify the altitude and was again ignored by the pilot. On the third call; the pilot responded and said we are descending for traffic to FL360. At this time I climbed the aircraft back to FL370 but separation was lost. Air Carrier Y also said at this time that he had responded to a TCAS RA and was returning to his assigned altitude of FL360. After the incident; we listened to the tapes and clearly heard the pilot read back Air Carrier X descending to FL360 for traffic. Both myself and the Trainee missed the readback and thus we are partially to blame for the resulting error. However; the pilot must also bear some responsibility for taking a correctly issued traffic call and misconstruing it to mean a descent clearance to the altitude the traffic was at. Further; the fact the pilot was remiss in monitoring the frequency; i.e.; I had to call 3 times to get a response; causing a delay in corrective action which resulted in an error.

Data retrieved from NASA's ASRS site as of April 2012 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.