Narrative:

Air carrier X abcd; a departure from hou; was cleared to FL390 by me. During the pilot's readback; there was another transmission from another aircraft which blocked air carrier's readback. What I heard was another aircraft departing the houston terminal area checking in climbing to FL230. What I did not hear was air carrier X abed; who was also on frequency; taking air carrier X abcd's clearance. I had not advised these two flights that similar sounding call signs were on the same frequency. Satisfied that the blocking transmission was the other aircraft coming from low altitude I went about my scan. I saw that air carrier X abed was 500 ft high about the same time that the conflict alert fired. I did not question if the aircraft was actually climbing or if I had simply gotten a bad mode C report for a hit or two and opted to wait another hit or two to see which it was. Once I realized that the aircraft was indeed climbing; I issued a descent due to the proximity of air carrier X abed and another aircraft already established at FL390. In my estimation; lateral separation was not lost between these two aircraft as the air carrier appeared to have sufficient time to establish vertical separation. The pilot apologized for taking company's climb clearance and did say that they too wanted FL390. Once air carrier X abed was safely clear of his traffic at FL390 he was cleared to FL390. Had I had the presence of mind to issue a similar call sign warning this would undoubtedly have not occurred. Had I stated after air carrier X abcd's readback was stepped on that the climb clearance was for that aircraft only then this would not have occurred. The pilot might have questioned an unsolicited climb clearance which was issued without explanation from ATC.

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Original NASA ASRS Text

Title: ZHU described a conflict event when an Air Carrier accepted a climb clearance intended for another aircraft; the Controller acknowledging his/her failure to issue a similar call sign warning.

Narrative: Air Carrier X ABCD; a departure from HOU; was cleared to FL390 by me. During the pilot's readback; there was another transmission from another aircraft which blocked Air Carrier's readback. What I heard was another aircraft departing the Houston terminal area checking in climbing to FL230. What I did not hear was Air Carrier X ABED; who was also on frequency; taking Air Carrier X ABCD's clearance. I had not advised these two flights that similar sounding call signs were on the same frequency. Satisfied that the blocking transmission was the other aircraft coming from low altitude I went about my scan. I saw that Air Carrier X ABED was 500 FT high about the same time that the conflict alert fired. I did not question if the aircraft was actually climbing or if I had simply gotten a bad Mode C report for a hit or two and opted to wait another hit or two to see which it was. Once I realized that the aircraft was indeed climbing; I issued a descent due to the proximity of Air Carrier X ABED and another aircraft already established at FL390. In my estimation; lateral separation was not lost between these two aircraft as the Air Carrier appeared to have sufficient time to establish vertical separation. The pilot apologized for taking company's climb clearance and did say that they too wanted FL390. Once Air Carrier X ABED was safely clear of his traffic at FL390 he was cleared to FL390. Had I had the presence of mind to issue a similar call sign warning this would undoubtedly have not occurred. Had I stated after Air Carrier X ABCD's readback was stepped on that the climb clearance was for that aircraft only then this would not have occurred. The pilot might have questioned an unsolicited climb clearance which was issued without explanation from ATC.

Data retrieved from NASA's ASRS site as of July 2013 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.