Narrative:

A lear jet was handed off from the det city departure controller climbing to 8;000. I issued 9;000 to the aircraft. I later cleared the lear direct to the annts intersection and heard two separate read backs. Since I was afraid that 2 separate aircraft read back the clearance (after listening to the tapes I believe that both pilots of the same aircraft responded to my direction) I reiterated that the clearance was for the subject lear jet. This time I instructed him to 'fly heading two four zero and when you're able go direct to the annts intersection'. The aircraft read back 'direct annts and uh; two four zero'. At the time I believed that this was a correct read back of the heading and departure fix. Although we have not yet spoken with the pilot; I believe now that the pilot intended to climb and maintain FL240. I later observed the aircraft climbing through 10;300 ft. Since there was conflicting traffic at 11;000 and 12;000 I instructed the pilot to descend immediately back to 9;000. I then took action to move traffic that was in front of him at 8;000 in case the immediate instruction caused him to break his altitude assignment on the way back down. Recommendation; I don't know that I would have done anything differently at the time. I honestly believed that the pilot intended to read back the heading and fix he was cleared to. My workload was fairly heavy and I went on to the next task. Perhaps the hesitation in the pilot's voice; coupled with the appearance that english was a second language for him should have raised a flag and I could have taken an extra moment to make sure we were on the same page.

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Original NASA ASRS Text

Title: D21 controller described altitude assignment vs. heading mix up that resulted in a near loss of separation event; the controller indicating language barrier as a likely causal factor.

Narrative: A Lear Jet was handed off from the DET City Departure Controller climbing to 8;000. I issued 9;000 to the aircraft. I later cleared the Lear direct to the ANNTS Intersection and heard two separate read backs. Since I was afraid that 2 separate aircraft read back the clearance (after listening to the tapes I believe that both pilots of the same aircraft responded to my direction) I reiterated that the clearance was for the subject Lear Jet. This time I instructed him to 'fly heading two four zero and when you're able go direct to the ANNTS intersection'. The aircraft read back 'direct ANNTS and uh; two four zero'. At the time I believed that this was a correct read back of the heading and departure fix. Although we have not yet spoken with the pilot; I believe now that the pilot intended to climb and maintain FL240. I later observed the aircraft climbing through 10;300 ft. Since there was conflicting traffic at 11;000 and 12;000 I instructed the pilot to descend immediately back to 9;000. I then took action to move traffic that was in front of him at 8;000 in case the immediate instruction caused him to break his altitude assignment on the way back down. Recommendation; I don't know that I would have done anything differently at the time. I honestly believed that the pilot intended to read back the heading and fix he was cleared to. My workload was fairly heavy and I went on to the next task. Perhaps the hesitation in the pilot's voice; coupled with the appearance that English was a second language for him should have raised a flag and I could have taken an extra moment to make sure we were on the same page.

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.