Narrative:

I took the position; runway 6 was active when I had previously worked the position; and we had switched to runway 36 since I last worked it. Aircraft X called inquiring about the ATIS and asked for weather. I read him the wind and altimeter. He then requested taxi to the active. I said 'taxi to runway 6 via alpha. *Pause* correction; taxi to runway 36 via alpha.' the pilot then responded 'aircraft X'. The pilot then began taxiing across runway 36. I told him to 'hold position; make 180 on taxiway a and hold short of runway 36.' my next transmission was; 'hold position for landing traffic on 36.' after the traffic landed; I told the pilot; 'continue holding short of 36 and contact tower on 125.075.' at this point; he contacted tower. The tower controller told him 'turn left on course; runway 36 cleared for takeoff.' the pilot responded; 'did I miss something? I thought I was cleared to 6.' the tower controller replied; 'aircraft X; it sounds like you were cleared to runway 36 via alpha.' the pilot said; 'ok; I did not catch the 36'. He then read back the takeoff instructions and departed. After reviewing the tapes; it appears that when I paused to make the correction; though I did not un-key; he may have keyed up to start reading back my instructions. The only response on the tapes is 'aircraft X'. He was not holding short of anything because he was going to runway 36; so per our directives he is not required to read back the instructions. I did not know that he may not have heard my correction instructions because there was no indication that he may have keyed up and talked over me. Also; because he is not required to read back instructions unless he is to hold short or if he is given an altitude assignment; I thought he was acknowledging me by giving his call sign. The other possibility is that he misheard my instructions or got confused because I originally told him to taxi to runway 6 then corrected his instructions to 36. The only thing for certain is that he crossed an active runway because he believed that he was to go to runway 6. This was a communication error. I took the response 'aircraft X' to mean that he heard/understood my directions to taxi to runway 36. His understanding was to taxi to runway 6. There was not an aircraft on the runway or even one in close proximity to it at the time of the incident; but if a misunderstanding like this took place under different circumstances; the result could be more severe. Recommendation; if he did indeed key up and begin talking over my transmission; having a tone or something to alert both of us that a transmission was 'stepped on' may have prevented the situation because the instructions would have been repeated. If he just misunderstood; or got confused because I made the correction; then just repeating the transmission; or requiring a full read back of the instructions may have prevented the situation.

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Original NASA ASRS Text

Title: ALO Controller described a confused clearance event when a taxiing aircraft apparently stepped on an ATC transmission; did not acknowledge the clearance issued; and crossed a runway without approval.

Narrative: I took the position; Runway 6 was active when I had previously worked the position; and we had switched to Runway 36 since I last worked it. Aircraft X called inquiring about the ATIS and asked for weather. I read him the wind and altimeter. He then requested taxi to the active. I said 'Taxi to Runway 6 via alpha. *Pause* Correction; taxi to Runway 36 via alpha.' The pilot then responded 'Aircraft X'. The pilot then began taxiing across Runway 36. I told him to 'hold position; make 180 on Taxiway A and hold short of Runway 36.' My next transmission was; 'Hold position for landing traffic on 36.' After the traffic landed; I told the pilot; 'Continue holding short of 36 and contact Tower on 125.075.' At this point; he contacted Tower. The Tower Controller told him 'Turn left on course; Runway 36 cleared for takeoff.' The pilot responded; 'Did I miss something? I thought I was cleared to 6.' The Tower controller replied; 'Aircraft X; it sounds like you were cleared to Runway 36 via Alpha.' The pilot said; 'OK; I did not catch the 36'. He then read back the takeoff instructions and departed. After reviewing the tapes; it appears that when I paused to make the correction; though I did not un-key; he may have keyed up to start reading back my instructions. The only response on the tapes is 'Aircraft X'. He was not holding short of anything because he was going to Runway 36; so per our directives he is not required to read back the instructions. I did not know that he may not have heard my correction instructions because there was no indication that he may have keyed up and talked over me. Also; because he is not required to read back instructions unless he is to hold short or if he is given an altitude assignment; I thought he was acknowledging me by giving his call sign. The other possibility is that he misheard my instructions or got confused because I originally told him to taxi to Runway 6 then corrected his instructions to 36. The only thing for certain is that he crossed an active runway because he believed that he was to go to Runway 6. This was a communication error. I took the response 'Aircraft X' to mean that he heard/understood my directions to taxi to Runway 36. His understanding was to taxi to Runway 6. There was not an aircraft on the runway or even one in close proximity to it at the time of the incident; but if a misunderstanding like this took place under different circumstances; the result could be more severe. Recommendation; if he did indeed key up and begin talking over my transmission; having a tone or something to alert both of us that a transmission was 'stepped on' may have prevented the situation because the instructions would have been repeated. If he just misunderstood; or got confused because I made the correction; then just repeating the transmission; or requiring a full read back of the instructions may have prevented the situation.

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.