Narrative:

I was training a cpc-it on local 1. We were down to a single runway operation due to construction. The far runway (21R) was in use and is very difficult to see without binoculars. The approach threshold is over 1 1/2 miles away. All aircraft were being taxied to this runway. R22 helicopters in particular are very difficult to locate. Three helicopters were in the pattern along with a C172 with other approaches and aircraft inbound. The C172 called for departure and was cleared for takeoff. I saw the R22 helicopter just cross the threshold out of the corner of my eye on the runway end video camera. Cpc-it had thought this helicopter had already departed and was on the upwind past the departure end. I immediately keyed up to tell the C172; 'disregard; hold short runway 21R'. The call was blocked while the cpc-it had to quickly make other transmissions due to the volume of traffic. When we were able to get back to confirm the C172's instructions; they had already taken the runway. The helicopter was already on the go from an option and separation was then appropriate but the C172 aborted their takeoff apparently aware that it didn't feel right. After reviewing the tapes; the C172 was reading back his takeoff clearance as I was telling them to disregard the takeoff clearance and hold. I should not have let the certified professional controller move on to talking to other aircraft assuming my last transmission was acknowledged. I should have confirmed the C172 was holding before moving on to the next priority. From a system perspective; obviously a taller tower (which has been on request for over 10 years) would have provided better visibility operationally to lower the possibility of these occurrences. Our approach end video camera was essential. It could be much improved however by using a wider angle of view and higher resolution. The cpc-it might have had a better chance of seeing the landing traffic from a better camera perspective.

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Original NASA ASRS Text

Title: Tower Controller described an unsafe situation when helicopter operations were difficult to observe due to the location and height of the Tower.

Narrative: I was training a CPC-IT on Local 1. We were down to a single runway operation due to construction. The far Runway (21R) was in use and is very difficult to see without binoculars. The approach threshold is over 1 1/2 miles away. All aircraft were being taxied to this runway. R22 helicopters in particular are very difficult to locate. Three helicopters were in the pattern along with a C172 with other approaches and aircraft inbound. The C172 called for departure and was cleared for takeoff. I saw the R22 helicopter just cross the threshold out of the corner of my eye on the runway end video camera. CPC-IT had thought this helicopter had already departed and was on the upwind past the departure end. I immediately keyed up to tell the C172; 'disregard; hold short Runway 21R'. The call was blocked while the CPC-IT had to quickly make other transmissions due to the volume of traffic. When we were able to get back to confirm the C172's instructions; they had already taken the runway. The helicopter was already on the go from an option and separation was then appropriate but the C172 aborted their takeoff apparently aware that it didn't feel right. After reviewing the tapes; the C172 was reading back his takeoff clearance as I was telling them to disregard the takeoff clearance and hold. I should not have let the Certified Professional Controller move on to talking to other aircraft assuming my last transmission was acknowledged. I should have confirmed the C172 was holding before moving on to the next priority. From a system perspective; obviously a taller Tower (which has been on request for over 10 years) would have provided better visibility operationally to lower the possibility of these occurrences. Our approach end video camera was essential. It could be much improved however by using a wider angle of view and higher resolution. The CPC-IT might have had a better chance of seeing the landing traffic from a better camera perspective.

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.