Skyhawk 172/Cutlass 172
Air Traffic Control Fully Certified
Situational Awareness
ATC Issue All Types
Air Traffic Control Issued New Clearance
ATC Equipment / Nav Facility / Buildings; Environment - Non Weather Related
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.
Runway 21 left; our main runway was closed. Runway 21R was the active runway for departing and arriving aircraft. The approach end of Runway 21R is over a mile and half from the Tower. Seeing a Robinson 22 short final is very hard to see with the naked eye. I was busy with four aircraft in the pattern a couple inbounds and departures waiting. I did not see the R-22 on the numbers of Runway 21R then cleared the C172 for takeoff. After I cleared the C172 for takeoff; my Instructor then keyed up and said 'NEGATIVE [C172]; HOLD SHORT OF RUNWAY 21R'. The C172 did not hear my Instructor's transmission because he was reading back his takeoff clearance. The two transmissions overlapped at the exact same time. The C172 was already passed the hold short lines so they turned onto the runway and exited at the next taxiway. By this time the R22 was already airborne with more than 3;000 FT available. If there is a next [time]; I will make sure that the aircraft I'm talking to received my transmissions and make that my number one priority. I will also use binoculars every time in these situations. Also a Tower with a better location so all runways are clearly visible and a communication system where Tower supersedes all other transmissions; even the aircraft keyed up talking.
Tower Controller described an unsafe situation when helicopter operations were difficult to observe due to the location and height of the Tower.