Walley2 asked
Another question of personal interest: Is radar and its effective cover an input parameter to FAA D Class tower analysis?
The short answer is that I think it isn't, but I'd welcome other inputs on the topic. I think its effect sits, unquantifiably, deep inside the analysis. Let me give a simplified answer for reasons of space.
The FAA analysis method is in the report: (1990) Establishment and discontinuance criteria for airport traffic control towers. Report FAA-APO-90-7. Federal Aviation Administration, Washington. These criteria are stated to be for
VFR towers. The FAA state that the primary responsibility of the VFR tower controller is to provide aircraft sequencing in the air and separation on the ground, and controllers determine aircraft position from pilot's reports and by direct observation of the aircraft.
There is no requirement for radar in executing this responsibility, and the criteria themselves do not provide for radar in the capital costing of the tower, nor do they provide any mechanism to check the value of radar. So it is simply not an input parameter.
The benefits of the tower in the analysis come from preventing collisions between aircraft, other prevented accidents and reduced flying time. To estimate those benefits, the FAA has developed a series of equations. For example, the reduction in collisions comes from the calculated difference using equations predicting collisions at towered airports and at non-towered airports.
These equations came from an analysis of all aircraft accidents on US soil using data from 1983-87, which were then screened to pick out accidents within 5 miles of airports. The final screening was to eliminate airports where there were less than 10,000 or more than 250,000 annual operations. A total of 2227 airports and 864 accidents were then used.
The equations for no tower/tower were found by analysis. The FAA tried some sophisticated methods to isolate out the effect of other variables such as ASPPCOV – the existence of radar approach control at the airport, and others such as the number of runways, but were not able to do so. So the fact that some (or was it many) airports with VFR towers have radar approach control is submerged somewhere in the computation of benefit, but it is not possible to see what the effect of radar is explicitly.
I don't know if there have been any later developments on this which bring the cost of radar into the analysis, or whether this tower analysis simply remains for VFR towers without the specific provision of radar. Anyone else got inputs? VoR? Philthy?