I am certainly not following this rational on wake separation...Perhaps some views on the thought or approved process.
Two aircraft less than 2.5nm apart on a clear day from a tower when both are established on precision approaches is perfectly comfortable.
The ICAO wake tables are based on a minimum of MRS between many and similar aircraft wake cats.
While these distances are based on ATC and radar control, I dont follow how that can be extrapolated to visual separation, while you can 'see' the aircraft..you cannot 'see' the turbulence....
How far does one take 'similar' aircraft in this scenario...an A380 2nm behind an A380?
Aircraft still make a vortex, and the mechanics of vortex creation are not fully understood nor developed. As an example, a lightly loaded 737-800, on final may only need flaps 30, while a heavily loaded 737-800 will need flaps 40. These flap settings create a completely different type, strength, and advection rate of the vortex pair.
Crosswinds do not only affect advection rates, it also influences creation of the vortex
I would note that it is well documented that the wake turbulence is stronger and lasts longer on a clear day.
Adding this later..the FAA RECAT program does not allow for visual sep between any wake cats, even same...
"Familiarity with RECAT wake separation standards is particularly important during visual approach operations as pilots assume responsibility for avoiding wake turbulence when cleared to visually follow preceding traffic."
http://www.faa.gov/other_visit/aviat.../SAFO12007.pdf