F.D. it's not about what was at the beginning the egg or the chicken but about what's the foundation? Once again the key figure is 90 M. That's the required visual segment throughout the takeoff roll proven to provide adequate directional control with external reference. All airbus models geometrically are above that value when it comes to the visual segment outta cockpit. The regulator has decided that 125 is lowest figure without PVI or HUD. Consequence is that RVR 125 merely satisfies 90 m visual segment requirement. Now, I need 90 M VS throughout the takeoff roll to control the aircraft. How can I be sure to achieve that? Can I line up and taxi down the runway to see if I can see 6 CL constantly? Well, I could but what are the odds it remains the same by the time I come back to takeoff position and how much delay will it cause? Is it gonna remain the same? All very much depends on local weather phenomena, doesn't it? Therefore I need a machine or human observer reporting RVR along the TODA. You, sitting in the cockpit or the human observer or transmissiometer at the takeoff position are looking at the same portion of the runway, no difference there. Moreover 125 RVR measurement in TDZ on 4000 M RWY does not guaranty the achievement of such over the whole TDZ. It's a reasonable assurance of what you might expect. Is 90 M VS the same as 125 RVR in geometrical terms, NO. Can I replace RVR measurement at the takeoff position with 90 M VS, YES coz that's all I need to perform LVTO with external references. 90 m VS and RVR 125 M aren't the same in geometrical terms but both have the same function namely safe takeoff. FAA approach is clearly allowing that. It's safe and sound.
