Hi found some more info on that one
Correlation between visual segment: SVR ; RVR
A formula can be used to compute the SVR required for the pilot to acquire visually the specified visual segment, at the DH for a particular operation.
v : visual segment (m)
h : pilot's eye height above ground level (m)
w: Cockpit cut-off angle (°) = Down vision angle (°) - pitch angle (°)
SVR = square root of ( v+(h * cotan w) )² + h² (purely geometrical relation)
Example of establishment of required RVR
For CAT II operations with auto pilot down to DH=100ft and manual control below,
the required visual segment is 90m. The required SVR for such operation is 220.7m
(using the example as shown in Figure 2.4).
Eye height = DH + 20ft =120ft = 36.6m
Cut-off angle = 20°
Pitch = 4°,
w = 16°,
Visual segment = 90m
Using above formula we find SVR =220.7 m
At 120ft above ground level, the SVR/RVR is expected to be 0.68 or more, so we obtain a required RVR equal 324.6m. RVR = SVR x 1/0.68 = 324.6 m (for SVR/RVR=0.68) The same method may also be used to evaluate the visual segment for a given RVR
Interesting observation is that the pilot sees the same segment of 90 m being at 100 ft as at the ground for LVTO. If one remembers, 90 m visual segment is required upon line up additionally to all the relevant RVR readings.
Cheers.
