Is it as easy as the differences in the Approach Lighting System which prescribes RVR for NPA vs. VIS?
Close, one cannot descend below the MDH unless elements of the approach lights, threshold or touchdown zone, markings or lights, or the visual glide slope. Runways that have precision approaches normally always have approach lights that extend from the runway out up to around 900m.
ICAO charts do not actually publish the visibility required on their charts, they display the OCA/OCH. The chart suppliers make it easy for pilots and convert this OCA/OCH into a useful MDA/DH/RVR/Visibility. I will run an example of how they do this.
To calculate visibility EU regulations AMC5 CAT.OP.MPA.110 Aerodrome operating minima DETERMINATION OF RVR/CMV/VIS MINIMA FOR NPA, APV, CAT I — AEROPLANES
(https://www.easa.europa.eu/system/files ... 202017.pdf
) provides the following formula
Required RVR/visibility (m) = [(DH/MDH (ft) × 0,3048)/tanα] – length of approach lights (m)
For this example I will use the RNAV 04 at EGSS (London Stansted)
ICAO charthttp://www.ead.eurocontrol.int/eadbasic ... -09-15.pdf
London Stansted Aerodrome - Textual Datahttp://www.ead.eurocontrol.int/eadbasic ... -01-04.pdf
EGSS AD 2.14 APPROACH AND RUNWAY LIGHTING
RW04 780m of approach lights
From the chart we can gather the following information, descent path is 3 degrees (5.24%), the OCA is 650', OCH 318', therefore threshold is 332' (same as Jeppesen chart).
If the OCH is 318' (96.93m), and the approach is 3 degrees/5.24%, the triangle for this to hit the end of the runway is 96.93/(5.24/100)=1849m, so if we were to touchdown on the end of the runway we would need to have a visibility of 1849m.
However we do not land at the end of the runway, we cross the end of the runway at the TCH, which for this runway is 53' touchdown point is 1011'/308.29m from the threshold. That means at the OCH we are only 1849-308m from the runway threshold which is 1541m.
The approach lights extend 780 m from the threshold 04, 1541-780=761m, which is rounded down to the Jeppesen RVR of 750m.
Now if we look at table 5 on page 67 of the EU regulation https://www.easa.europa.eu/system/files ... 202017.pdf
For a DH of 318, with the FALS out the NALS RVR is 1400m (1541-210 (maximum)=1331), Jeppesen RVR ALS out 1400m.