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Is it required to have both RVR and visibility to be more than minimum for ILS CAT 1 or one of them is acceptable. If both required which one has priority.

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If RVR (which is both runway-specific and more precise) is available, then it is controlling. If not then visibility (single value for the airfield) is controlling.

For example, typical Cat I minimums might be 1800 RVR or 1/2 Statute Mile (when the runway has an ALSF-II approach lighting system). If the RVR is near 1800 for one runway, the visibility may be low enough at the sensor for the airport that the prevailing visibility is reported as 1/4 SM -- too low to allow you to fly the approach (Part 121 rules, at least). But with the RVR, since it's measured at your runway (and often, right next to the touchdown zone of your runway), you're legal to fly the approach, since that much visibility "right here" means you have an excellent chance of seeing the approach lights at your DA.

Also, for all ILS approaches below Cat I mins (Special Cat I, Cat II, Cat III), only RVR matters. The difference between each of those mins (1600 for Special Cat I, 1200 for Cat II, 600 for Cat III) are too fine to be captured by a "prevailing visibility" sensor.

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