MANTHRUST, “Why ban approaches in such a scenario just because the auto RVR meter indicates a low value!”
”Fog is often patchy and the best person to decide if a landing can be safely achieved is the Captain at decision height."
The reasons against change are because the scenarios are unpredictable, often with misleading visual cues. Furthermore human judgement is often flawed or the perceptual system confused. The RVR system was developed to overcome disparities in visibility measurement; I flew on many of the flight trials in various fog conditions. By far the most challenging were the periods of fog formation or dispersal, most often associated with Cat 1 becoming Cat 2, or Cat 3 improving to Cat 2.
When ‘shallow’ fog forms there are successive periods of layering and subduction; each layer associated with different visual ranges, vertical extent, water content, etc. The conventional view of shallow fog is a single layer, thus over flying at height you can see down through it to the runway, but during an approach the visibility reduces as the aircraft enters the layer. This effect alone has cause many accidents.
In reality a shallow fog layer may have many layers and the visual effects are similar to entering and exiting many stratus layers, which alternately reduce and brighten the approach lights and have similar effect on the slant visual range. Thus, the far point of visibility can oscillate towards or away from you; it is very disconcerting.
When fog begins to clear, often from a stable homogeneous deep layer (that is why Cat 3 conditions are more predictable), the clearance pattern is similar to the formation of small cumulus clouds. Even where the sun burns through the fog some areas remain thick and dense with greatly reduced visibilities. Flying through these ‘cumulus’ causes rapid changes in visual range, approach light intensity, and ambient light level.
Don’t mess with the unstable periods of fog formation or dispersal. Cat 2 (RVR) is one operation where you can make a valid decision to land and then find that you are wrong before you get to the threshold.
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