Originally Posted by
safetypee
Airman, "
It still leaves doubts in mind as it leaves one in the position where something can be legal, but doubtful as regards safety. Namely, a situation where the touch down zone RVR is ok but the mid point is below 125 in cases of a manual roll out or less than 75, leaving one wondering whether taxiing would be safe". A good point
From research and experience in the late 70s, the variation of RVR in stable, thick fog (Cat III) was rarely less than one measurable division i.e. 175 and 75 very unlikely. [n.b. in transition fogs, forming, and particularly dispersing conditions, > 250 m, Cat II, the range of reported values can be large and change quickly. ]
It is a good point, however, there are two sides to the story here.
The approach itself is executed by an automated system that doesn't need external visibility. For all that matters, it can handle the approach in 0/0/0 conditions (with rollout guidance). The "non-normal" is a failure case of either the guidance system on the ground, or the system on the aircraft, where the pilot is to interrupt the approach and "get-out". The visibility requirement is there because of the human factor, who needs to provide some kind of confirmation. The 75m requirement to me has always been one of "the section of the runway where - at the expected speed - should be able to stop manually and evacuate safely".
The taxi "RVR" is a different ballgame. The decision making process is different, as nothing stops you from stopping, setting the parking brake, and requesting guidance when the provided guidance is not sufficient to you. We are talking visibility here, not RVR. Because yes, as you mentioned, ground traffic has limitations as well. I recall a landing down to the minimums with a dense bank of fog over the apron. It requires you to stop, request assistance and even request if the guy on the ground in his car needs you to put on the lights so he can see you and not drive into your engine or gear... and the safety report will be your a answer to management and authorities, which in turn might change things around the airport. But the main ”get-out" here for you as a PIC is to stop and set the parking brake. Even on the runway if you feel you need to.