I am intrigued, increasingly concerned, as to why many views across Pprune start from a legal position.
Flying is about safety, thus the primary position is to consider if the proposed action is safe, then compare with the requirements.
In the very rare event the choice of violating the rules, then the safety view is a basis for legal defence, if there is an unwanted outcome.
Safety first - providing we understand what safety means.
A worthy debate, but elsewhere.
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. ]
Re taxiing; 50 m (now 75) was taken as a limiting factor for fire rescue services.
Experimrnts showed that day taxing in 60 m was feasible with green centreline lights, but at night 150 m reported was increasingly difficult. The point being that these values of RVR cannot be equated: in the above examples the met vis 'what it is really like' was very low and sufficiently low to shut down road transport; taxi, bus, fire rescue, refuelling, etc.
Question; does the basis of EASA Cat III fail-passive still allow manual landing after system failure below DH ?
My experience has been with the certification of the first 'super fail-passive' Cat III operation (50ft DH - AVRO RJ), which depended on extended proof of concept, technical evaluation - flight and technical simulation, and human factors of cockpit view and height above the runway.
Assuming that recent advances in technology would reduce the frequency of system failure, changing flight deck view and height (the human aspects) to include larger aircraft might not reduce the overall risk.
But then, certification risk assessment is a black art, i.e. because something is less likely to happen - failure below DH, then the human issues should not be used to offset the overall risk (discuss the different viewponts with Boeing, FAA, (737), and EASA; the concerns about experience, training, #4, might indicate that EASA's judgement is ill founded).
A previously judged very low risk, but now in a very safe industry, still deserves thought; at least to provide some clarity of what was considered and the basis of the certification judgement.
The majority of operational situations have some component of risk, the uncertainty, and not having experience and confidence.
Based on the initial operations with the AVRO RJ, some landings violated the rules - land after failure below DH in restricted RVR. From the investigations these incidents identified a minor hardware fault - AP disconnect button set too high, thus 'white knuckling' pilots were more likely to inadvertently disengage the AP, and that their judgement to land was not a safety issue - 'it was obviously safe to land'.
Some of the previous research did indicate that humans may more capable than imagined by certification and testing, but not always; we do not know how this relates to the situations which pilots misinterpret
Safety, risk, is the amount of uncertainty to be managed.
Ask the question, but don't loose any sleep over the probability.