Baro Cat II evolution to RA
@aterpster - Sounds like you may well have known my old pals W.R. and Larry DeC ! If so, you're well aware of the reasons for the eventual transition to RA (even at TWA), and the fact that the RA transition had nothing whatsoever to do with any Baro shortfall. In fact the transition was driven largely by needs of AFDS, the PB-20 (and PALS) on the B707, and the soon to come GPWS (bless Don B's heart) [remember TWA 514?]. We both loved the L1011 and its pioneering Cat III approvable autoland, but it is too bad it never fully incorporated either the MONA AMBAC, or AINS-70, or we otherwise could have probably started with [real] RNP two decades sooner. As to LPV, it is nearly useless and obsolete, and contributes nothing now, or will ever, to [real] RNP, being both unnecessarily angular and airspace wasting straight-in, and no better (and probably a lot worse) than [real] RNP [filtered inertial] core performance in big air transport jets. [Real] RNP is already vastly better than any LPV or APV will ever be for a broad range of reasons, and as a result it is much safer. Just for a few examples, it addresses all relevant risk factors, isn't exclusively based on (flawed Gaussian) extrapolations, and it fully protects the core down to the TDZ and back out. With both the present scientific VNAV VEB and RA floor available, it is the best offering out there, for any ops which do not have an ILS or GLS reference. Down the road, especially when we add another 30 to 60 SVs (e.g., Galileo et al), and with networks of GLS/GBAS on the way, it gets even better. That's why we already (for a decade now) have provision for RNP .003 in AC120-28D (see Table 4.5.1-1 Page 17). Further, RNP ALWAYS addresses both rare-normal and suitable non-normal conditions as well as normal performance (TERPS and PANS-Ops don't, can't, and shouldn't). That is just one more reason why hardly any real RNP airlines even use FAA's pseudo RNP (lite) at this point.