IAN W:
In order to have what the FAA these days calls an Approach with Vertical Guidance (APV), which could be either an ILS, LNAV/VNAV, LPV, GLS, or RNP AR, requires that an area between DA and the runway threshold be fairly flat. This is the Glideslope Qualification Surface (GQS). Because of the terrain in this area for KBHM Runway 18, an APV is not possible. If it was, the RNAV approach to Runway 18 at KBHM would have both an LPV and LNAV/VNAV line of minima.
As to RNP AR and its values as low as RNP AR 0.10, the accuracy and integrity of such an approach is not as good as an unrestricted ILS within the last approximate one and one-half miles of the runway. The advantage of RNP AR approaches are the use of RF legs to avoid terrain. If there is no significant terrain above several hundred feet, RNP AR offers no advantage whatsoever and requires an inordinate amount of aircraft avionics, particularly to values of less then RNP 0.30 or RNP AR missed approaches with RNP of less than 1.0
KBHM has two RNP AR approaches, one to Runway 6, and one to Runway 24.
These RNP AR IAPs have DAs and visibilities considerably higher than the Runway 6 CAT I and II IAPs, and the Runway 24 LPV IAP.
Outside of the airline world RNP AR is essentially a myth. OTOH, where runways qualify LPV is ubiquitous, but even LPV (thus far) cannot compare to a CAT II and especially a CAT III ILS.
Finally, thus far there are no RNP AR approaches that use LPV for the final segment. RNP AR was a concept to use air carrier avionics in use at the time; i.e., IRUs and Baro VNAV.
Last edited by aterpster; 17th Aug 2013 at 13:32.