C100Driver:
Tom, look up Baro VNAV and you will find the biggest limitation to current RNP operations.
The real advantage of RNP AR is the lateral accuracy and RF legs to get access with reasonable minimums at airports with significant terrain issues.
Baro VNAV became the vertical component because of politics; i.e. the airlines all had Baro VNAV and none of them had SBAS as a sensor.
Until a better altitude measuring device is used RNP operations will not approach the accuracy and reliability of an ILS. WAAS and LAAS can increase the accuracy but it is the vertical that limits it.
To the contrary, WAAS (SBAS) provides a far better vertical solution than Baro VNAV. In the US we now have LPV approaches to 200 1/2. The future will see RNP AR that switches from linear RNP and Baro VNAV to LPV at the FROP (final roll-out point). The result will be greatly improved vertical and lateral accuracy to values that cannot be achieved with linear RNP. With LAAS there is no reason that an auto-land couldn't be a part of the future RNP AR IAP. The limiting factor today is airline economics rather than technology.
Also human failure to report the correct QNH will affect the Baro VNAV solution. Human failure in setting, reading and transmittim QNH is about 10 -3 reliability verse FMC reliability at 10 -7.
At terrain rich locations the lateral solution is far less forgiving of screw ups than the vertical component. Altimeter missettings started with the Wright Brothers.
Make no mistake I am a big fan of RNP AR operations and fly them almost every day of my working life in some very interesting places, but it still does not replace an ILS yet as a better approach. It is close but for most operations it is not quite there.
Until LAAS becomes part of the RNP AR solution, it will be "apples" and ILS will be "oranges."
What concerns me is that the "greenies" have kidnapped the U.S. RNP AR program. The concept a few years ago was "RNAV everywhere and RNP AR only where needed." Now, it has become a vehicle where very complex equipment is required to save a few pounds of jet fuel on each approach. Unless abated this will place the non-RNP AR operators (the majority of business aviation) at an inappropriate and unfair disadvantage. And, at airports without terrain or serious obstacle issues the use of VNAV and RF legs on a conventional RNAV platform will provide the "green solution" without requiring all the conditions and restriction imposed by RNP AR. But, the FAA thus far has been resistive to what I call RNP AR overlays with conventional state-of-the-art RNAV. For example, they won't permit non AR state-of-the art RNAV to perform RF legs in the final approach segment or within two miles of the FAF. That, plain and simple, is regressive policy.
RNP AR is great at a places like Rifle, Colorado, Glacer Park, Montana Runway 20, or Monterey, California Runway 28L. But, it is like shooting flies with an elephant gun at places like Baltimore, Maryland, and such.