Originally Posted by
KC10 FATboy
I have a question you might be able to answer.
AC 90-108 Use of Suitable Area Navigation (RNAV) Systems on Conventional Routes and Procedures discusses when you can and can't use RNAV systems to substitute for an inoperative NAVAID.
For example, you can't substitute an RNAV system for a out-of-service NAVAID when the navigation or procedure is NA (not authorized), when substituting for a final approach course, or substituting for a LOC or BCLOC final approach course.
With that being said, with the invention of RNAV RNP procedures, why would you still need to have raw data on a final approach course? The tolerances for an RNP approaches are tighter and safer than raw data VOR or NDB. Most operators today fly VOR and NDB approaches in LNAV/VNAV or similar fashion. If combined with RNP procedures, who cares if the NAVAID was working or not? Maybe not the right question. The FAA apparently cares. How about, why do they care? What am I missing?
I can only tell you why initially the FAA would not let you fly the APCH if the underlying navaid was ots. It was because when FMS, LORAN and GPS was first started all the existing non-rnav approaches were coded by the database providers i.e. Jeppesen, Swissair etc and dumped into the ARINC 424 data. These approaches were referred to as overlay approaches and were never flight checked by FMS or GPS aircraft in NAV to verify the database coding was correct. Back then flight check aircraft flew the approaches using the underlying navaid as guidance not as a NAV approach, not sure how they do it now. Because of this the FAA mandated you must be able to verify the course with RAW data in case the coding was wrong. When GPS or RMAV procedures are created they are tested using a FMS or GPS input and flown in NAV to verify the database coding is correct. I was fortunate to be a part of the ARINC 424 committee back in the early days when the industry was trying to figure out how to get where we are today.