FMS Approaches

The two most frequently cited sources are AC 90-108, Use of Suitable Area Navigation (RNAV) Systems on Conventional Routes and Procedures, and the AIM, in Paragraph 1-2-3. As the current AIM language is basically a repeat of portions of AC 90-108, let’s focus on the AC.

As it turns out, AC 90-108, in Paragraph 8 b, specifically prohibits the use of the FMS/RNAV system as a substitute for the lateral guidance provided by the ground based nav-aid during the final approach segment. It seems pretty clear to this reader, and to others with knowledge of the subject with whom we have spoken.

So what’s the problem? If there is a problem, it may be that very few of us are actually observing that prohibition. Let’s face it, which procedure is easier to use, more accurate, and arguably safer—flying a raw data VOR approach to a manually calculated VDP or flying it as an FMS approach with VNAV to the MDA? The answer to that is a no-brainer.

While the AC prohibition was established with TERPS-related issues in mind, it seems today a lonely beacon from the past in a world moving rapidly towards all things PBN. So here we have one of those interesting dichotomies between the regs and what’s happening in the real world of operations, similar, let’s say, to the regulatory requirement for a pilot to wear a truly painful and unsuitable piece of emergency equipment (oxygen mask) during normal operations (above specified altitudes). In both these cases, some would observe that it ain’t happenin’.

Meanwhile, the FAA has been alerted to the FMS approach issue and is looking at it. The easy fix will be to include the prohibition from AC 90-108 in the AIM. Hhmm. Stay tuned.