AC 90-108 Conundrum

The FAAs Advisory Circular 90-108 just feels wrong. In it, section 8 itemizes circumstances where a suitable RNAV system (including GPS and GPS/WAAS equipment) cannot be used.

Fred Simonds

Monday, December 15, 2014 at 12:00 AM ET

Verified

Key Takeaways:

The FAA's AC 90-108 imposes seemingly illogical and overly conservative restrictions on using GPS (RNAV) for navigation, specifically prohibiting its substitution on the final approach segment and requiring "raw LOC data" reference for lateral navigation on localizer-based courses.
These conservative rules pose significant compliance difficulties for modern Electronic Flight Information Systems (EFIS), which struggle to display raw localizer data concurrently with GPS as the primary navigation source.
The article criticizes these regulations as illogical and overly bureaucratic, arguing they unnecessarily complicate pilot procedures for GPS-equipped aircraft without enhancing safety, given GPS's superior precision.

See a mistake? Contact us.

The FAA’s Advisory Circular 90-108 just feels wrong. In it, section 8 itemizes circumstances where a “suitable RNAV system” (including GPS and GPS/WAAS equipment) cannot be used.

One prohibition is substitution on a final approach segment, FAS. We know this rule well and compliance is easy. Before reaching the FAS, most often denoted by an FAF, change the CDI source button from GPS to, as the AC says, “the NAVAID (for example, a VOR or NDB) providing lateral guidance for the final approach segment.”