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 Advisory Circular 90-108 imposes what the author considers overly conservative restrictions on GPS use, such as mandating a switch to less precise conventional NAVAIDs on final approach segments, despite GPS's superior accuracy.
The AC's requirement for "raw LOC data" when navigating on localizer-based courses is problematic for modern Electronic Flight Information Systems (EFIS), which struggle to display this raw data alongside GPS navigation.
These regulations create practical difficulties for pilots using advanced avionics, often forcing awkward workarounds to comply and highlighting a disconnect between FAA rules and current aviation technology.

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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.”