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 is criticized for its conservative and seemingly illogical restrictions on using GPS during localizer (LOC) approaches.
Key prohibitions include forbidding RNAV substitution on the final approach segment and mandating "raw LOC data" for lateral navigation on LOC-based courses, even for initial interception.
While compliance is straightforward for older aircraft with separate analog instruments, these rules create significant practical challenges for modern Electronic Flight Information Systems (EFIS) which cannot easily display raw localizer data alongside GPS.
The article argues these regulations are outdated, questionably enhance safety, and force pilots into awkward workarounds despite GPS being a more precise and primary navigation method for most aircraft.

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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.”