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:

FAA Advisory Circular 90-108 mandates specific, restrictive rules for using RNAV/GPS on localizer-based approaches, requiring a switch to traditional NAVAIDs on the final approach segment and referencing raw localizer data for lateral navigation on LOC courses.
These regulations create significant compliance difficulties, particularly for aircraft equipped with modern Electronic Flight Information Systems (EFIS) which often cannot simultaneously display raw localizer data alongside GPS navigation.
The article questions the logic and safety benefits of these conservative rules, arguing they are counter-intuitive and outdated given the high precision and widespread adoption of GPS in modern aviation.

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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.”