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 restricts RNAV (GPS) use for conventional approaches, specifically prohibiting its substitution on the final approach segment and lateral navigation on LOC-based courses without reference to "raw data."
While compliance is possible for older aircraft with separate instruments, it is often awkward and inefficient, forcing pilots to switch from precise GPS to less accurate NAVAIDs.
Modern Electronic Flight Information Systems (EFIS) struggle to meet the "raw data" requirement, as they typically cannot simultaneously display primary GPS navigation and raw localizer information on the HSI.
The article questions the logic and safety benefits of these conservative rules, highlighting the inefficiency and difficulty they create for pilots, especially given the widespread adoption and accuracy of GPS.

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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.”