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 overly conservative rules restricting the use of precise GPS systems, especially for substituting on final approach segments and for lateral navigation on Localizer-based courses without reference to raw LOC data.
Compliance with the requirement to display "raw LOC data" while using RNAV is challenging, particularly for modern Electronic Flight Information Systems (EFIS) which often cannot simultaneously present both raw localizer and GPS data on the primary display.
Pilots are forced to implement awkward workarounds, such as utilizing separate conventional instruments or creatively interpreting the regulations, leading to frustration over the perceived bureaucratic hindrance to efficient and safer navigation with modern GPS 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.”