When we think of GPS-based instrument approaches, we usually lump all the approach minima into a single “GPS approach” bucket. We tend to mentally order the types with WAAS as better than non-WAAS, and vertical guidance better than those without. After all, that’s how the magic box picks the type of service to provide.
Obstacle Clearance
Key Takeaways:
- GPS approach minima don't always follow an intuitive hierarchy; sometimes, non-precision LNAV minimums are lower than vertically guided LNAV/VNAV or LPV approaches.
- These discrepancies arise from distinct obstacle evaluation standards (TERPS) and the unique Obstacle Clearance Surfaces (OCS) designed for each approach type, with LNAV/VNAV's complex OCS (derived from Baro-VNAV limitations) sometimes being penetrated by obstacles that clear a flat LNAV OCS.
- GPS navigators prioritize vertically guided approaches (like LNAV/VNAV) over LNAV even if LNAV offers lower minimums, as they do not incorporate minima data into their selection logic.
- Pilots are permitted by AIM guidance to disregard higher LNAV/VNAV minimums and instead fly to a lower LNAV MDA, treating the glidepath as advisory, when specific conditions warrant it.
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