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 a logical hierarchy (e.g., LNAV/VNAV may have higher minimums than LNAV, or non-WAAS LNAV/VNAV higher than WAAS LPV) because each type is evaluated independently.
- These anomalies arise from differences in Obstacle Clearance Surface (OCS) designs for various approach types; for instance, LNAV/VNAV's complex, sloping OCS (developed from Baro-VNAV limitations) can be penetrated by obstacles that do not affect the flat OCS of an LNAV approach.
- While GPS navigators automatically select vertically guided approaches, pilots are permitted by AIM 5-4-5 (i)(5) to disregard a higher LNAV/VNAV DA and fly to a lower LNAV MDA, treating the glidepath as advisory, when such situations occur.
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