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 do not always follow a "better is lower" hierarchy; LNAV (non-precision) can sometimes have lower minimums than vertically guided LNAV/VNAV approaches, and non-WAAS LNAV/VNAV can be lower than WAAS LPV.
- These discrepancies arise because each GPS minima type is evaluated independently using different obstacle clearance standards (TERPS), resulting in distinct Obstacle Clearance Surfaces (OCS).
- The LNAV/VNAV's complex, partially sloping OCS, often influenced by its Baro-VNAV origins and associated temperature restrictions, can be penetrated by obstacles close to the runway that might not affect the flat LNAV OCS, leading to higher LNAV/VNAV minimums.
- While navigators automatically select vertically guided approaches, pilots can legally choose to fly to lower LNAV minimums, treating the glidepath as advisory, when such situations occur.
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