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:
- The common assumption that vertically-guided GPS approaches (like LNAV/VNAV or LPV) always offer lower minima than non-precision LNAV is often incorrect due to independent obstacle evaluation for each approach type.
- Differences in obstacle clearance surface (OCS) design, particularly the sloping LNAV/VNAV OCS originating from Baro-VNAV and its allowances for temperature errors, can result in LNAV/VNAV minimums being higher than LNAV, especially with obstacles close to the runway.
- Pilots can override the GPS navigator's default selection of a vertically-guided approach and opt to fly to lower LNAV minimums by treating the glidepath as advisory, as authorized by FAA guidelines for these unique situations.
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