+V Advisory Glidepath TERPS Clearance Violations

WAAS-capable GPS receivers compute an advisory glideslope (shown as “+V” on the annunciator) for LNAV-only RNAV (GPS) approaches. This glidepath is not part of the FAA’s published procedure—it is a synthetic straight line drawn by the receiver from the Final Approach Fix (FAF) altitude to the runway threshold crossing height (TCH), ignoring any stepdown fixes. Because the +V path is not obstacle-checked by FAA TERPS designers, it can descend through terrain that the published stepdown altitudes are designed to clear.

This study identifies RNAV (GPS) approaches in the FAA’s Coded Instrument Flight Procedures (CIFP) where the +V advisory glidepath violates the TERPS 250′ minimum terrain clearance in the final approach segment. For each LNAV-only approach (those without a published Vertical Descent Angle), terrain is sampled every 30 m along the FAF-to-threshold ground track and the +V glidepath altitude is compared to the terrain at each point. Sampling stops 0.8 NM from the threshold—on flat terrain with a 3° glidepath, the +V altitude at that distance is already approximately 250′ AGL, so closer points cannot produce meaningful violations.

The +V Clearance column shows the minimum clearance between the +V glidepath and terrain along the final approach segment. The Procedure Clearance column shows the terrain clearance at that same point if the pilot instead follows the published LNAV stepdown altitudes (and MDA where applicable)—demonstrating the margin of safety the published procedure provides that the +V path does not.

Each approach has a Profile chart showing the approach in cross-section. The red line is the +V advisory glidepath from FAF to threshold. Blue horizontal lines show the published LNAV stepdown altitudes and MDA. Brown shading is terrain along the approach course; red shading highlights where the +V glidepath is less than 250′ above terrain. The purple vertical line labeled Proc Clearance is drawn at the point of minimum +V clearance and shows the terrain clearance a pilot would have at that same point if following the published LNAV procedure instead of the +V glidepath.

Terrain elevations come from the Shuttle Radar Topography Mission (SRTM), a near-global elevation dataset collected by NASA in 2000. This study uses SRTM1 (1 arc-second, ~30 m resolution) with bilinear interpolation between grid points. SRTM represents bare-earth radar reflectance and may understate true obstacle height where trees, towers, or buildings are present; conversely, it may overstate ground level in areas of dense canopy. Approach geometry—fix coordinates, altitudes, and threshold crossing heights—is extracted from the FAA’s Coded Instrument Flight Procedures (CIFP) (ARINC 424 format).