WAAS Alternate

This forum thread captures a persistent point of confusion among instrument-rated pilots regarding the interaction between WAAS (Wide Area Augmentation System) equipage and IFR alternate airport planning under AIM and TERPS criteria. The specific question centers on how the FAA's guidance for selecting an alternate airport differs when the pilot is using a WAAS-capable GPS versus conventional avionics, particularly around the nuance of LNAV or circling-only minimums at the alternate. The poster correctly identifies the well-known rule that non-WAAS aircraft need at least one airport in the flight plan (departure, destination, or alternate) served by a non-GPS-based approach, guarding against total GPS/WAAS system failure. Where the confusion arises is in a related but distinct provision: for WAAS-equipped aircraft, when using a GPS approach at the alternate for planning purposes, pilots must plan to the LNAV or circling minimums (not LPV or LNAV/VNAV), because WAAS vertical guidance is not guaranteed to be available everywhere if the WAAS system suffers a degraded state. This is a separate consideration from the standard alternate minimums calculation (the "800-2" or "600-2" rule for non-precision vs. precision approaches, or the airport-specific alternate minima published on the approach plate).

This matters operationally because these two rules serve entirely different risk-mitigation purposes and get layered together in practice, which is exactly why pilots get tripped up. The 800-2/600-2 standard alternate minima (or NA/nonstandard minima published on the plate) governs whether weather forecasts support filing that airport as an alternate at all — it's a go/no-go filter based on ceiling and visibility forecasts. The LNAV/circling-minimums-only rule for WAAS receivers at the alternate is a contingency planning rule: it assumes that if the aircraft ever needs to fly the missed approach and divert to the alternate, WAAS augmentation could be unavailable (due to a satellite fault, ionospheric disturbance, or FAA-published WAAS NOTAM), so the aircraft must be able to still get in using only baro-VNAV-free, GPS-only LNAV or circling minimums — which are typically higher and less precise than LPV or LNAV/VNAV lines. Pilots must therefore evaluate the forecast weather at the alternate against the LNAV or circling minimums, not the more favorable LPV minimums, when determining if the alternate qualifies. Missing this distinction can lead a pilot to legally file an alternate that looks fine using LPV minimums but would not actually be legal or safely flyable to under the required contingency planning if WAAS augmentation were lost.

For working pilots — whether flying single-pilot IFR in a piston or turboprop, or crewing a business jet or Part 135/121 operation — this distinction is a recurring gotcha in checkride prep, IPCs, and real-world dispatch planning. Many training materials and even some flight-planning software abbreviate or oversimplify the AIM 1-1-19 guidance, and pilots moving between different avionics suites (WAAS vs. non-WAAS GPS, or aircraft with FMS-based RNP vs. simple TSO-146 WAAS units) must actively verify which rule applies to their specific equipment and route. Dispatchers and part 135/121 operations control centers dealing with fleet mixes must also track which tail numbers are WAAS vs. non-WAAS to correctly apply alternate minimums during flight release, since an aircraft's specific avionics installation — not just "does it have GPS" — determines which minima apply.

More broadly, this thread reflects the ongoing challenge of keeping pilots current on the regulatory nuances introduced as GPS and satellite-based augmentation systems have become the primary means of instrument navigation rather than the backup. As the National Airspace System continues its shift toward PBN (Performance-Based Navigation) and away from legacy ground-based navaids — with VOR decommissioning accelerating and ILS critical-category installations shrinking — pilots will increasingly rely on GPS/WAAS for both primary and alternate planning. This makes a solid working knowledge of AIM 1-1-19, TERPS alternate criteria, and the fine print differences between WAAS and non-WAAS operations not just an academic exercise but a genuine safety-of-flight issue, especially for single-pilot operators without a dispatcher double-checking their alternate selection logic.