RNAV Approaches
Area Navigation — uses GPS (and sometimes inertial nav) to fly approaches without ground-based navaids. Modern airports often have RNAV approaches in addition to or in place of ILS. Five common minimums lines on RNAV charts: LNAV, LNAV+V, LNAV/VNAV, LP, and LPV — each with different equipment requirements and minimum altitudes.
RNP — the underlying performance standard
Required Navigation Performance defines the accuracy required for a procedure or airspace. Expressed as a number (the lateral accuracy in NM) and a confidence level (95%).
- RNP 0.3 means navigation must be accurate to within 0.3 NM 95% of the time.
- Common RNP values: 1.0 (en route), 0.3 (most approaches), 0.1 (high-precision).
- Higher RNP numbers = looser accuracy requirement = easier to comply with but lower minima allowed.
WAAS-equipped GPS receivers automatically meet RNP 0.3 requirements. Older non-WAAS GPS receivers may not, requiring RAIM checks before flight.
The five minimums lines
- LNAV — Lateral navigation only (non-precision). Flies to a Minimum Descent Altitude (MDA). Available on basic GPS approaches without WAAS.
- LNAV/VNAV — Lateral + vertical navigation. Uses barometric VNAV — flies to a Decision Altitude (DA). Approach-like, not quite precision. Requires baro-VNAV-capable equipment (typically WAAS GPS or FMS).
- LNAV+V — LNAV with advisory vertical guidance. Still flown to MDA — the +V is a courtesy vertical path, not authorization to descend on it. Many WAAS GPS receivers display +V on basic LNAV approaches.
- LPV — Localizer Performance with Vertical guidance. WAAS-required. Effectively a precision approach with DA as low as 200 ft HAT (height above touchdown) — comparable to ILS minimums.
- LP — Localizer Performance. WAAS lateral-only, no vertical guidance. Used where terrain prevents publishing LPV vertical minimums but lateral accuracy is improved over LNAV.
WAAS vs non-WAAS
- WAAS-equipped aircraft can fly any RNAV minimums line. LPV in particular requires WAAS.
- Non-WAAS (basic IFR GPS) is limited to LNAV minimums, and only after a RAIM check confirms signal integrity.
- Most modern panel-mount IFR GPS receivers (Garmin GTN 750, GNS 530W, etc.) are WAAS-equipped.
- WAAS service may not be available everywhere — outages or equipment limitations sometimes force LNAV-only operations.
The +V trap
"LNAV+V" displays an advisory glide path on a normal LNAV approach. Many pilots descend on the advisory path until reaching minimums, treating it like a precision approach. Two reasons that's wrong:
- The +V advisory path is computed by the GPS, not the FAA. It may not satisfy obstacle clearance assumptions of the LNAV procedure.
- You're still on an LNAV approach with an MDA, not a precision approach with a DA. You can't legally descend below MDA based on advisory guidance — you need visual references per 91.175.
Use +V as a glide-path aid for stabilization, not as authorization to descend.
Step-down altitudes
Vertical guidance from the VNAV portion of RNAV is not part of a non-precision approach. Published step-down altitudes must still be observed. The vertical advisory or VNAV path is an aid, not a replacement for charted minimums.
Always cross-check the GPS's vertical guidance against the published step-down altitudes. If they disagree, the chart wins.