13.4 APV Approaches: LPV, LNAV/VNAV & Baro-VNAV

APV Approaches: LPV, LNAV/VNAV & Baro-VNAV

Quick Answer: Approaches with Vertical Guidance (APV) provide lateral and vertical electronic guidance flown to a Decision Altitude (DA), but are classified under ICAO/FAA standards as APV rather than full precision approaches. LPV (Localizer Performance with Vertical Guidance) uses WAAS for angular lateral scaling (narrowing to $\pm 40\text{ m}$ at threshold) and geometric vertical guidance, delivering minimums as low as 200 ft HAT and 1/2 SM. LNAV/VNAV provides linear lateral guidance ($\pm 0.3\text{ NM}$) and vertical guidance via WAAS or Baro-VNAV (which is subject to cold-temperature altitude errors). Advisory vertical guidance (+V on GPS displays) is strictly advisory—the approach remains an NPA and the MDA is the legal hard floor.

The widespread deployment of satellite navigation and the FAA's Wide Area Augmentation System (WAAS) has revolutionized instrument flight procedures. Today, thousands of airport runways across the National Airspace System that never had ground ILS transmitters feature instrument approaches with vertically guided, stabilized descent paths.

These procedures fall into the regulatory classification of Approaches with Vertical Guidance (APV). Understanding the operational differences between LPV, LNAV/VNAV, Baro-VNAV, and advisory vertical guidance (+V) is critical for flight safety and FAA exam mastery.

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Defining Approaches with Vertical Guidance (APV)

An Approach with Vertical Guidance (APV) is an instrument approach procedure based on a navigation system that provides both lateral course guidance and vertical glidepath guidance, but does not meet the stringent international ground-based monitoring and integrity requirements of ICAO Annex 10 precision approaches (such as Category I ILS, MLS, or GLS).

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|                        APPROACH CLASSIFICATION SPECTRUM                                 |
|                                                                                         |
|  PRECISION APPROACH (PA)    APPROACH WITH VERTICAL GUIDANCE (APV)  NON-PRECISION (NPA)  |
|  • Ground ILS / GLS / MLS   • LPV (WAAS)                           • LNAV (GPS)         |
|  • Lateral + Vertical       • LNAV/VNAV (WAAS / Baro-VNAV)         • LP (WAAS)          |
|  • Flown to DA              • LDA with Glide Slope                 • VOR / LOC / SDF    |
|  • Standard: 200' & 1/2 SM  • Flown to DA (Decision Altitude)      • Flown to MDA       |
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Key Characteristics of APV Procedures

1. Flown to a Decision Altitude (DA): Because certified vertical guidance is provided along a stabilized glidepath, APV approaches are flown to a Decision Altitude (DA) rather than an MDA. The pilot does not level off; an immediate missed approach is executed at DA if visual references are not acquired.
2. Stabilized Flight Path: Delivers a continuous $3.00^\circ$ descent path directly to the runway touchdown zone, drastically reducing CFIT risk.
3. Regulatory Classification: While flown identically to an ILS in the cockpit, the FAA legally classifies LPV and LNAV/VNAV as APV under 14 CFR Part 91/135 operational authorizations.

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LPV: Localizer Performance with Vertical Guidance

LPV represents the highest performance tier of satellite-based instrument approaches in the National Airspace System.

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|                           LPV LATERAL AND VERTICAL GEOMETRY                             |
|                                                                                         |
|                                          Angular Lateral Containment                    |
|   FAF (⚡)                                Narrowing to ±40 meters                        |
|   ───●───────────────────────────────────────────────┐ at Runway Threshold              |
|       \                                              ▼                                  |
|        \ Geometric WAAS Vertical Glidepath (3.00°)  ┌──────────────────┐                |
|         \                                           │ Runway Threshold │                |
|          \                                          └──────────────────┘                |
|           ▼ DA: Down to 200 ft HAT (1/2 SM Visibility)                                  |
+-----------------------------------------------------------------------------------------+

Technical Architecture of LPV

• Avionics Requirement: Requires an advanced WAAS / SBAS receiver certified under TSO-C145 (airborne navigation sensors) or TSO-C146 (stand-alone avionics, e.g., Garmin GTN 650/750, G1000 WAAS).
• Angular Lateral Guidance: Unlike standard GPS linear CDI deflection, LPV lateral guidance mimics an ILS localizer. Lateral full-scale sensitivity increases as the aircraft approaches the runway, narrowing to $\pm 40\text{ meters}$ ($\approx 131\text{ feet}$) at the runway threshold.
• Geometric Vertical Guidance: Vertical guidance is computed directly from WAAS satellite trilateration and geodetic ellipsoidal models (WGS-84), completely independent of barometric pressure or atmospheric temperature.
• Minimums: Where runway infrastructure permits (runway markings, lighting, obstacle clearance), LPV approaches provide minimums identical to a Category I ILS: 200 feet Height Above Touchdown (HAT) and 1/2 statute mile visibility (2,400 ft RVR).

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LNAV/VNAV: Lateral Navigation / Vertical Navigation

LNAV/VNAV approaches were the original satellite-based vertically guided approach procedures, designed prior to the full implementation of WAAS.

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|                             LNAV/VNAV vs. LPV COMPARISON                                |
|                                                                                         |
|  Feature              | LNAV/VNAV                         | LPV                         |
|  -------------------- | --------------------------------- | --------------------------- |
|  Lateral Guidance     | Linear: ±0.3 NM full-scale        | Angular: Narrows to ±40m    |
|  Vertical Source      | Certified Baro-VNAV or WAAS       | WAAS Geometric Only         |
|  Temperature Limits   | YES (when using Baro-VNAV)        | NO (WAAS is temperature-free)|
|  Typical Minimums     | 350 - 500 ft HAT, 3/4 to 1 SM     | 200 ft HAT, 1/2 SM          |
|  Minima Line          | DA                                | DA                          |
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Technical Architecture of LNAV/VNAV

• Linear Lateral Guidance: Full-scale CDI deflection remains linear at $\pm 0.3\text{ NM}$ from the Final Approach Fix down to the runway threshold. Because lateral tolerance does not narrow near the ground, the obstacle evaluation area is wider, resulting in higher DA minimums (typically 350 to 500 feet HAT) than LPV.
• Dual Vertical Guidance Sources: Vertical glidepath guidance may be provided by:
  1. WAAS GPS (TSO-C145/C146): Satellite geometric glidepath.
  2. Baro-VNAV (TSO-C129a / TSO-C115b): Flight Management Systems (FMS) computing a vertical profile from aircraft barometric air data computers.

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Baro-VNAV Principles and Temperature Limitations

Barometric Vertical Navigation (Baro-VNAV) systems generate vertical guidance by sampling ambient atmospheric pressure through the aircraft's pitot-static system and barometric altimeters.

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|                        BARO-VNAV TEMPERATURE PHENOMENON                                 |
|                                                                                         |
|  EXTREME COLD TEMPERATURES ("High to Low, Look Out Below")                               |
|                                                                                         |
|  True Flight Path in Cold Air (Dangerously Low!)                                        |
|  ═══════════════════════════════════════════════╗                                       |
|                                                 ▼                                       |
|  - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Intended / Indicated Path    |
|                                                                                         |
|  ▲ Obstacle Penetration Risk!                                                           |
|  ■■■■■■■ (Terrain / Towers)                                                             |
+-----------------------------------------------------------------------------------------+

Cold Temperature Physics & Altimeter Error

• Standard altimeters are calibrated to the ICAO Standard Atmosphere (ISA) (sea level pressure $29.92\text{ inHg}$, temperature $+15^\circ\text{C}$, lapse rate $2^\circ\text{C}$ per $1,000\text{ ft}$).
• In Cold Air: Air is denser and pressure levels compress closer to the surface. The barometric altimeter registers a higher altitude than the aircraft's actual True Altitude above the ground ("High to low, look out below").
• Impact on Uncompensated Baro-VNAV: When flying a Baro-VNAV glidepath in sub-standard cold temperatures, the avionics fly an actual vertical flight path that is substantially lower than charted, compromising obstacle clearance buffers along the final approach segment!
• In Hot Air: The aircraft flies a true glidepath that is steeper than intended, potentially causing high descent rates on final.

Charted Temperature Limitations

• Approach charts for LNAV/VNAV procedures depict temperature limits in the briefing notes:

"Baro-VNAV NA below -15°C (5°F) or above 48°C (118°F)."

• Operational Rule: Aircraft relying on Baro-VNAV for vertical guidance cannot fly the LNAV/VNAV line of minima when the airport surface temperature is outside the published temperature envelope. They must revert to the non-precision LNAV line of minima (MDA).
• WAAS Exemption: Aircraft equipped with WAAS (TSO-C145/C146) receivers are exempt from Baro-VNAV temperature restrictions because WAAS vertical guidance is derived geometrically from satellites rather than barometric pressure.

Cold Temperature Restricted Airports

• The FAA publishes a list of Cold Temperature Restricted Airports in the Terminal Procedures Publication. When the airport temperature is at or below the published threshold, pilots must apply temperature correction tables to all intermediate, step-down, and missed approach altitudes.

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Advisory Vertical Guidance: The "+V" Distinction

Modern glass-cockpit GPS navigators (such as Garmin G1000, GTN 650/750, GNS 430W/530W, and Avidyne IFD series) generate artificial, advisory vertical descent paths on non-precision approaches. When active, the annunciator displays LNAV+V or LP+V.

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|                        CERTIFIED APV vs. ADVISORY VERTICAL GUIDANCE (+V)                |
|                                                                                         |
|  Feature              | Certified APV (LPV, LNAV/VNAV)    | Advisory Guidance (LNAV+V, LP+V) |
|  -------------------- | --------------------------------- | -------------------------------- |
|  Regulatory Status    | Approved Instrument Approach (APV)| Non-Precision Approach (NPA)     |
|  Charted Minima Line  | LPV or LNAV/VNAV                  | LNAV or LP                       |
|  Operating Floor      | Decision Altitude (DA)            | Minimum Descent Altitude (MDA)   |
|  Level-off at Minima? | NO (Go-around immediately at DA)  | YES (Must NOT descend below MDA) |
|  Step-Down Fixes      | Protected by design glidepath     | Pilot MUST cross-check step-downs|
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The Golden Regulatory Rule of +V

$\mathbf{+V \text{ is ADVISORY ONLY. The approach is legally a Non-Precision Approach.}}$

1. The MDA is a Mandatory Hard Floor: When flying an approach annunciating LNAV+V or LP+V, the pilot must not descend below the published LNAV or LP MDA unless the visual criteria of 14 CFR § 91.175 are satisfied.
2. Do Not Fly +V as a DA: Treating an LNAV+V advisory glidepath like a Decision Altitude and flying through the MDA without visual references is a serious regulatory violation and CFIT hazard.
3. Step-Down Altitude Cross-Checks: While the navigator calculates an advisory $3^\circ$ path, the pilot remains legally responsible for cross-checking the barometric altimeter at every charted step-down fix between the FAF and the MDA.