Descend with caution

The latest avionics even provide an advisory glidepath on many VOR approaches. Recent incidents, however, have prompted renewed discussion of when pilots should follow advisory vertical paths and revealed that many pilots misunderstand both the types of vertical guidance available and how their avionics display and use those descent paths.

It’s important first to distinguish the approved vertical guidance associated with 3D (precision) approaches to a decision altitude (DA) from the advisory vertical guidance available on 2D (nonprecision) approaches to a minimum descent altitude (MDA). (For more information about the evolving definitions of precision and nonprecision approaches and the distinction between a DA and an MDA, see “Let’s Be Precise” (April 2025 AOPA Pilot) and “Decision Time” (September/October 2025 Flight Training).

Approved vertical guidance provides a constant-angle descent path clear of obstacles from the final approach fix (FAF) to—but not below—a decision altitude. At the DA you must be able to continue visually to the runway or begin a missed approach. When obstacles are present, the DA and visibility may be well above the 200 feet and one-half statute mile for a standard ILS or comparable RNAV (GPS) approach to LPV minimums.

In light GA, “approved vertical guidance” means an ILS glideslope or an LPV glidepath. WAAS-capable systems can also provide an LNAV/VNAV glidepath to a DA, but the systems in our panels display LNAV/VNAV glidepaths only for the increasingly rare 3D approaches that have only LNAV/VNAV minimums. The FAA brochure “Required Navigation Performance (RNP) Approaches (APCH)” explains that LNAV/VNAV minimums were developed before WAAS was certified to allow aircraft equipped with a flight management system (FMS) and certified baro-VNAV systems, usually installed only in twin turboprops and jets, to fly to a DA.

The WAAS-capable avionics in our panels can also provide advisory vertical guidance, typically displayed as +V, to help us establish and maintain a constant-angle descent when flying 2D approaches to LNAV minimums published as an MDA. But a +V advisory glidepath, also called a vertical descent angle (VDA), is just that—an aid—and you must monitor the altimeter to ensure that you meet all published altitudes at stepdown fixes, including those inside the final approach fix. Most importantly, you must not follow a +V advisory glidepath below the MDA unless you have obstacles and the runway environment in sight and can continue visually to the threshold per FAR 91.175.

Understanding that limitation is critical when you use an autopilot or flight director in APR mode. An autopilot does not distinguish between approved and advisory vertical guidance and will follow an advisory glidepath below the MDA, even if you set the MDA in the altitude pre-select window. You must be ready to level off at the MDA or begin a missed approach if you cannot meet all the requirements of FAR 91.175. (See “Visual Descent Points As Decision Points,” January 2026 AOPA Pilot.)

AIM 5-4-5 Instrument Approach Procedure (IAP) Charts emphasizes that last point: “Obstacles (both lighted and unlighted) are allowed to penetrate the visual segment obstacle identification surfaces…There is no implicit obstacle protection from the MDA/DA to the touchdown point. Accordingly, it is the responsibility of the pilot to visually acquire and avoid obstacles below the MDA/DA during transition to landing.”

The limitations of +V advisory guidance stem from how those glidepaths are generated. Procedure designers apply the standards in FAA Order 8260.3 (one of the so-called TERPS handbooks) and use trigonometry to draw a constant-angle descending line from the FAF crossing altitude to the threshold crossing height. Whenever possible, the glidepath angle is 3.0 degrees. But when obstacles or the locations of waypoints require a nonstandard angle, designers can move fixes and adjust the gradient, which can be as steep as 5.7 degrees for Category A aircraft flying final at 81 to 90 knots and 4.2 degrees for Category B airplanes.

But even with such adjustments, an advisory glidepath still does not guarantee obstacle clearance below the MDA. The discussion deep in AIM 5-4-5 explains that when obstacles intersect an advisory glidepath inside the FAF, the FAA does not show a VDA on the chart, and the note “Visual Segment–Obstacles” is added to the profile view. That note is an additional warning that you must not continue below the MDA unless you can see and easily avoid hazards between you and the runway. However, even when a VDA is not published on an FAA chart, the angle still may be encoded in the avionics database and create +V guidance, and third-party publishers may show a VDA on their charts.

At the April 2026 session of the FAA-industry Instrument Procedures Group, a representative from the National Business Aviation Association (NBAA) reviewed the issues with +V guidance and the confusion that existing policy and guidance creates among pilots. NBAA recommended that a new note, “Use of VNAV below MDA NA,” replace the current profile view warning. NBAA also urged the FAA to remove VDAs from databases when obstacles intrude below the MDA and that the FAA evaluate and redesign such procedures. The group is considering the proposals.

In addition, be aware that as approach charts are updated, the FAA is gradually removing notes that explain that altitude restrictions at stepdown fixes within the final approach segment apply only when you’re flying the 2D version of an approach to LNAV minimums (an MDA). For more details, see “Watch Your Step (Downs),” (May 2025 Flight Training) and the Aeronautical Chart User’s Guide.

Finally, when you’re flying a WAAS-based approach, understand that the glidepaths are generated internally by the box in your panel. As noted in the AIM, that path is a fixed line in space and is not affected by temperature and pressure deviations from standard conditions. The latest draft of the forthcoming AC 90-119, which consolidates FAA guidance about RNAV systems, notes that “Temperature and pressure deviations…do not affect WAAS vertical guidance, unlike the vertical guidance…from baro-VNAV.” That AC also explains, however, that you “must comply with all charted procedure altitudes, including those in the intermediate segment, even if using the LPV, LNAV/VNAV, or ILS/GLS glidepath prior to the FAF.”

Large temperature-induced altimeter errors may change the altitude and position at which you intercept a glidepath or glideslope and require you to apply altimeter corrections and adjust the aircraft’s configuration for a stable descent.

Bruce Williams is a CFII and specializes in IFR training and instruction in aircraft with advanced avionics. He owns a Beechcraft Bonanza A36.

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