Plus-V Guidance, Stepdown Fixes, and Why Temperature Changes the Answer

Advisory vertical guidance on an LNAV approach - the plus-V your GPS navigator displays - looks and flies like a certified glidepath. It is not. Plus-V is a mathematical extrapolation computed from baro altitude and database geometry, carrying no FAA obstacle clearance analysis. In cold temperatures, baro altitude itself becomes unreliable, compounding the risk in ways that aren’t obvious from the cockpit display.

What Is Plus-V Guidance, and Where Does It Fit in the Approach Hierarchy?

Understanding plus-V requires understanding where LNAV approaches sit among instrument approach types.

ILS approaches are precision approaches. The glideslope is a physical radio signal broadcast from a ground antenna, calibrated and flight-checked. Obstacle clearance is engineered into the glideslope angle itself. When you’re on glideslope, continuous obstacle clearance from top to bottom is guaranteed.

LPV approaches (Localizer Performance with Vertical guidance) deliver precision-approach performance from GPS satellites. The satellite-based glidepath meets the same engineering standard as an ILS, with decision altitudes as low as 200 feet above the touchdown zone.

LNAV/VNAV approaches provide approved vertical guidance computed barometrically. The FAA applied obstacle clearance criteria to the path. Minimums are typically higher than LPV, but the vertical path is certified.

LNAV-only approaches have no certified vertical guidance at all. The traditional technique is “dive and drive” - descend to each published stepdown altitude, level off, look for the runway, then descend again.

A GPS navigator can compute a constant-angle descent path from your current position to the runway threshold on any LNAV approach. That computed path is the plus-V. It drives a vertical deviation indicator. It flies like an ILS. It carries no FAA obstacle clearance analysis. It is advisory only.

Does Plus-V Guidance Clear Stepdown Fix Altitudes?

Not necessarily - and this is the central point every pilot using plus-V needs to internalize.

Stepdown fix altitudes exist for one reason: obstacle clearance. Each altitude was chosen because terrain, a radio tower, or another charted obstruction required the aircraft to be at or above that altitude when crossing that fix. The altitude is not arbitrary. Something lives below it that the approach protects you from.

On an ILS or LPV, there are no stepdown fixes because the certified glidepath provides continuous obstacle clearance all the way down. LNAV approaches were designed around the stepdown structure. Those altitudes are the clearance architecture.

The plus-V glidepath may or may not pass through each stepdown altitude at the required crossing point. Depending on fix location and the computed angle, the advisory path can put you below a required crossing altitude - not dramatically, but below it.

The FAA is explicit: plus-V guidance is advisory only, and pilots are required to comply with all published altitude restrictions on the approach regardless of what the computed path shows. If a stepdown requires 2,300 feet and the plus-V targets 2,100 feet, you fly 2,300 feet. The plate governs. Always.

Before flying any LNAV approach, pull up the stepdown fix altitudes and compare them to where the plus-V will be at each crossing point. You cannot determine this from the cockpit display alone.

How Does Cold Temperature Affect Your Altimeter?

Your altimeter measures atmospheric pressure and converts it to altitude using the International Standard Atmosphere (ISA) model published by ICAO. That model assumes a standard sea-level temperature of 15°C (59°F) and a lapse rate of approximately 2°C per thousand feet.

The real atmosphere does not conform to this model. In cold conditions, air is denser and pressure levels compress closer to the earth’s surface than the ISA assumes. The result: your altimeter reads higher than your actual altitude. The instrument is measuring pressure correctly - but pressure altitude and true altitude diverge when temperature departs significantly from standard.

The mnemonic is old and worth keeping: from high to low or hot to cold, look out below. In cold conditions, you are lower than your instruments indicate.

The error is not trivial. At -30°C, a fix requiring a crossing altitude of 1,500 feet above airport elevation may need to be flown at closer to 1,900 feet above airport elevation to provide the charted obstacle clearance. That is a 400-foot error on an approach designed around specific obstacle margins.

Controlled flight into terrain accidents have occurred where cold temperature altitude error was a contributing factor. Crews flew the charted crossing altitudes. In actual cold conditions, those altitudes were insufficient. The aircraft was lower than the crew believed.

What Are Cold Temperature Restricted Airports (CTRAs)?

The FAA analyzed years of accident data and designated airports where cold temperature errors are operationally significant enough to require published corrections. These are Cold Temperature Restricted Airports (CTRAs).

At a CTRA in qualifying cold conditions, pilots are required to apply published altitude corrections for specific approach fixes - this is not optional. The CTRA list is in the Chart Supplement (formerly the Airport Facility Directory). If you’re operating in unfamiliar territory in winter conditions, check it before you depart.

How Do Cold Temperatures Affect Plus-V Guidance Specifically?

Plus-V is computed using baro altitude. The glidepath your navigator draws is based on what your altimeter reports combined with approach database geometry.

On a standard day, baro altitude reasonably represents your actual position, and the plus-V path is a reasonable approximation of your physical trajectory relative to terrain.

On a cold day, baro altitude reads higher than true altitude. The navigator’s computation thinks you’re higher than you are. You can track the plus-V perfectly - sitting right on that advisory line - and still be physically below the crossing altitude the approach requires at a stepdown fix. Your display says you’re on profile. Your altimeter says you’re at the required altitude. Neither tells you the whole story.

The combination of advisory-only guidance and baro error in cold temperatures can stack against you in a way that neither factor alone would suggest.

How Should You Preflight an LNAV Approach in Cold Conditions?

Know your avionics. Some modern navigators, including newer Garmin units, include cold temperature compensation that either prompts you to confirm a correction or applies one automatically based on sensed outside air temperature. Read the pilot operating handbook supplement for your specific navigator. Software versions, database editions, and aircraft configurations vary. Do not assume your avionics behave identically to another installation you’ve flown.

Preflight the corrections - before departure. If your system doesn’t apply corrections automatically, look up the cold temperature correction for the approach and apply it to your crossing altitudes on the ground. ForeFlight and similar planning tools include cold temperature correction features. Use them and cross-check against Chart Supplement data.

Compare plus-V against the plate. For every LNAV approach you brief, identify the stepdown fix altitudes and reason through where the plus-V will be at each one. This takes thirty seconds on the ground and nothing on the approach.

Check the CTRA list. If you’re flying into an unfamiliar airport in winter conditions, verify whether it carries a CTRA designation. The Chart Supplement is authoritative.

Key Takeaways

• Plus-V is advisory guidance only. It carries no FAA obstacle clearance analysis. Stepdown fix altitudes are regulatory minimums that must be complied with regardless of where the advisory path sits.
• Stepdown fix altitudes are the clearance architecture of LNAV approaches. They exist because something in the terrain requires you to be at or above that altitude at that specific fix. The plate governs when plus-V and a stepdown conflict.
• Cold temperatures cause baro altitude to read high. At significant cold deviations from standard, the error can exceed 400 feet. You are lower than your instruments indicate.
• Baro-based plus-V guidance inherits that error. In cold conditions, you can track the advisory glidepath perfectly and still be below a required stepdown crossing altitude in the real world.
• Know your avionics and preflight corrections early. Verify whether your navigator applies cold temperature compensation, check your destination against the CTRA list, and apply corrections before departure - not in the clouds.