​Exploring the Corner Case Approach for a Pilot

LNAV/VNAV minimums are a bit of a mystery to most U.S. pilots. That’s probably because they’re rarely encountered, except at the relatively few airports with RNAV (GPS) approaches that have LNAV/VNAV minimums but no LPV minimums.

However, as will be explained, I encountered them while flying an approach with LPV minimums when the satellite signal quality was insufficient to support LPV minimums.

This Article First Appeared in FLYING Magazine

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LNAV/VNAV minimums are less of a mystery to pilots in other parts of the world. For example, I flew my first approach to LNAV/VNAV minimums when teaching at a Cirrus Pilot Proficiency Program in Australia in 2019. To have LPV minimums, there must be a set of ground reference stations nearby to generate the WAAS correction signals required to fly to LPV
minimums. Australia doesn’t yet have that infrastructure, so the lowest minimums commonly available for its RNAV (GPS) approaches are LNAV/VNAV. 

One of the most overlooked aspects of flying an RNAV (GPS) approach is that pilots often fail to look and see what minimums the system has authorized.

With a modern WAAS-capable GPS receiver, as soon as the final approach fix (FAF) becomes the active waypoint, the WAAS receiver evaluates the GPS satellite and WAAS correction signals. If the receiver’s analysis shows the signals are of sufficient quality to fly to LPV minimums, the TERM annunciator turns to a magenta LPV annunciator. Other common annunciations are L/VNAV (for LNAV/VNAV), LNAV, LNAV+V, LP, and LP+V. In Garmin G1000, G3000, G5000 and Perspective systems, these annunciators appear inside the HSI. 

A yellow LPV annunciator means the current satellite signal quality isn’t yet adequate for the approach. That’s an early indication LPV minimums may not be approved, unless the satellite signal improves. If satellite signal quality at the FAF is insufficient for LPV minimums, the approach minimums will be automatically downgraded, and typically the LNAV minimums annunciator and a message saying the approach was downgraded will appear. 

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Approaches with LPV minimums of 250 feet agl and higher and LNAV/VNAV minimums both require the same 50-meter Vertical Position Level (VPL) signal quality. If that limit isn’t met, the approach will downgrade to LNAV minimums. Approaches with LPV minimums of less than 250 feet agl require a VPL of better than 35 meters, so those approaches could downgrade to LNAV/VNAV. 

Another scenario under which an approach could downgrade from LPV to LNAV/VNAV minimums is if the 40-meter Horizontal Alarm Limit (HAL) is exceeded. This is unlikely, since horizontal accuracy in the continental U.S. is extremely good and usually better than vertical accuracy. However, if the HAL limit were exceeded, the approach would downgrade to LNAV/VNAV minimums.

The big difference in flying to LNAV/VNAV minimums is that the glide path indicator appears as a magenta pentagon. From the FAF to the missed approach point, the LNAV/VNAV glide path must be flown within a plus-or-minus, 75-feet vertical window. The limits are displayed on the vertical deviation indicator as vertical white lines above and below the window, and the length of the lines vary as you approach the MAP.

When you’re flying the approach correctly, the magenta pentagon glide path indicator stays within the gap between the two white lines. If the pentagon glide path indicator moves outside the gap and points at one of the lines, you have excessive deviation, and the lines and the pentagon glide path indicator turn yellow.

Note that this is very different from flying to LPV minimums. LPV vertical guidance is angular, like an ILS, and becomes progressively more sensitive inbound. In certified avionics, that sensitivity is commonly limited to a minimum full-scale value of plus-or-minus 15 meters (plus-or-minus 49 feet) near the runway. The difference between 49 feet and 75 feet might not seem that great. But it’s important to recognize that the 49 feet for LPV representsfull-scale deflection, while the 75 feet for LNAV/VNAV requires staying between the white lines, which is far less than full-scale deflection.

A variety of equipment can be used to fly LNAV/VNAV minimums. GA pilots typically use a modern WAAS-capable GPS receiver. But airliners certified to fly these approaches can use barometric-VNAV systems, sometimes known as Baro-VNAV systems, which combine an IFR GPS receiver, which doesn’t have to be WAAS capable, with an additional barometric pressure input. These systems use barometric altitude information to compute a descent path between two waypoints, or an angle from a single waypoint.

There are many limitations, found in RNAV GPS instrument chart notes, that apply to Baro-VNAV systems. However, these notes don’t apply when flying with a WAAS-capable GPS receiver.

For example, you’ll find notes for high and low outside temperature limits for flying the approach. Also, remote altimeter settings are not permitted for use by airliners with Baro-VNAV systems, since the farther you are from the altimeter setting source, the greater the possible error when you use that altimeter setting. But with a WAAS-capable receiver you can use a remote altimeter setting listed on an approach to fly to LNAV/VNAV minimums.

KHWD RNAV (GPS) 28L

I recently experienced an approach to LNAV/VNAV minimums at Hayward, California (KHWD) while flying the RNAV (GPS) 28L approach in a Cirrus Vision Jet (see Chart Wise on page 20).

At some point early in the approach, the LPV annunciator in the HSI began flashing yellow. At about the same time, the autopilot’s vertical navigation, controlled by the VNV key, kicked off, and it would not make the initial stepdowns on the approach automatically. So, I had two issues to deal with.

The first thing I did was use my time machine—the thrust lever. I pulled it back to slow down and gain time. Dealing with the vertical navigation was the simpler of the two issues. Instead of using vertical navigation, I used the VS key to do the stepdowns individually. 

The GPS downgrade was a little more complex. After LPV flashed yellow for a while, L/VNAV appeared, and it too was flashing yellow. But then after a time it turned to magenta, and I knew the overall signal quality was sufficient to fly to LNAV/VNAV minimums. However, we were initially outside the white lines, above the glide path, with a yellow vertical deviation indicator. By slowing and increasing my descent rate, I got between the white lines, and the indicator turned magenta. 

Why the unusual downgrade to LNAV/VNAV? This occurred during a solar storm on January 19 that disrupted radio communications and greatly shrunk the area in which LPV service was available. As pilots, we need to be prepared for the unexpected. Knowing all you can about GPS approaches can help when the unexpected occurs.

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Airports Without LPV Minimums

Here are a few of the airports in the U.S. with RNAV (GPS) approaches for which there are LNAV/VNAV but not LPV minimums: 

• Treasure Coast (KFPR), Florida: RNAV (GPS) RWY 28L 

• Stewart (KSWF), New York: RNAV (GPS) RWY 27 

• Peoria (KPIA), Illinois: RNAV (GPS) RWY 22 

• Carbondale/Murphysboro (KMDH), Illinois: RNAV (GPS) RWY 18L 

• Harvey (5H4), North Dakota: RNAV (GPS) RWY 29 

• Akutan (PAUT), Alaska: RNAV 27 

• Kona (PHKO), Hawaii: RNAV (GPS) RWY 35

If you fly into one of these airports, you’ll want to understand how flying to LNAV/VNAV minimums differs from flying to LPV minimums.

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This column first appeared in the July Issue 972 of the FLYING print edition.