RNAV (GPS) Lines of Minima

"We're going to use LNAV/VNAV minimums" - Student "Why?" - Tizi "I don't know" - Student

How low we can go on an approach is dependent on the line of minima we use. In the case of RNAV (GPS) approaches, there are various options available:

LPV - Localizer Performance with Vertical guidance

LNAV/VNAV - Lateral NAVigation with Vertical NAVigation

LNAV - Lateral NAVigation

LP - Localizer Performance

These line of minima are not always available on every chart. Their availability depends on the Terminal Procedures (TERPS) architecture of the airport. Engineers many not be able to have LPV available at a field, but only LNAV/VNAV, just do to the geometry of the runways, airports, obstacles, etc. So don't expect them on every RNAV approach.

What type of guidance you will receive on these various "lines of minima" is described in this article. Please read it to fully understand them.

Which minimums you will fly (i.e., LPV vs. LNAV, etc.) is dependent on the equipment you have on board. The FAA now recognizes that each line of minima is considered a separate and distinct "type of approach". Your guidance is different, how you fly it is different, and how low you can go is different.

Ok, so we said that it's a matter of equipment. Most avionics, by design, will load by default the best minimums available (i.e., the lowest). You should therefore expect your GPS to automatically load LPV whenever you select an RNAV (GPS) approach. I mean, why wouldn't they, right?

Garmin will show you the lowest minimums available when selecting an approach, as shown here for KSAV. All the RNAVs have LPV minimums available.

Let's talk about each line of minima and what equipment is needed to fly them.

LPV - Localizer Performance with Vertical Guidance

LPV approaches provide angular guidance, both lateral and vertical. They are pretty much just like an ILS, except GPS based. As such, the lateral and vertical paths "exist in space" as a GPS path. The airplane just needs to catch that path and follow it down.

LPV approaches require Wide Augmentation Area System (WAAS). The system uses antennas located all over the United States to enhance the position solution of the GPS unit. GPS units that are not WAAS capable are significantly cheaper, but will not be able to execute an LPV approach for you. Smaller avionics solutions, like the Garmin Touch Navigator (GTN) series, do not allow the user to choose their line of minima. So, if you don't want to shoot LPV minimums, you will have to manually deactivate WAAS from the Satellite Based Augmentation System (SBAS) menu in the settings (that will degrade to LNAV minimums with an MDA).

LNAV/VNAV - Lateral Navigation with Vertical Navigation

LNAV/VNAV approaches provide linear guidance, both lateral and vertical. Although they are GPS based, the path is built by the airplane's flight management system (FMS), and doesn't exist in space.

LNAV/VNAV approaches are rather tricky. LNAV/VNAV approaches require either:

1. WAAS, or

2. A Baro-VNAV system

If you have WAAS, LNAV/VNAV will look like an LPV approach, even though the precision is reduced (so don't expect the needles to get more sensitive closer to minimums) and the minimums are higher. If you don't have WAAS, the system uses baro-VNAV technology to compute the path. The path is based on barometric altitudes. Therefore, it changes with baro setting. This is really important, because the wrong baro setting will drastically change your path, potentially guiding you straight into the ground before you even reach the runway.

A baro-VNAV system is typical of integrated flight decks (IFDs) where the avionics have the computing power to provide a GPS built path. This is not to be confused with the VNAV autopilot mode, which does the same thing but in the enroute segment of flight.

If an airplane has both WAAS and a baro-VNAV system, the system should automatically degrade from LPV to LNAV/VNAV whenever a GPS Loss of Integrity (LOI) is detected by the aircraft. You should see an 'LOI' annunciation somehwere alerting you of that state. Other avionics systems, like the Honeywell PlaneView and Apex systems, will allow the pilot to manually switch to and from any available line of minima.

LP - Localizer Performance

Just like LPV, WAAS is required for these approaches. They are lateral guidance without vertical guidance, but because the geometry is angular, they require. higher precision with WAAS.

LNAV - Lateral Navigation

All you need to fly LNAV approaches is an IFR certified GPS. That's it. No WAAS or baro-VNAV stuff. They don't provide vertical guidance, and are fairly simple to fly. Just remember your step-down fixes!