What do you need to get LNAV/VNAV?

7 Answers

1. 
   

   
   

   Mark Kolber on Apr 07, 2015

   Either a certified barometric VNAV (baro-VNAV) system (not typically in GA) or GPS with WAAS. You can see a discussion of it in AIM 5-4-5 and other FAA publications.

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2. Best Answer
   
   

   
   

   John D Collins on Apr 07, 2015

   LNAV/VNAV APV procedures are designed to be used by aircraft equipped with a Baro-VNAV system. This type of system derives its glidepath by a barometric pressure sensor. The system derives the altitude from the barometric pressure and uses GPS to determine the lateral path. A GS is computed by this equipment to provide the vertical guidance. With a Baro-VNAV system, WAAS is not required. Since MSL altitude is also a function of temperature, these systems will have limits for both cold and hot temperatures.

   All that said, most GA aircraft don’t have a Baro-VNAV system .Although the G1000 is capable of Baro-VNAV, it is not implemented for approach guidance except on a very few aircraft (example King Air). However, the WAAS GPS TSO permits using a WAAS GPS for vertical guidance on LNAV/VNAV procedures, so they may be flown to the LNAV/VNAV DA using WAAS. When using WAAS for the vertical guidance, the temperature limits do not apply as the WAAS GS is unaffected by temperature.

   In order to use a WAAS GPS to fly the LNAV/VNAV DA minimums, one of these annunciations is required: LPV or L/VNAV. An LNAV annunciation does not provide approval to fly the LNAV/VNAV. If the WAAS GPS annunciates LPV, any of the charted minimums are permitted to be flown, including LPV, LNAV/VNAV, or LNAV, as it is the pilot option. In most cases, the LPV will offer the best minimums. In some cases, there are RNAV approaches that do not have an LPV option, but do have a LNAV/VNAV option. In this case, as long as the WAAS GPS annunciates L/VNAV, the LNAV/VNAV may be flown to its DA.

   All downgrades are from the most accurate APV service to LNAV without any vertical guidance. So a LPV annunciation will never downgrade to L/VNAV, but may downgrade to LNAV. There are a few RNAV procedures with LNAV/VNAV minimums that are outside of the WAAS service volume that provides adequate vertical guidance, example in Puerto Rico, and the chart has a note that “WAAS VNAV NA”.

   So for most situations, I teach my students they can ignore the LNAV/VNAV minimums with their WAAS GPS when the approach has LPV minimums available. For the few procedures still in existence where an LPV is not on the approach chart, but LNAV/VNAV is charted, as long as you see the annunciation L/VNAV, you may fly to the associated DA in the same way you fly an ILS or LPV.

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3. 
   

   
   

   Drew on Apr 07, 2015

   John,

   Does a downgrade mean that you suddenly lose the GPS GS in the approach, similar to the loss of GS on an ILS? Or is that for the procedure development folks side to worry about?

   So, LPV is the only APV that absolutely requires WAAS? Baro-VNAV is not enough?

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4. 
   

   
   

   John D Collins on Apr 07, 2015

   You normally have a warning prior to a downgrade if you are paying attention and know what to look for. The normal green LPV annunciation will display as amber. Downgrades occur when you are 1 minute from reaching the FAF, where you will get both a message and the annunciation will change to LNAV. You may continue the procedure if you are briefed for a LNAV MDA, but you will not have any vertical guidance and you must respect any LNAV procedure minimums and its MDA.

   The only two procedures that require WAAS are LPV and LP. If you are using the WAAS GPS for the LNAV/VNAV vertical guidance, you will also need WAAS, but if you are using a Baro-VNAV system for the vertical guidance, WAAS is not required.

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   Drew on Apr 07, 2015

   I fly in a C172SP with G1000. I’m unfamiliar with the “normal green LPV annunciation”. I know I need to check the CDI APR mode to verify it’s in the APR mode I want; however, that is in magenta. So, downgrades is like a LOI except for RNAV APVs?

   Thank you.

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6. 
   

   
   

   John D Collins on Apr 07, 2015

   I don’t have a C172 G1000 Manual, but in the G36 manual it states:

   When GPS is the selected navigation source, the flight plan legs are sequenced automatically and annunciations appear on the HSI for the flight phase. Flight phase annunciations are normally shown in magenta, but when cautionary conditions exist the color changes to yellow.

   Also, you can look on the Satellite Status page. There is a parameter VFOM which stands for Vertical Figure of Merit. It is a value measured in feet and represents the 95% containment area +/- for the actual vertical position. If this value is above 60 feet, then LPV with a DH greater than or equal to 250 feet, L/VNAV, and LNAV+V vertical guidance is not likely to be available and a downgrade to LNAV is likely to occur. If it is above 40 feet, then for LPV with a DH less than 250 feet is not likely to be available and a downgrade to LNAV is likely.

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7. 
   

   
   

   Mark Kolber on Apr 08, 2015

   When GPS is the selected navigation source, the flight plan legs are sequenced automatically and annunciations appear on the HSI for the flight phase.

   They do. If I recall correctly (it’s been a while for me with the G1000), the HSI “LPV” annunciation appears in magenta (the same color as the GPS nav source) on the HSI. The white(armed)-to-green(engaged) annunciation in the second line of the PFD only announces the it’s in AP mode and glideslope interception. You can use the G1000 sim to test it out but there’s a decent video on YouTube that shows the process and the annunciations. Searh for “G1000 LPV Instrument Approach demonstration” on YouTube if you want to see it.

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