Fly The Wing

Just when you thought you had VOR and NDB approaches figured out, they came up with more alphabet soup acronyms to confuse you! I'll try to break it all down simply and succinctly without making your eyes glaze over.

First, RNAV refers to 'Area Navigation', and that means anything other than the ground-based 'point-to-point' navigation you get with VOR's, ADF's and NDB's. Think 'space-based'. Yes, it could relate to LORAN or MLS (Microwave Landing System), but in the real world of single-engine Cesnnas and Pipers in which most of us reside, think 'GPS'. So, when you see an instrument approach with the letters 'GPS' or 'RNAV' in the title, you can think of them both as simply GPS approaches, as that's going to be the only thing you’re going to have onboard your airplane to fly that approach.

NOW IT GETS A LITTLE TRICKY

So here's where it starts to get a bit confusing. Those minimum descent altitudes and visibility requirements you see at the bottom of the approach --- those are 'lines of minima', not actually different types of approaches. Remember how you might have an 'S-ILS' line of minimums on a precision approach, as well as an 'S-LOC' line? That simply defined two different sets of minimums for that same approach - the first if you had glideslope; the second if

you didn't have glideslope (higher minimums, no vertical navigation, resulting in a localizer-only non-precision approach).

Well, the LP, LPV, LNAV and LNAV/VNAV are very much in that same vein. You're still flying the same approach on that plate; those different lines of minima simply define various altitudes (DA or MDA) and visibility requirements based on your equipment capabilities.

A PICTURE’S WORTH 1,000 WORDS

I put together a one-page table and diagram that will help you digest the elements and features of these various ‘lines of minima’. You may download the PDF by clicking here: RNAV Approach Table.pdf.

(TRYING TO) KEEP IT SIMPLE

When you see an LNAV/VNAV and LPV approach, think of them as relatively the same, at least in terms of both requiring a WAAS-enabled GPS. (Technically, you could fly the LNAV/VNAV without WAAS if you had a baro-VNAV and FMS, but I'm pretty sure you don't have that in your Skyhawk, so you need WAAS.)

Once you figured out LNAV/VNAV and LPV minimums, LP approaches began popping up; (the RNAV approaches at Gillespie and Lindbergh are the only two LP’s in Southern California).

One thing to keep in mind regarding all the aforementioned approaches is that they don't rely on any ground-based Navaids (other than the WAAS component; but don't worry about that right now). And here's the beautiful thing about that concept. When you're using a highly-accurate 'space-based' mode of navigation - GPS - you don't really need to concern yourself with your proximity to ground-based Navaids (VOR's and their associated radials, NDB's, Marker Beacons, Fan Markers, etc.). Now, obviously we don't want to hit any terrain, buildings or towers - that's the whole point of a successful approach, right? But remember approaches where you would 'cross a VOR at 1,500', then descend'; or 'at the 177-degree radial, begin a descent to 4,000', etc? Your altitude was derived from an altimeter, which you'd set based on a ground-based pressure-reading from between 1-59 minutes ago, at an airfield who-knows-how-many miles away. (Refresh your memory of what happens to absolute vs. true altitude during cold weather operations; remember 'hot to cold, look out below'?)

Think about a traditional ILS approach with a Glideslope. Other than the Glideslope Intercept Altitude (and later the DA), you could essentially cover the altimeter (but don't try that at home). Once you're on glidepath, you use the G/S Indicator to maintain that glidepath. The Glideslope is very much like a Localizer turned sideways; keep that horizontal needle centered and it will keep you clear of all obstacles and take you down as close to 200' above touchdown altitude. Although the glideslope is obviously ground-based (it's located beside the runway), it is nonetheless an electronic radio signal that is always correct, no matter what the barometric pressure or altimeter might indicate.

'V' IS FOR VERTICAL

Now imagine if that Glideslope signal was even more sensitive; based on space-based GPS signals able to define your exact position in the approach - laterally and vertically - within just a few feet. There you have the resultant approaches above that contain the letter 'V' (V stands for vertical). Laterally you are kept on centerline of the approach using the GPS in a manner we've been doing for years.

But with the increased sensitivity and accuracy of the WAAS component, we now have reliable enough information to design a stabilized, continuous vertical descent to the runway. Unlike the step-downs on non-precision approaches, which require the 'Dive and Drive' technique, GPS approaches with vertical descent allow for a smooth, continual descent toward the Missed Approach Point. There is no MDA; it's a DA just like on an ILS. When you get to that magical Decision Altitude, no leveling off - you either immediately see the runway or the required runway environment - or you go missed; them's the rules.

An LPV is the GPS equivalent of an ILS; it is the GPS approach that will get you the lowest. About 1/3 of the LPV’s will take you down to 200’ HAT. There are now more LPV approaches in the US than there are ILS approaches. You need a WAAS-enabled (TSO-C145A/146A) GPS and a notation in your Airplane Flight Manual that it is specifically approved for LPV approaches. Essentially, if you’re using the Garmin 430/530W, your software revision must be version 3.30 or greater, which would have an install date of July 2009 or later. If your equipment meets this standard (WAAS, RAIM, 6.2 second notification, etc.) you may use it as Primary Means of Navigation (no need for working VOR or ADF in the airplane).

LP - WITHOUT THE ‘V’

Stay with me now on this… The letters ‘LP’ stand for Localizer Performance. Your GPS acts like a Localizer. Remember the ‘funnel’ type signal of a Localizer? It’s called ‘angular deviation’. As you get closer to the airport, the signal becomes more narrow, and the needle on your OBS more sensitive. That’s Localizer Performance. The newer LP approaches allow you to get (a bit) lower than an LNAV approach (which uses the standard GPS Linear Lateral Guidance - 5nm / 1nm / .3nm).

Now, LPV adds the vertical descent component, so of course you will be able to go much lower with an LPV than an LP (just like you go lower on an ILS vs. a Localizer approach). However, LP is not a ‘fail-down’ mode for LPV. In other words, you don’t just ‘kick back’ to an LP if you can’t do the LPV, like you do when the Glideslope is out on an ILS, and you ‘kick back’ to the Localizer minimums. To avoid confusion like this, they will never publish an ‘LP’ line on the same approach plate as an LPV or LNAV/VNAV, so don’t expect to see it there!

CAVEATS

Although an LNAV/VNAV allows you to descend lower than a simple LNAV, the visibility requirement will many times be higher for the LNAV/VNAV approach. Why? Well the 'precision' aspect of the VNAV results in giving you a DA - when you get there you either see the runway environment or you immediately go missed. The LNAV is a non-precision; hence you have an MDA that you fly at until you reach the MAP.

WHASSUP WIT WAAS?

WAAS (Wide Area Augmentation System) takes GPS accuracy to new heights. There are about 40 permanently installed ground-based systems throughout North America, Mexico, Canada, Alaska, Hawaii and Puerto Rico.; one in each ARTC. For SoCal (ZLA), our WAAS station is in Palmdale near the airport. These ground stations provide timing and accuracy correction to the space-based (satellite) component of GPS. The WAAS element puts the position of your airplane within 25-feet, 95% of the time, and reduces the probability of erroneous GPS data to less than 3-seconds per year. And those kinds of stats are enough to be able to get your small airplane down to a runway without hitting anything.

As of December 13, 2012, there were 3,029 new Wide Area Augmentation System (WAAS) Localizer Performance with Vertical guidance (LPV) approach procedures. Currently, there are also 395 Localizer Performance (LP) approach procedures in the U.S.

RNAV Instrument Approach Charts

Charts may contain as many as four lines of approach minimums: global landing system (GLS), WAAS and LAAS, LNAV/VNAV, LNAV, LP, LPV and circling. During a transition period when GPS procedures are undergoing revision to a new title, both RNAV and GPS approach charts and formats will be published. ATC clearance for the RNAV procedure will authorize a properly certificated pilot to utilize any landing minimums for which the aircraft is certified.

While WAAS avionics certified under TSO-C145B or TSO-C146B may be configured for LP, avionics certified under prior technical service orders (TSO) may require updates in order to properly depict LP procedures. The flight manual for your avionics will indicate whether your WAAS receiver is LP ready.

MORE THAN YOU WANTED TO KNOW

You will see things that you can probably just ignore for now, as you (the pilot) and your airplane will not be qualified for. One of these is approaches that contain 'RNP' - Required Navigation Performance. This is special equipment and crew training that enable you to conduct Category II and III approaches; the latter which can literally take you down to the runway in near 0/0 conditions. If you have 'RNP', it's unlikely you would even be reading this article.

You'll start to see GLS in the coming years. This incorporates an 'LAAS' - Local Area Augmentation System, which is a VHF component that corrects satellite timing errors much like WAAS but to the tenth-degree. It is being tested at airports like Chicago's O'Hare. Rather than needing ground-based localizer, glideslopes, marker beacons, etc. for every single runway, one LAAS situated on the ground can provide all the necessary data to create GLS approaches for all 14 runways! Think of the savings in hardware and maintenance.