What is RNAV?
RNAV in simple terms is GPS. RNAV stands for “Area Navigation” (thanks acronym maker guy). Bottom line, if you are planning on using a RNAV approach plate, you are going to be doing an approach that uses GPS instead of a ground-based navigation system, such as a VOR or Localizer/Glideslope.
What are the two flavors?
RNAV approaches come in two flavors as noted at the top of the approach plate, either RNAV (GPS), or RNAV (RNP)
The most common you will deal with is RNAV (GPS). RNAV (RNP) is more common in airlines and jets. RNP ultimately means Required Navigation Performance, and is extra equipment onboard the aircraft that monitors the integrity of the satellite signals received. Since RNP equipment is big, heavy, and expensive, it is more commonly found on larger aircraft. For that reason, let’s just focus on the RNAV (GPS) approaches for this TOPIC.
Well, GPS approaches can be EXTREMELY precise. When backed up with RAIM and or WAAS, the aircraft can be guaranteed to be within just a few feet or even inches of the intended course (compared with greater errors associated with VORs or ILS/LOC systems). Although they are very precise THERE ARE NO GPS APPROACHES THAT QUALIFY AS A “PRECISION” APPROACH. This is rather new technology (in aviation terms, I mean come on, we just stopped using NDBs last year, and they were invented in the early 1900’s). ICAO Annex 10 establishes the criteria (International Civil Aviation Organization) for what a “precision” approach is, and currently all of the GPS approaches published in the United States do not meet the criteria for “precision”. (okay, in total disclosure here, there is one GPS approach that is a precision approach, a GLS approach that has a ground based station to refine the satellite and provide precision at or greater than that of an ILS, but only the new 747-8 and 787 Dreamliner (and a few airbuses) have this equipment installed on board, you won’t find it in your 172 from the FBO).
RNAV Unique Features
One of the unique features of an RNAV approach is the publishing of TAA’s or TERMINAL ARRIVAL AREAS. These “pie slice” shaped areas replace the typical MSA (minimum safe altitude) altitudes we are familiar with from VOR, LOC/ILS, and other types of approach plates. The value of a TAA over a MSA, is that the TAA is considered a part of the “PUBLISHED” approach, thus it is treated the same as if you are physically following a thick black line like you would on a ILS/LOC plate.
(Remember, thick black lines are published segments of the approach with minimum altitudes that we can descend down to.) The TAA acts as a large area that has been surveyed by the FAA, and no matter where you are within that area, when you are “cleared for the approach”, you are also subsequently cleared to descend to the minimum altitudes published inside the TAA (of course ATC won’t explicitly tell you this, they expect you to know that your “clearance for the approach” is also a descent clearance to then comply with the minimum altitudes on the plate, all the way down to the MDA (or DA if applicable).
Looking at the example above, if you were approaching FOVTA from the Southeast (or anywhere between the bearings from the fix shown), you could descend when cleared for the approach according to the TAA. For example: You are 35 miles southeast of FOVTA at 6,000′. ATC isn’t too busy, so they give you an approach clearance when you are still WAY far from the airport. Here’s how it would read:
Cessna 8MA 35 miles from FOVTA, Descend and maintain 3,000, cleared RNAV runway 13 approach into Venice, report IFR cancellation on this frequency in the air or on the ground, frequency change approved
So with that clearance, you can immediately start out of 6,000′ for 3,000′, then once you are within 30nm of FOVTA, you can descend to 2,800′, then when you are within 20nm of FOVTA you can descend to 2,000′. Of course you would have to do a course reversal reaching FOVTA before proceeding inbound on the approach, and then once established inbound on the thick black line you can descend down to 1,600′ until passing JIVGA, then descend all the way down to your MDA.
There are several different types of minimums or “minima” as it’s often referred to on RNAV approach plates. As you can imagine, the more precise the level of GPS used, the lower the minimums will take you. Let’s go ahead and take a look at each type, what equipment you will need to be able to use those “minima” and what exactly is a “precision” or “non-precision” approach.
LNAV mean lateral guidance. This is something you could get from a GPS equipped with RAIM, it would not need to have WAAS installed or working to be able to legally use the LNAV Minima. You will have an MDA (minimum descent altitude) with these minima.
LNAV / VNAV (DA)
These minima use “baro vnav”, meaning there is a little barometer (altimeter) combined with your GPS unit to provide vertical guidance while the GPS provides lateral guidance. You won’t encounter airplanes equipped with BARO/VNAV in GA much. WAAS can also be the source of the “vertical” guidance. With a properly certified WAAS GPS you could use these minima (you will have a simulated “glideslope” to follow for vertical guidance).
Localizer Performance w/ Vertical Guidance (LPV). This uses GPS with WAAS to give lateral and vertical guidance. The WAAS vertical guidance simulates a glideslope, while the Lateral guidance mimics that of a localizer rather than the preset CDI deflections you get with LNAV. Check out the two diagrams below for it to make sense. The first one on the left is your typical LNAV (where the CDI has preset sensitivity depending on how far you are from the airport you have programmed into your GPS). The second one on the right, is how LPV works, mimicking a localizer up to a certain point, then the CDI “corridor” is simply 700′ wide from the threshold all the way down the runway (350′ left or right of centerline would be full-scale deflection). We know from experience the localizer associated with an ILS gets more and more and more sensitive the closer you get to the array (antenna), but coincidently enough, when you cross the runway threshold flying an ILS, the course width is roughly 700′ (350′ to the left or right of centerline would have the “LOC Needle” (CDI) go full-scale deflection.
Just like on any other circling approach you fly, you have an MDA you cannot go below until you are in a position to land. You simply need a GPS with RAIM to fly down to these minima on an RNAV GPS approach (no WAAS required).
How do you know what your GPS is certified to?
Well, a sticky note that says “WAAS GPS” scribbled in crayon and stuck to the panel isn’t exactly going to cut it. You need to look in the AFM (airplane flight manual) and see if it has a TSO number of TSO C-145 or TSO C-146. Once you have the TSO number you can reference that to the AIM (airmen information manual) to understand what approaches you can fly with it, and what alternate airport requirements need to be met when planning on using GPS approaches.
As of now, a GPS WAAS receiver TSO’d under TSO C-145 or C-146 (such as a Garmin 430W) would allow you to file to a destination airport that required an alternate due to weather (1-2-3 rule) and file an alternate airport, planning just GPS approaches at both airports. You would not need to have an alternate airport that had a “ground-based” approach (like a VOR or ILS). Both airports you file to could have GPS only approaches (unless otherwise stated with the .
Alternate Airport Requirements
Like we said above, when you have a WAAS GPS TSO’d under C-145 or C-146, then both airports (when you require an alternate legally) can be airports with only GPS approaches. If your GPS in the aircraft is certified under TSO-C129() and TSO-C196(), then one airport (either the destination or the alternate) must have something besides a GPS approach to get you in. The other option if you don’t have the WAAS GPS is just to wait until the weather gets better so you do not legally need an alternate airport, or you can descend from the MEA under VFR conditions to land at the alternate airport (regardless of what approach types it has).
CDI Course Widths
As you get closer to the airport, the CDI (needle) will get more sensitive. It is not progressive like a VOR or Localizer, it happens in set increments, where full scale deflection is not a matter of “degrees” off course, but a matter of exact “width” off course. See the diagram below.
Review: When can you go below DA or MDA?
When you are on an instrument approach, the regulations (14 CFR 91.175(c)(3)) state that you may descend below MDA or DH only if:
- The aircraft is continuously in a position allowing for a normal rate of descent to a landing on the intended runway;
- The flight visibility is at or above the visibility required to complete the approach; and
- At least one of the following visual references for the intended runway is distinctly visible and identifiable to the pilot:
- The approach light system, (except that you may not descend below 100 feet above the touchdown zone elevation using the approach lights as a reference unless the red terminating bars or the red side row bars are also distinctly visible and identifiable)
- The threshold.
- The threshold markings;
- The threshold lights;
- The runway end identifier lights;
- The visual approach slope indicator;
- The touchdown zone or touchdown zone markings;
- The runway or runway markings;
- The runway lights.