Understanding RNAV approaches

While perusing the searches people use to find my blog, I often notice that many are looking for information on RNAV approaches. My earlier posts on this topic were done using examples from a PC simulator when LPV approaches were first becoming available. Now that the number of LPV approaches outnumber the ILS approaches available and I've flown hundreds of different RNAV approaches, it's clearly time to revisit this complex and popular topic. So here's practical information on RNAV approach design, naming conventions, the different approach minima that you might encounter, the types of vertical guidance that may be offered, how ATC will get you established on the approach, and some pre-flight planning considerations.

Tomaeto, Tomahto

RNAV stands for aRea NAVigation and encompasses a variety of aircraft equipment described in U.S Terminal and En Route Area Navigation (RNAV) Operations. Appropriately certified GPS units are considered RNAV as are many Flight Management Systems. Older VOR/DME RNAV units are also consider RNAV units, but in a much more limited way. In simplest terms, an IFR-certified GPS unit is most often the straw that stirs the RNAV drink for most GA aircraft.

For RNAV-equipped aircraft, it's easy for a pilot to navigate directly to a VOR, NDB, intersection, or approach waypoint. That's why RNAV approaches are often designed with a Terminal Arrival Area or TAA (not to be confused with a Technically-Advanced Aircraft). The TAA is a T-, Y- or L-shaped arrangement of Initial Approach waypoints designed to simplify the interaction between ATC and the pilot. A good, detailed description can be found in the Aeronautical Information Manual section 5-4-5(d).

What's in a Name?

Any approach title items contained in parenthesis are omitted when referring to the approach, so both the pilot and ATC would refer to the approach shown below as the "RNAV Runway 12 approach" - GPS is left out because it appears in parenthesis. A stand-alone GPS approach, like the Rio Vista GPS RWY 25 approach would be called a GPS approach. Confusing? Yeah, but supposedly all GPS approaches are eventually going to be renamed to RNAV approaches, it will just take some time.

If you desire an RNAV approach, think like a controller and include your approach request when you check in. Controllers usually appreciate this as it is unambiguous and it saves time.

Santa Barbara approach, Barnburner 123, 7000, request Santa Maria RNAV 12, direct WINCH, with information Foxtrot.

Some RNAV approaches contain "RNP" in parenthesis, which stands for Required Navigational Performance. These SAAAR (Special Aircraft & Aircrew Authorization Required) approaches are not available to us mere mortals.

Some RNAV approaches contain the letter Z or Y and the reason is simple: FMS databases can't handle two approaches to the same runway using the same navigational system, so the letters Z or Y are added to prevent ambiguity. Some kludge, eh? The deal is this: RNAV Z approaches usually provide lower approach minima (typically LPV) than RNAV Y (typically LNAV and LNAV/VNAV) approaches, but sometimes the opposite is true. More on approach minima later.

So what approaches can you fly with an old VOR/DME RNAV unit like the venerable King KNS80? The only RNAV approaches you can fly with these units are the ones named
"VOR/DME RNAV ..." There are 60 to 70 of these approaches in the US, like the UKI VOR/DME RNAV or GPS-B approach. Someone could write a master's thesis on the details behind the naming of that approach!

Cleared Direct ...

A controller will typically clear you to the nearest IAF and then provide an approach clearance. The pilot loads the RNAV approach with the IAF transition specified by the controller, activates the approach, and then follows the guidance to each of the waypoints in the sequence that make up the approach. Consider the Santa Maria RNAV (GPS) RWY 12 approach.



There are three IAFs: OVMAF, WINCH, and LILWU. ATC will usually clear you to the IAF nearest to your position. If you are approaching from the Southeast, "direct WINCH" would be a safe bet and you'd need to fly the HILO (hold in-lieu of a procedure turn) to reverse course. The transitions from OVMAF and LILWU all say "No PT" and you should not fly the HILO without ATC's permission (see 14 CFR 91.175(j) Limitation on Procedure Turns).

Many pilots I've spoken to are confused by TAAs that have a 90 degree turn from the Initial Approach segment to the Intermediate Approach segment. Relax because GPS units provide turn anticipation and the TAA waypoints are fly-by waypoints (you're not required fly right over them). The GPS knows your ground speed, actual track, and the number of degrees of turn required, so just pay attention to your GPS, start the turn when it tells you to turn, and you should end up right on intermediate approach course.

*** Edited 4/22/09, based on NTC comments ***

What about the Vectors-To-Final option for loading an approach? Use it with caution because ATC is restricted from clearing you direct to any waypoint inside the Intermediate Fix (IF) or vectoring you any closer than 3 miles from the FAF on an RNAV approach. Not that this hasn't stopped some controllers from doing otherwise. Vectors-To-Final will only display the the FAF and MAP and I'm not sure why Garmin units even provide you this option for RNAV approaches. I guess it could be useful in an emergency, but not in normal operations.

***

Many GPS units also contain VOR receivers so ensure that your HSI or CDI is displaying the GPS course. Otherwise you might get confused when the course doesn't come alive and ATC starts asking you what the heck you are doing.

When to Descend?

The Santa Maria RNAV (GPS) RWY 12 approach depicts the Minimum Safe Altitude in sectors based on the waypoints WINCH and LIWLU, but many RNAV approaches have a conventional MSA depiction. Nice of the FAA to keep it simple, eh? You reference the MSA altitudes like the one depicted on the Santa Maria approach once you're cleared for the approach without any altitude restriction from ATC.

Let's say you're approaching from the Northwest and Santa Barbara approach clears you direct WINCH. You load the approach with WINCH as the transition and activate the approach. Your GPS may then ask you if you want to load the hold and you say ... wait for it ... NO!

Your GPS says the desired track to WINCH is 142 degrees and ATC clears you for the approach. You are at 7000 feet and 9 miles from WINCH, so you can descend to 5500 feet. Once you are within 6 miles of WINCH, you can descend to 3300 feet. Passing WINCH, follow the altitudes listed on the profile view of the approach chart. Simple, once you understand the conventions.

The "How Low?" Lowdown

RNAV approach charts may have as many as four different types of approach minima (ceiling and visibility) and this is probably the thing than confuses most pilots who are new to RNAV approaches. The possible minima are labeled:

• LNAV - lateral navigation only, no descent guidance
• LNAV/VNAV - later navigation with advisory descent guidance
• LPV - Localizer Performance with Vertical guidance
• CIRCLING - used straight-in minima are not published or circling is desired.

If your GPS is non-WAAS (TSO C129), then life is pretty simple: You only get LNAV minima and you fly the approach like any other non-precision approach, descending as indicated on the profile view of the approach chart. A good practice is to perform a RAIM check on a TSO C129 unit prior to departure and again prior to reaching the IAF.

If you have a WAAS GPS unit (TSO C145 or 146) you can skip the RAIM check, but you should check for WAAS outage NOTAMs for your destination as part of your preflight briefing. With a WAAS unit, the approach chart minima you will use will depend on the course sensitivity the GPS unit displays when your are flying the approach, a few miles outside the FAF. This course sensitivity depends on the WAAS signal integrity and may vary from day to day and hour to hour.

When you activate the approach, your WAAS unit will probably display TERM sensitivity - a full-scale deflection of the course needle (left or right) represents a 1 mile displacement (left or right) from the desired track.



Somewhere before the FAF, usually at the last intermediate fix before the FAF, the sensitivity will change to either LNAV, LNAV+V, L/VNAV, or LPV. (Some approaches list GLS minima as N/A, but this is just a placeholder, it will be replaced eventually with LPV.)

LNAV+V, L/VNAV, or LPV course sensitivities all offer basically a 0.3 mile full-scale deviation and they also provide vertical guidance, but there are crucial differences between the type of vertical guidance provided.

Anytime vertical guidance is provided, be aware that the glidepath may be provided all the way to the surface. Therefore the pilot must ensure they (or the autopilot) do not descend below the MDA or DA appropriate for the course sensitivity displayed by their GPS unless the appropriate visual references described in 14 CFR 91.175 are present.

Advisory Guidance

LNAV+V provides only advisory guidance and this is considered a non-precision approach: You need to ensure you do not descend below any step-down altitude listed on the approach chart's profile view. You may see LNAV+V on some RNAV approach charts that only have LNAV minima, but you may also see it on an RNAV approach where the required signal integrity for LPV is unavailable. RNAV approaches with only circling minima and with an approach course that is more than 30 degrees out of alignment with any runway will not display advisory guidance. The advisory vertical guidance should be a constant glide angle required to get you to MDA a bit before the missed approach point. If you are an adherent to the "dive and drive" style of non-precision approach flying (I am not, by the way), then you can ignore the advisory guidance all together and fly a less-than-stabilized approach.

L/VNAV

This sensitivity is Garmin's way of telling you that this is an Approach with Vertical Guidance (APV): If you follow the glidepath and the lateral guidance to the Decision Altitude, you won't hit anything. I suspect Garmin chose L/VNAV because 1) they didn't have enough characters available to display LNAV/VNAV and 2) they wanted it to be distinguishable from LNAV/+V. Find that confusing? You're not alone!

L/VNAV vertical guidance is provided all the way to the surface. Therefore the pilot must ensure they (or the autopilot) do not descend below the DA unless the appropriate visual references described in 14 CFR 91.175 are present.

LPV

This sensitivity is also to a Decision Altitude and is considered an Approach with Vertical Guidance (APV). The LPV approach provides lateral and vertical guidance similar to an ILS, but usually to a DA no lower than 250 HAT and no less than 1/2 mile visibility.

LPV sensitivity will be annunciated at the last fix before the final approach fix. On the Oakland RNAV (GPS) RWY 27L, this is also where the glidepath will begin being displayed on most GPS units. Interestingly, glidepath intercept is depicted on this chart's profile view at the FAF. For an ILS, the makes sense because of the physical construction and limitations of the ILS. RNAV glidepaths don't have these limitations, so I don't see any risk in following the LPV glidepath as soon as it appears. Just verify your altitudes at each waypoint.

Glidepath to Where?

Pilots have asked me if the glidepath provided for L/VNAV or LPV approaches, like an ILS glideslope, would take them to the touchdown zone. I'm not certain, but my understanding is that the glidepath (or glideslope) for CAT I approaches takes the aircraft to a Threshold Crossing Height (TCH). Look at any RNAV approach chart that provides LNAV/VNAV or LPV minima and you should find that a glideslope angle and TCH are listed, just as you'd find for an ILS.

In order to Serve you Better

If you see an error message saying that the GPS is unusable while flying an RNAV approach and still outside the FAF, you need to execute the missed approach. If this happens inside the FAF, the regulations say you can continue the approach. The only way I'd continue is if I already had the required visual references or some sort of emergency.

When planning to fly an RNAV Z approach, you'd best have the RNAV Y version of the approach handy, too. If the required WAAS signal integrity is not available, your GPS may inform you that the approach has been downgraded and that you should use the LNAV minima - those minima won't be shown on your RNAV Y approach chart, but on the RNAV Z approach chart. Nice curveball, huh?

Absence Makes the Pilot Go Missed

You loaded and activated the approach correctly, you identified the correct minima to use, you got to the MDA or DA and you don't see the required visual references. It's time for the missed approach and all IFR-certified GPS units (except older GNS480) will suspend waypoint sequencing at the MAP. You'll need to press a button (and perhaps set a new desired track on your CDI) to start navigating on the missed approach segment. For most Garmin units, you press the OBS button or softkey. For many King units, you press the Direct button.

Too Complex?

Many a pilot has complained to me that RNAV approaches are just too complex. I agree. I think the approach designers and the RNAV avionics designers have created their own treehouse with some pretty complex rules, dependencies, and exceptions. The pilot guides for these products try to describe these operational subtleties, but this is some complex @#%& for single-pilot IFR.

If you've made it this far, congratulations: It's a rare instrument pilot who can stomach this much minutiae. If there's something I forgot to cover, email me or post a comment. And lastly, this post took a fair amount of time to craft. If you found it useful, please click on the donate button on the upper right corner of this page. The amount you donate is up to you, but every little bit helps.