Aviation Mentor_ Understanding RNAV Approaches

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aviationmentor.blogspot.com/2009/04/understanding-rnav-approaches.html

A gold seal flight instructor and former freight dog shares flying tips & techniques with an occasional bit of humor.

Aviation Mentor

Saturday, April 18, 2009

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 establishedon

the approach, and some pre-flight planning considerations.

*** Note: After reading this, you may want to check out a more recent post on Radius-to-Fix legs>>>

Tomaeto, Tomahto

RNAVstands 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 Sys tems. 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 ArrivalArea 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 theAeronautical 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 notavailable 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!

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Cleared Direct ...

A controller will typically c lear 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 s imple, 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.

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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!

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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 - lateral navigation with advisory descent guidance

LPV- Localizer Performance with Vertical guidance

CIRCLING- when 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 WAASunit, 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 act ivate the approach, your WAAS unit will probably display TERMsensitivity - a full-scale

deflection of the course needle (left or right) represents a 1 mile displacement (left or right) from the

desired track.
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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 LPVcourse 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+Vprovides only advisoryguidance 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 advisoryvertical

guidance should be a constant glide angle required to get you to MDA a bit before the missedapproach 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 Alt itude 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.
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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.
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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?
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