Rnav Approach

[G7USL 117]

Can anyone explain why this approach may sometimes be required please?  I fly the NGX in P3D.

[rondon9898 84]

Because it is far easier than a non-precision approach, particularly the NDB, which in my opinion should have gone the way of the dinosaur years ago. As long as you have sufficient navigational accuracy and the vertical and lateral trajectories for the approach have been carefully checked, the RNAV approach is much easier than using an NDB as it's not susceptible to interference and has vertical deviation information just like the ILS.

[G7USL 117]

Thanks John.

[Mike777 163]

 

 

Can anyone explain why this approach may sometimes be required please? I fly the NGX in P3D.

 

Hi, Dave,

 

I'm not sure they are ever actually "required," but they certainly can be useful.

 

Often for an offset approach -- for example one approach for SFO 28R, or DCA 19, which involves a fairly sharp low level right turn. 

 

SFO:

http://flightaware.com/resources/airport/SFO/IAP/RNAV+(GPS)+PRM+X+RWY+28R/pdf

 

DCA:
http://flightaware.com/resources/airport/DCA/IAP/RNAV+(RNP)+RWY+19/pdf

 

Also any runway that either does not have GS or localizer, or for which one or both are not working.  Especially if there is no GS or it is inoperative, it is useful to have the vertical guidance of an RNAV approach.

 

Both the SFO and DCA approaches end in visual final approaches, which I believe is always the case for an RNAV approach. 

 

Somewhere on the PMDG forums there is a discussion of why you wouldn't want to combine an RNAV approach with an ILS approach, and why, for example the 777 won't autotune the ILS if an RNAV approach is selected.

 

Not meant to be an exhaustive explanation -- just a few examples.

 

Mike

[G7USL 117]

Thanks Mike. I thought maybe it was something to do with local terrain etc.?   Thanks for your explanation, much appreciated.

[scandinavian13 5,192]

 

 

Can anyone explain why this approach may sometimes be required please?

 

Airports are equipped with approaches based on necessity, along with other fluid requests (I'll explain the latter as we go).

 

 

 

The type of approach is highly dependent on the type of airport, and the types of operators into that airport. If it's a small general aviation airport, then it usually gets (at best) an RNAV approach, and/or perhaps a VOR approach if one is nearby. NDB approaches are also occasional, but usually only for airports that have been around for quite some time, and the NDB itself hasn't died yet (at which point they're simply decommissioned). Why do even the smallest of GA airports get an RNAV approach? They're the cheapest due to the lack of equipment. There's no LOC array, glide slope antenna, monitoring equipment, and so on.

 

Stepping up to a larger GA airport with more traffic and larger operators, you start to see LOC and ILS approaches. The reason is that, as mentioned earlier, these are more expensive to develop, set up, and maintain. Either the FAA or local government - or more likely, a combination of both - have a business and safety reason to install the equipment and maintain it. At this level, though, there's usually only one ILS approach to the most frequently used runway, with an RNAV approach on the reciprocal side.

 

Moving up to a regional airports and above, you usually see that there's generally a business reason to commission and maintain multiple approach types. Additionally, these approaches are set up to both ends of the runways, and potentially multiple runways. The RNAV approaches remain to handle ILS outages, or offer different routing options around terrain. As an example, KROA is buried in (low) mountains, which means that the "ILS" to Runway 6 has to be offset due to terrain, making it an LDA. Runway 34 has an ILS because the mechanics of the approach actually worked out. Runway 24 wasn't found to be used enough for landing to require setting up an LDA approach, but enough that an RNAV could be set up. Runway 16 can't have an approach at all due to terrain on the north side making the approach too steep. In this case, while the airport has more sophisticated options available, the curved paths of the RNAV are available to handle the terrain.

 

At international airports, you generally only see ILS approaches to each runway, along with RNAV for the same reasons as regional airports. There may be one or two VOR approaches as a last ditch effort back up. RNAV approaches are generally overlay-type approaches that are just about the same thing as the ILS approach, but navigated via GPS/INS. These approaches generally also have curved entries that help to smooth traffic flows off of the STAR, or navigate complex paths. An example of the latter case here is the RNAV Runway 19 approach to KDCA, which follows a relatively complex curvy approach following the Potomac River to the airport.

 

Taking this last point to a larger extreme are certain operators who are utilizing RNAV approaches to get into specific airports in specific situations. Alaska Airlines has a number of approaches that they have specifically sponsored up in Alaska (and since they've footed the bill, they're not public). Similarly, jetBlue paid a pretty penny to create an RNAV approach into Runway 13L at JFK, which they can use when conditions are jamming up operations in NY airspace. If JFK goes to the 13s, LGA also has to go to 13, which then causes issues for EWR for both the 4s or the 22s. As such, ILS 13L at JFK is avoided at all costs. jetBlue's RNAV approach, though, cuts in a lot closer to the airport than would be required for the ILS, so they can use a more favorable runway while other operators take a delay in trying to get in on the ILS, or take a less wind-favored runway.

 

 

 

Short version?

Contrary to the common sim belief that the ILS is the King of All Approaches, the RNAV offers a more consistent and wide-spread approach to airports. The only true advantage that you get with the ILS is its current approval for autolands, if your aircraft is so equipped and it's necessary (another sim misconception...autolands are very very infrequent). RNAV hasn't been approved for autoland yet.

[G7USL 117]

Thanks Kyle, I was rather hoping you'd notice this one. lol

[scandinavian13 5,192]

Thanks Kyle, I was rather hoping you'd notice this one. lol

 

You're welcome.

 

...and to tack on - again - to Mike's earlier post, the SFO example is actually pretty unique and important. Since the runways are so close together, maintaining proper radar separation is tough if you want to keep the arrival rates up to help avoid huge delays ("Oh SFO is delayed today? Color me surprised..." is a common line for me at work when I look at the OIS). To combat this, they have a few different tools, from an offset ILS approach and an offset RNAV approach, to a further offset - beyond the capabilities of an ILS at this airport - RNAV approach that allows the aircraft to get below the clouds, visually acquire the leading ILS approach, and proceed visually. This, of course, is something that we have the advantage of doing over here that isn't something you'd see elsewhere, as many other places still require minimum radar separation even when pilots have visual contact with the preceding traffic.

[Mike777 163]

RNAV approaches are generally overlay-type approaches that are just about the same thing as the ILS approach, but navigated via GPS/INS.

 

Hi, Kyle,

 

Thanks for your very thorough discussion.  I am curious though as to why many airports have RNAV approaches that are virtually identical to the ILS approaches to the same runways.  Are these only for equipment failures, either on the ground or on the aircraft?  Is there any reason why, in the particular situation of a working ILS/GS, that a pilot or controller would prefer an RNAV approach to an ILS one for the same runway?

 

Thanks,

Mike

 

PS: Kyle, I think I got the particular RNAV SFO 28R approach from one of your earlier posts.

[scandinavian13 5,192]

 

 

Thanks for your very thorough discussion.  I am curious though as to why many airports have RNAV approaches that are virtually identical to the ILS approaches to the same runways.  Are these only for equipment failures, either on the ground or on the aircraft?  Is there any reason why, in the particular situation of a working ILS/GS, that a pilot or controller would prefer an RNAV approach to an ILS one for the same runway?

 

Part of it is just the idea that most of the work is already done. An ILS approach is a lot more stringent in terms of requirements than an RNAV, so the site survey is already complete for the environmental impact, noise studies, and so on, so you might as well get a "free" approach out of it. As a controller, you can't assign an approach that doesn't exist, and if you have an ace up your sleeve in the case equipment goes out (intentionally or unintentionally), or something else of the sort, you have a little more fault tolerance. For a good number of them, the fixe names don't even change.

 

 

 

PS: Kyle, I think I got the particular RNAV SFO 28R approach from one of your earlier posts.

 

Probably - SFO and JFK were big examples we used when we were floating the idea of "Best Equipped Best Served" over the "first come first served" concept. The other one was a MDW and ORD example, but it got less discussion time.

[Budbud 227]

Similarly, jetBlue paid a pretty penny to create an RNAV approach into Runway 13L at JFK, which they can use when conditions are jamming up operations in NY airspace. If JFK goes to the 13s, LGA also has to go to 13, which then causes issues for EWR for both the 4s or the 22s. As such, ILS 13L at JFK is avoided at all costs. jetBlue's RNAV approach, though, cuts in a lot closer to the airport than would be required for the ILS, so they can use a more favorable runway while other operators take a delay in trying to get in on the ILS, or take a less wind-favored runway.

 

Hi Kyle,

 

Strangely, I have had the impression that the 13L (visual, VOR or RNAV) approach is used quite often when the wind is from the sector E-SSW when looking at the traffic in flightradar24 but statistics would be interesting. I can see even traffic coming from Europe directed to the 13L instead of rwy 22s which doesn't look logical...

And by the way, the RNAV approach is published so all traffic other the Jetblue may use it also but do you know what is the most given approach on the 13L? VOR, RNAV or visual?

[BlueStar 323]

 

 

Because it is far easier than a non-precision approach, particularly the NDB, which in my opinion should have gone the way of the dinosaur years ago.

 

John,

 

How many real world NDB approaches have you flown?

 

blaustern

[icaruss 18]

Because it is far easier than a non-precision approach, particularly the NDB, which in my opinion should have gone the way of the dinosaur years ago. As long as you have sufficient navigational accuracy and the vertical and lateral trajectories for the approach have been carefully checked, the RNAV approach is much easier than using an NDB as it's not susceptible to interference and has vertical deviation information just like the ILS.

RNAV approaches, including those with LPV minimums are non-precision. RNAV approaches with Ground based augmentation system (LAAS) also known as GLS approaches are the only satellite based precision approaches.

[skelsey 765]

 

 

Is there any reason why, in the particular situation of a working ILS/GS, that a pilot or controller would prefer an RNAV approach to an ILS one for the same runway?

 

Not entirely sure whether it's still the case but at Gatwick any aircraft following an A380 is/was offered the RNAV.

 

The reason is because the A380 infringes the ILS critical areas for a much larger period of time when vacating the runway and therefore much greater spacing (14 miles) would be required between a landing A380 and a following aircraft to ensure that the critical area is protected. Putting the following aircraft on the RNAV approach means that the ILS critical areas no longer need to be protected and therefore the spacing can be reduced to the usual 8 miles.

[Mike777 163]

 

 

Strangely, I have had the impression that the 13L (visual, VOR or RNAV) approach is used quite often when the wind is from the sector E-SSW when looking at the traffic in flightradar24 but statistics would be interesting. I can see even traffic coming from Europe directed to the 13L instead of rwy 22s which doesn't look logical...

And by the way, the RNAV approach is published so all traffic other the Jetblue may use it also but do you know what is the most given approach on the 13L? VOR, RNAV or visual?

 

Hi, Romain,

 

I don't see an RNAV approach for 13L at Flightaware.com - just the VOR, the Parkway Visual and the ILS.  Strangely, there is an RNAV approach to 13R, a runway normally used for takeoffs.  It starts much more to the west than the 13L Canarsie and Parkway approaches -- also odd, since this brings it close to the approach path for LGA 4.   My assumption is that the Canarsie VOR and Parkway Visual, among other things, provide for noise abatement since, like 4L/R AND 31L/R they are flown mostly over water.

 

 

 

The reason is because the A380 infringes the ILS critical areas for a much larger period of time when vacating the runway and therefore much greater spacing (14 miles) would be required between a landing A380 and a following aircraft to ensure that the critical area is protected. Putting the following aircraft on the RNAV approach means that the ILS critical areas no longer need to be protected and therefore the spacing can be reduced to the usual 8 miles.

 

Thanks, Simon.  Very interesting example!

 

Mike