phatfarmlines
Posts: 1231
Joined: Thu Sep 27, 2001 12:06 pm

Rnav Approaches

Tue Dec 09, 2003 12:45 am

I have been reading up on literature with the procedures involving an RNAV approach, but I am not clear on certain items:

-Is this a precision approach?
-If it is a precision approach, how would the GS mechanism be initiated on the panel (like APPR or AP2 for ILS)?

Thanks from a non-pilot.
 
AAR90
Posts: 3140
Joined: Fri Jan 21, 2000 11:51 am

RE: Rnav Approaches

Tue Dec 09, 2003 1:35 am

Is this a precision approach?

No, not yet.

...how would the GS mechanism be initiated on the panel (like APPR or AP2 for ILS)?

VNAV switch. There is a "vertical path deviation indicator" [goes by many different names] on the Navigation Display similar to GS deviation indicator.



*NO CARRIER* -- A Naval Aviator's worst nightmare!
 
phatfarmlines
Posts: 1231
Joined: Thu Sep 27, 2001 12:06 pm

RE: Rnav Approaches

Tue Dec 09, 2003 4:25 am

AAR90:

Thanks for the quick response. You state that this is a not a precision approach yet. Is there not much difference between a RNAV approach and a GPS approach as it stands?
 
Woodreau
Posts: 1182
Joined: Mon Sep 24, 2001 6:44 am

RE: Rnav Approaches

Tue Dec 09, 2003 10:59 am

The main difference in my mind between a GPS approach and a RNAV approach is that a GPS approach can only be flown with an approved GPS receiver, whereas an RNAV approach can be flown if you have equipment which will allow you to achieve a specified Required Navigation Performance, (GPS, FMS, LNAV/VNAV) which allows airliners to fly these approaches with their flight management equipment which may or may not use GPS.

Woodreau
Bonus animus sit, ab experientia. Quod salvatum fuerit de malis usu venit judicium.
 
AAR90
Posts: 3140
Joined: Fri Jan 21, 2000 11:51 am

RE: Rnav Approaches

Tue Dec 09, 2003 1:07 pm

You state that this is a not a precision approach yet. Is there not much difference between a RNAV approach and a GPS approach as it stands?

I think Woodreau has it pretty close. From my position [AA B737 CA] we fly all non-precision approaches using RNP only procedures. That means we can fly a LOC approach when the LOC is inop or ADF approach even though there is no ADF installed in the plane [no "tunes" enroute].  Angry AA 757s have been flying RNAV approaches into Rocky Mountain airports for a number of years with no RNP capability... strictly an RNAV approach using an approved area-navigation system.  Laugh out loud
*NO CARRIER* -- A Naval Aviator's worst nightmare!
 
Guest

RE: Rnav Approaches

Tue Dec 09, 2003 2:39 pm

I can't speak for the airline types, but us corporate types have been able to use R-Nav and GPS approaches on a regular basis for many years now. We use our FMSes to fly these approaches.

The are several different manufacturers of FMSes, the ones that I'm personally most familiar with are the UNS-1Cs and the Global GNS-XLS. Depending upon the specific aircraft installation and interface and the particular software version being employed, the UNS-1s provide pseudo-ILS capability in conjunction with VOR, LOC, LOC-BC, GPS, and NDB non-precision approaches. The long and short of it is that every instrument approach that we make is, for all intents and purposes, just another ILS - its cockpit presentation is identical to an ILS and it's flown identical to an ILS. We even have the capability to create and fly (fully coupled if we wanted to) "VFR" approaches to any runway (or for that matter, to any set of lat/long coordinates) in the world.

As to how they work, the FMSes simply use GPS and some magic to calculate a glidepath that will meet all of the minimum crossing restrictions published for the approach. From a safety point of view, they are excellent. You are able to set up a stabilized approach from the point that you are turned onto the final approach course. There is none of the "dive and drive" that can be so destabilizing. The only time we have to fly the various approaches the "old" way is during recurrent training and checkrides, but even then they're starting to ease up a bit.

The main difference between a conventional ILS and a non-precision approach employing a "pseudo-glideslope" is at the "bottom" of the approach - you end up with a "DH" situation rather than a "MDA". The FMS brings you down a 3 degree (more or less, depending upon the particular approach) and you'll hit the minimum altitude before you arrive at the published missed approach point - which is typically at the runway threshold. You end up with a VDP that corresponds quite nicely to the standard 3 degree descent path. It's just like an ILS - when you get to the DH you look for the runway. If you see the runway you land, if you don't you go somewhere else. (In practice I guess you could level off and continue on to the MAP, but why? We're not in a helicopter or a Super Cub and we'd have a difficult time landing and getting a jet stopped from that point.) These pseudo-ILS approaches aren't always available. There are a few where they won't work and the box won't let you do them. The one place that comes readily to mind is Aspen, Colorado. However, I've been flying with this equipment for 15 years and hundreds of actual approaches and I can count on one hand the number of places where the box won't give you a glideslope.

Additionally, the FMS's will also also fly holding patterns and DME arcs. On a typical non-precision approach in a non-radar environment, the "box" will intercept and fly the arc, then fly the entire approach approach (complete with pseudo-glideslope). In the event of a miss, it will fly the entire missed approach procedure, including entering the hold. All we have to do is relax and monitor the approach and make sure that the gear and flaps were down. Pretty amazing stuff when you think about it. Its neat to watch it fly the arc. It flys it by varying the bank angle rather than with a series of straight legs. It will stay within .1 NM of the arc at all times - even interception.

As always, there is always "good news" and "bad news". The good news is that pilot work load is greatly reduced. There is little or no need to perform the "dive and drive" vertical profiles that are inherent in many of the non-precision approaches. This makes for a much more stabilized approach. Situational awareness can also be greatly enhanced. (It's pretty neat to be able to watch yourself being vectored onto the final approach course! It's fully displayed on your EFIS.) Now for the bad news. It's easy to allow yourself to become too complacent - you still need to pay close attention to what's going on around you. And finally, FMSes are wonderful tools, but like any other computer it's garbage in, garbage out. In other words, you had better know how use the equipment and program them correctly. It's not difficult, but you do have to do it right.

Jetguy
 
zak
Posts: 1926
Joined: Mon Sep 22, 2003 12:17 pm

RE: Rnav Approaches

Tue Dec 09, 2003 5:55 pm

btw to my understanding RNAV is very much a boeing thing. they use LNAV (lateral) and VNAV (vertical). so i think the RNAV name might be a boeing thing, however practically its all the same with different names. airbus doesnt have these explicit system modes but a different way of running things in their mdcu even though the result is about the same, enter a holding patter give it the direction etc and watch it follow the perfect path it previously plottet.
10=2
 
Guest

RE: Rnav Approaches

Wed Dec 10, 2003 12:46 am

Actually, RNav isn't a "Boeing thing", but rather an FAA accronym for Random area NAVigation.

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