# Thread: Rnav horizontal guidance

1. Member
Join Date
Jul 2014
Posts
115
Blog Entries
1

## Rnav horizontal guidance

Hi,
What is the horizontal guidance for an RNAV approach?

2. Originally Posted by AF330
Hi,
What is the horizontal guidance for an RNAV approach?
Duh...since it's a GPS approach, guidance comes from the GPS.

3. Originally Posted by AF330
Hi,
What is the horizontal guidance for an RNAV approach?
Horizontal guidance is the left and right indications from the course needle on the primary flight displays. The other, vertical guidance, is provided by the pointer on the right hand side of the display that has a vertical scale. With both, indications center you are on course horizontally, and on glide path vertically.

4. Member
Join Date
Jul 2014
Posts
115
Blog Entries
1
Thanks... yes it's with GPS! But what is the vertical guidance for an RNAV approach?
Just like the glide-slope frequency for an ILS (G/S + LOC)!

Thanks

5. Originally Posted by AF330
Thanks... yes it's with GPS! But what is the vertical guidance for an RNAV approach?
Just like the glide-slope frequency for an ILS (G/S + LOC)!

Thanks
With an RNAV approach it's not like an ILS. The display looks the same but you don't tune any radios to get the vertical guidance. The glide path is usually derived by math using the altitude at the final approach fix and any step downs between the FAF and the runway. There is no radio station that provides this information during an RNAV approach except from GPS satellites calculating your position and position from waypoints on the approach.

6. Member
Join Date
Jul 2014
Posts
115
Blog Entries
1
Oh, so do you mean that the plane (Airbus) knows the FAF vertical profile, just like a VOR approach?

7. The FMS, yes should know it depending if it is VNAV capable. Some are, some are not. Some have the VNAV capable but can't be coupled to the autopilot. Now, as far as the sim goes (Airbus) I have no idea. I don't fly the Airbus. But the concept should be the same.

8. evm
Senior Member
Join Date
Sep 2012
Location
Germany
Posts
1,683
Last edited by evm; 09-22-2014 at 06:36 PM.

9. The default gps will guide you on verticle guidance to the runway or at least it will give you a good idea as to what kind of descent you should be on. The default verticle descent for fsx GA aircraft is -+700 and -+1800 for the default jets. There is a VSR indicator on the gps that guides you vertically towards airports, runways, and intersections. When I'm on a flight, I'll wait till the VSR reads -30. When you change the altitude setting on the default jets the verticle descent defaults to -1800. -30VSR = -1800. (30X60=1800feet) This will change as your aircraft speed changes. Generally if the VSR reading is - then you need to descend and if it's a + then you need to ascend. As you fly, keep a watch on the VSR. You can always set the autopilot to the verticle speed that the gps suggests, just multiply the reading by 60///301900Z...nevermind...I missed this was a real world post...my bad. I'll go back into my FSX world now.
Last edited by mqytn; 09-30-2014 at 03:03 PM.

10. There is a VSR indicator on the gps that guides you vertically towards airports, runways, and intersections. When I'm on a flight, I'll wait till the VSR reads -30. When you change the altitude setting on the default jets the verticle descent defaults to -1800. -30VSR = -1800. (30X60=1800feet)
What the GPS CAN do isn't necessarily what is published for the actual approach, when you're talking real life (as I presume this is, given the forum section we're in here), rather than the sim.

Note that RNAV stands for area navigation (originally Random NAVigation), and may be done with inertial systems, LORAN or VOR/DME, in addition to GPS. It's the notion of not being tied to traveling directly to/from a ground station.

I'm not aware of any RNAV approaches using the above systems, but there are plenty of GPS approaches. Not all GPS approaches have vertical guidance, but those that do are LPV (Localizer Performance with Vertical guidance) or LNAV/VNAV (Lateral NAVigation/Vertical Navigation), both of which require WAAS (Wide Area Augmentation System) in addition to the GPS itself.

You can get a lot more from the FAA itself at this link. There's also a lot more to be found on a web search.

#### Posting Permissions

• You may not post new threads
• You may not post replies
• You may not post attachments
• You may not edit your posts
•