So I'm looking at the charts for the Rnav approach and LOC approach to RNW 10, to a regional airport in West Point, Virginia, KFYJ.

The approaches are very similar yet different. They both use slightly different intersections and missed approaches. However I think the GS is 3.5 on both and the distances and elevations are almost the same to the various intersections, yet vary slightly. They both start at an IAF at 2000 feet yet the LOC indicates 94 degrees and the Rnav has 97 degrees for runway 10. I see a note that the LOC has a 3 degree offset. Why is this? Also a note "VGSI and ILS glidepath not coincident" They both start out at a IAF ~14 miles away, yet again there is a slight difference between them.

So as a not-trained IFR person asking a silly question, why aren't they the exact same. I get why the minimums are different, but the approaches are other wise very similar. 

An ILS approach is guided by equipment on the ground.. sometimes this limits what can be constructed..

RNAV approaches are guided by satellites, and can be laid out differently.

As Bert mention localizer is based on physical transmitter on the ground and RNAV based on GPS signal.  "VGSI and ILS glidepath not coincident" means vertical guidance and visual  glide path are no the same. For example you get out of clouds at minims  PAPI may shows that you "high" or "low" instead of "on glide path " 

Localizer by nature is none precision approach and don't have vertical guidance. In contrast, RNAV approach can have multiple variants with or without vertical guidance.If you look at the chart below for RNAV RW10.  LNAV provides only horizontal guidance while LPV  gives both vertical and horizontal guidance (almost like ILS )

You can compare minimums for  LNAV MDA: 380ft and for LPV DA: 264ft . 100+ feet difference 

ILS is an actual signal from the ground whereas RNAV is a bunch of waypoints with assigned altitudes. Obviously RNAV is more flexible as it can be designed however needed such as curves around mountains etc. FWIW there is a newer one called GBAS/GLS (Ground Based Augmentation System).  GLS approach charts look similar to ILS, however the GBAS System on the ground has only one transmitter for all runways. Whereas obviously ILS needs separate transmitters for each runway. Not many airports have GBAS (see below). Also interesting to note that Zibo Mod 737 for XP can do GLS approaches, using the 737 Multi Mode Receiver (MMR). It has had it for a while.  PMDG does not have it. As for MSFS I'm not sure if it supports GBAS/GLS. But I think the data is there in Navigraph at least.  Look for an update in future regarding GBAS/GLS support.  The Pmdg etc should get an update too.

https://skybrary.aero/articles/gbas-landing-system-gls

Edited July 12, 20223 yr by Greazer

3 hours ago, mikegrr said:

. I see a note that the LOC has a 3 degree offset. Why is this?

Due to TERPS maybe? And has fate would have it, anything with an offset more than 3 degrees can't be called "Localizer approach" but will be labelled as an LDA approach instead - with even higher minimums.

And the two approaches are more different when you dissect them (aside from minimas)

RNAV : 3 degree glidepath,  5.1NM from threshold, (KULEY 1700ft)

LOC: 3.50 degree glidepath 4.4NM from threshold (NEZZZ 1700ft). As you cross the threshold DME will read 0.8NM.  The PAPI is calibrated for this glidepath (crossing height of 53ft)

Edited July 12, 20223 yr by SAS443