SR22: Upgrade to WAAS

Peter wrote:

Have they fixed the issue that if there is a BARO-VNAV approach charted for a particular airport, the +V approach (“advisory glideslope”) is suppressed? Almost no light GA can fly BARO-VNAV so this is a cockup. Garmin were aware of it and promised a fix. The +V is a great option to have for non-LPV (and non-ILS obviously) airports because you basically get LPV (well sort of, subject to not descending below the NPA MDA ).

It is not as simple as that.

SBAS receivers, like a GNS430W, is capable of flying a Baro-VNAV (LNAV/VNAV) only approach with vertival guidance (glide slope). So it, sort of, makes sense to not show the advisory glide slope for approaches that have LANV/VNAV minima.

91 APV Baro-VNAV procedures are allowed to be flown using EGNOS vertical guidance in Germany and Czech Republic. Other countries, including France, also authorise the use of EGNOS for APV Baro-VNAV.

The above quote is taken from egnos-portal.gsa.europa.eu/news/european-gnss-drives-general-aviation

Now, only more states than Germany, France and the Czech Republic need to allow it…

The German AIP supposedly (I do not know how to access the German AIP) contains the following text:

USE OF EGNOS ON RNAV (GPS) OR RNP APPROACH PROCEDURES WITH VERTICAL GUIDANCE
The use of the European Satellite Augmentation System (SBAS) “EGNOS” has been approved for operations on the RNAV(GPS)/RNP approach procedures published for APV procedures (APV baro-VNAV up to the published “LNAV/VNAV” minimum, APV-SBAS up to the published “LPV” minimum).
Class 2, 3 or 4 approved SBAS aircraft avionics are a requirement for the use of vertical guidance with “EGNOS” on APV baro-VNAV procedures (LNAV/VNAV minimum).
Information on the system availability of “EGNOS” will be published by NOTAM in the case of negative availability. Otherwise, it may be assumed that the “EGNOS” system is ready for operation.

TJ wrote:

Cambridge EGSC, which has a 10DME arc for Rw23. I was really looking forward to using it – Cambridge is often a procedural service and starting the ILS from the NDB at the airport often involves quite a lot of extra, unnecessary flying.

The DME Arc uses the DME I-CMG. This DME is displaced the read 0 at the threshold. That is the reason a GNS430(W) will not show the DME Arc.

SBAS receivers, like a GNS430W, is capable of flying a Baro-VNAV (LNAV/VNAV) only approach with vertival guidance (glide slope). So it, sort of, makes sense to not show the advisory glide slope for approaches that have LANV/VNAV minima.

I don’t know much about this but I don’t think the above is applicable to light GA, because the aircraft installation is not BARO-VNAV approved. I read somewhere that only some King Air types (and above) are approved for it.

Bizjets routinely have this capability and they use it for coupled “ILS” approaches to absolutely everywhere where there is any IAP, even just an NDB. There are posts here explaining that EASA didn’t approve it in some cases.

Peter wrote:

I don’t know much about this but I don’t think the above is applicable to light GA, because the aircraft installation is not BARO-VNAV approved.

This is indeed a tricky subject. But I am not talking about Baro-VNAV equipped aircraft, where barometric altitude is used to generate the glide slope. I am talking about flying Baro-VNAV approaches where the glide slope is generated by GPS altitude.

The GTN650 FAA STC says this:

There is no distiction between LPV and LNAV/VNAV when it comes to system capability.

The EASA STC Supplement to the FAA STC even mentions APV Baro-VNAV operations in the additional references section.

This use of Baro-VNAV is not accurate. Baro-VNAV is a calculated glidepath based on barometric altitude that can provide vertical guidance in a non-precision approach scenario.

LNAV/VNAV and LNAV+V both use GPS altitude for the glidepath. The latter of course only advisory and not an official procedure. This is not Baro-VNAV.

JasonC, I am not sure which of my use of the term Baro-VNAV that you do not think is accurate.

The PPL/IR PBN Manual this about approaches with LNAV/VNAV minima.

There they only use the term Baro-VNAV to refer to how the glide slope is computed in the aircraft.
But the term APV Baro-VNAV is used when refering to approach procedures, i.e. approaches with LNAV/VNAV minima. So, Baro-VNAV in that sense is not only about how the glide slope is calculated in the aircraft.

Anders wrote:

But the term APV Baro-VNAV is used when refering to approach procedures, i.e. approaches with LNAV/VNAV minima. So, Baro-VNAV in that sense is not only about how the glide slope is calculated in the aircraft.

I don’t agree. APV is approaches with vertical guidance. These can be to LNAV/VNAV minima (either using Baro-VNAV or SBAS) or LPV approaches.

LNAV/VNAV APV approaches are designed based on Baro VNAV for the vertical guidance. LPV APV approaches are designed based on SBAS for the vertical guidance. When SBAS vertical guidance became available, there were few vertically guided procedures available based on WAAS/SBAS. The design of how the vertical CDI deflection on a WAAS/SBAS system was limited to a maximum of +/- 150 meters to accommodate concerns with using this technology on an LNAV/VNAV procedure that was never designed to be used with this class of equipment. So with this accommodation, the WAAS/SBAS glidepath generation was considered acceptable to fly the LNAV/VNAV procedures as long as satisfactory SBAS signal was available. We have some RNAV (GPS) approaches in Puerto Rico that have LNAV/VNAV minimums, but are not approved for WAAS/SBAS vertical guidance because of the lack of an adequate WAAS/SBAS signal. In the US, AC 90-105 specifies that WAAS/SBAS vertical guidance provided by a TSO C146/145(AR) System is acceptable. LNAV/VNAV using Baro VNAV are temperature restricted, whereas with SBAS vertical guidance, the temperature restrictions don’t apply.

Approaches designated as LPV or LNAV/VNAV are considered APV, Approaches with Vertical guidance and have a DA, not an MDA. Non precision approaches with an MDA don’t have vertical guidance, but advisory vertical guidance can be used on many such approaches. WAAS/SBAS GPS systems provide a form of advisory vertical guidance, but it only applies to approaches that are based on GPS for lateral guidance and they must have LNAV or LP procedures. If an LPV or LNAV/VNAV is collocated on the same procedure as an LNAV, the vertical guidance is officially part of the procedure and there is no advisory vertical guidance for the LNAV, after all what is the point, you already have a vertically guided procedure. The advisory vertical guidance is indicated by the +V (LNAV+V or LP+V). Aircraft with Baro VNAV systems often support both LNAV/VNAV approaches as well as advisory vertical guidance on approaches with conventional navigation (VOR or NDB) or GPS.