A FAS data block only applies to LPV and LP approaches.
From RTCA DO-229D
SBAS approaches include the use of horizontal and vertical instrument guidance and failure monitoring. High accuracy SBAS Approaches rely on the concept of using a Final Approach Segment (FAS) data block. There is only one FAS per approach procedure that contains precise information for conducting LPV or LP approaches. It is recommended for SBAS equipment to use the FAS when flying LNAV/VNAV approaches that are co-located with LPV or LP. The approach type available for use is determined by the ability of the SBAS system to provide the necessary level of integrity to support the charted approach types.
The four approach types are defined below:
Approach operations requiring the Final Approach Segment (FAS) data block:
a) Localizer Performance with Vertical guidance (LPV) to alert limits consistent with decision altitudes as low as 200 ft above the threshold. At locations where the SBAS service or airport infrastructure doesn’t support the 200 ft decision altitude, LPV will use the same horizontal alert limits, but less stringent vertical alert limits with decision altitudes as low as 250 ft.
b) Localizer Performance without Vertical guidance (LP) to horizontal alert limit consistent with LPV criteria.
Approach operations not requiring the FAS data block:
c) Lateral navigation/vertical navigation (LNAV/VNAV) approaches with a less stringent horizontal alert limit and vertical alert limit than LPV. The vertical alert limit is consistent with barometric VNAV.
d) Lateral navigation (LNAV) approaches to horizontal alert limit consistent with LNAV/VNAV but no vertical capability.
With LNAV and LNAV/VNAV approaches, the integrity settings are fixed for all segments of the procedure (final approach lateral is +/- 0.3 NM and VAL for the VNAV is 50 meters. This is not the case with the LPV and LP procedure types. The HAL and VAL can be set on an approach basis. Also, the additional precision of the LPV and LP procedures may require a specific SBAS system (EGNOS or WAAS or …) to provide the corrections, whereas procedures that are solely based on GNSS can use RAIM or other systems. The FAS data block also includes a CRC to validate the data in the data block. The data block is generated, protected by application of a CRC
and validated by the appropriate authorities before distribution. Data in the FAS datablock includes SBAS Service provider, airport ID, runway, suffix, Path data selector (a unique number and alpha codes that references the approach), and other parameters to define the path such as LTP location and elevation, TCH, GPA, course width at threshold, flight path alignment point.
Although if an LNAV/VNAV or LNAV procedure is co-located with an LPV, the same data may be used, it is not required.
In the case of an LNAV with +V, the +V is advisory only and is usually based on the same VAL that would be used with an LNAV/VNAV if it existed, The LNAV+V annunciation is not an LNAV/VNAV.
Jeppesen.
What is interesting is that Jeppesen (or Garmin) actually disabled the GS guidance for UK airports, to implement the CAA ruling.
Timothy wrote:
It is a remarkable bit of Gold Plating over the top of any other state I am aware of. And it significantly impacts safety. The CAA should be ashamed of themselves. I doubt they are.
It would make some kind of sense if the CAA had requested that the GPS box simply ignored the VNAV part. Then the approach would be LNAV+V and it wouldn’t matter if the glide path didn’t agree with the Baro-VNAV glide path as it would be advisory anyway and all stepdown altitudes would still apply.
It is a remarkable bit of Gold Plating over the top of any other state I am aware of. And it significantly impacts safety. The CAA should be ashamed of themselves. I doubt they are.
I have to say though, I’m surprised that it’s not more widely publicised that this feature has been downgraded by the CAA.
The only official reference I can find to this is from CAP773, section 2.7.3;
“WARNING! At the time of writing this document (Autumn 2014), the use of SBAS to provide augmentation for the VNAV element of a notified LNAV/VNAV approach is not permitted in the UK. Notwithstanding any certification for RNAV VNAV approach operations using SBAS, aircraft which are not certified for the use of barometric VNAV data are currently precluded from flying approach operations to LNAV/VNAV minima.
These aircraft are not authorised to continue RNAV approach operations below the
published LNAV-only minima."
However (to me at least) this doesn’t clearly suggest to a potential buyer of a GTN avionics package that the vertical guidance wouldn’t actually be available! Just that one would fly to the higher LNAV minima.
Join the queue of those of us rolling our eyes and making tutting sounds in the general direction of the CAA.
The OP should also check that EGNOS had been selected on the SBAS page
Thanks Anthony, it certainly is.
Thanks to the information on the previous pages I’ve now concluded that the GTN is indeed set up correctly, and the reason that UK RNAV approaches with LNAV/VNAV mimima are only showing up as LNAV are down to the CAA restrictions.
The OP should also check that EGNOS had been selected on the SBAS page
Technically one should do that on any 146() equipment, but a rule more honoured in the breach than the observance, I fear.
