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R 9.4 Approach Question |
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aoffen 
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Joined: 29 Jan 2010 Location: Brisbane Qld Oz Status: Offline Points: 46 |
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 Topic: R 9.4 Approach QuestionPosted: 15 Nov 2018 at 4:54pm |
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I have a question re R9.4 and the LP+V approaches it enables. I own a Cirrus in Australia with R9, DFC100 and APX340. Currently in Australia we have no WAAS or EGNOS, so all our GPS approaches are non precision RNAV approaches. With R9.3, a coupled non precision RNAV approach with no WAAS satellite means no GS indication on the PFD. Tim Preusser from Germany (the COPA R9 expert) recently mentioned that he thought R9.4 changed that and if we upgraded, we would get a GS indication on non precision RNAV approaches as well as on Baro VNAV (LP+V) approaches, even with no WAAS.
Would anyone at Avidyne be able to advise if that is true or what 9.4 delivers in terms of approaches that is different from 9.3. Thanks Andrew
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AviSteve
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Posted: 16 Nov 2018 at 10:02am |
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The GPS must be in SBAS to get LP or LP+V. Without SBAS, you'll only get LNAV and LNAV+V. You'll only get LNAV+V if the approach has a defined flight path angle in the database. Are you saying you get LNAV+V, but don't get a glideslope indication? 9.3 has LNAV+V capability.
With respect to approaches, the only thing that 9.4 added was advisory VNAV to LP approaches (i.e. LP+V).
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Steve Lindsley
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aoffen
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Posted: 16 Nov 2018 at 5:41pm |
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Thanks Steve The acronyms don't really line up which is pretty confusing. What I refer to as the new “Baro VNAV” approaches here are actually classified as LNAV/VNAV on the Jepp charts E.g. attached is the chart for YMMB RNAVZ RWY 17L which has mins of 470’ on an LNAV/VNAV approach or 560’ on an LNAV. These are just being rolled out now and are not that widespread yet, but as far as I recall I have never done a GPS approach here and received a GS indication. I am not sure if that is because we have no SBAS or WAAS available here or because I actually haven't done one yet with an LNAV/VNAV specified minimum. If what you describe as LNAV + V corresponds to LNAV/VNAV then from the sounds of it I should get a GS with R9.3 on those approaches? Will I get an advisory GS on all RNAV approaches with 9.4? Does that sound right or are LNAV+V and LNAV/VNAV two different things? Thanks Andrew
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AviSteve
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Posted: 18 Nov 2018 at 7:32pm |
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You thought you were confused already... just wait... Precision GPS approaches have something called a "FAS Record" which contains the parameters for the Final Appproach Segment. Think of the FAS record as defining the mathematical equivalent of an ILS "beam". In order to utilize a FAS record, the system must be in SBAS. Generally, an LNAV/VNAV approach has an associated FAS record, just like an LPV would. However, there is a class of approaches called "standalone LNAV/VNAV" that do not have an associated FAS record. Those approaches have a defined flight path angle that allows the FMS to construct a glideslope, but the vertical guidance is advisory-only. While standalone LNAV/VNAV is recognized by the regulations (RTCA DO-229D, if you're interested), LNAV+V is a feature that is provided by the Avidyne system as an aid to LNAV only approaches. The +V guidance is also advisory-only. You would use published LNAV/VNAV minimums for a standalone LNAV/VNAV approach, but for LNAV+V you would still use published LNAV minimums. Interestingly, though, standalone LNAV/VNAV and LNAV+V are computed using the exact same algorithms. I looked up the YMMB RNAV-Z 17R in the raw nav database, and it is a standalone LNAV/VNAV approach. You should get an advisory glideslope indication for that approach, even with 9.3. Again, 9.4 only adds advisory vertical guidance to LP approaches (i.e. LP+V). Without SBAS (e.g. WAAS), LP+V won't do you any good.
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Steve Lindsley
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aoffen
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Posted: 18 Nov 2018 at 7:45pm |
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You thought you were confused already... just wait...
Your right!!!! So the take away for me then is I should get an advisory GS already using R9.3 for those "stand alone" LNAV/VNAV approaches here. Looks like 9.4 will only help us when someone stumps up the money to put up a WAAS satellite over Australia and we get the genuine LP+V set up. Is that correct? One more question (sorry to test your patience). People refer all the time to "Baro VNAV" approaches. Where does that fit into this little acronym hell we have? Cheers Andrew
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AviSteve
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Posted: 18 Nov 2018 at 9:11pm |
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9.4 contains a few other new features, but with respect to approaches, it's just LP+V. You're correct that you'll never get LP+V until you get an SBAS satellite(s) there. But the real benefit of SBAS would be the eventual availability of RNAV precision approaches (LPV and LP) and LNAV/VNAV (i.e. not "standalone"). Baro-VNAV is another subject altogether. Baro-VNAV allows the operator to conduct LNAV/VNAV operation using barometric altitude as the basis for vertical navigation. The R9 system is not approved for baro-VNAV operations.
Edited by AviSteve - 19 Nov 2018 at 8:13am |
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Steve Lindsley
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aoffen
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Posted: 19 Nov 2018 at 1:21am |
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Thanks Steve,
Really appreciate your patience ion running through all this stuff. It is pretty confusing so it helps to talk to someone who know the ins and outs.
Will hunt down a stand alone LNAV/VNAV somewhere and conduct an approach to confirm I get a GS and let you know. Thanks again Andrew |
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aoffen
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Posted: 01 Jul 2019 at 5:36am |
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Steve
Hi. I finally got the chance to venture out and try a LNAV/VNAV approach for real. I tried doing the YMYB RNAV-Z Rwy 17 approach twice but unfortunately did not get the outcome I was hoping for. Both times, instead of an indicative GS, I got an error message saying : L/VNAV Unavail : GPS integrity is insufficient for L/VNAV Approach I had full GPS reception both times and second time I made sure of it by doing a straight in approach via MBHNE to eliminate any turns (sometimes we get GPS integrity issues when doing turns here due to the position of the satellites low in the sky). There was also no RAIM issue at the time. I will attach a pic of the error message and the GPS Sensor status page at the time I was doing the second straight in approach. Would appreciate any comments guidance or feedback you might have on why it is so! Thanks and Regards Andrew   |
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AviSteve
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Posted: 01 Jul 2019 at 10:25am |
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Could be one of several factors. Unfortunately, your second picture cut off the top of the GPS Status page where HPL and VPL are shown. If HPL exceeds HAL or VPL exceeds VAL, then you'll get that alert.
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Steve Lindsley
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aoffen
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Posted: 01 Jul 2019 at 6:53pm |
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Bugger. So you think it was likely an actual GPS integrity issue rather than a software/approach design issue (or the fact we don't have WAAS down here)? I guess I will have to try it again - and this time get a better shot!!
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AviSteve
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Posted: 02 Jul 2019 at 9:51am |
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Well, if the GPS is not in SBAS, you'll get that message too. Based on the bar graph, the two WAAS satellites are not being used (hollow bars) so you're not in SBAS. The mode shown at the top of the screen would allow you to verify that as well.
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Steve Lindsley
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aoffen
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Posted: 02 Jul 2019 at 10:17pm |
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Unfortunately we still don't get SBAS in Australia at all. They are still running tests!! That sounds like it won't work for us without it. Is that correct?
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AviSteve
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Posted: 03 Jul 2019 at 9:01am |
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Yes, that is correct.
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Steve Lindsley
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aoffen
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Posted: 03 Jul 2019 at 6:10pm |
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OK.....sorry to go on and on with this one but....re-reading the conversation above (19 November), I had understood that "stand alone" LNAV/VNAV approaches were ok without SBAS, but the LNAV+V required SBAS to work. As the Australian LNAV/VNAV approaches were all "stand alone" we would be OK even without SBAS active yet down here.
Is that incorrect?
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AviSteve
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Posted: 03 Jul 2019 at 10:47pm |
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My apologies. I was doing OK in my post of Nov 18 up until I said "you should get an advisory glideslope indication for that approach". LNAV/VNAV approaches, whether standalone or not, do indeed require SBAS corrections. I went back and checked both DO-229D and the software just to be sure. This stuff can get so confusing, but that's no excuse. I should have gotten it right back in November. Again, sorry about that. |
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Steve Lindsley
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aoffen
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Posted: 04 Jul 2019 at 5:28am |
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No problems. I just really want to get it into my head.
So in summary, no GS on LNAV/VNAV approaches without SBAS which i guess means we can't use the lower minima. Thanks for all the help. Regards Andrew
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raybo412
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Posted: 29 Mar 2021 at 10:41pm |
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Hello Steve
I have a similar issue with my IFD 410, where we have LNAV/VNAV charted as available at the airfield. When selecting the approach it gives a CAS of LNAV/VNAV minima not available use LNAV MDA. This article by the Australian Regulator doesn't talk about SBAS for the LNAV/VNAV approach. The only thing I'm missing is the air data computer. So could that be the reason that I'm not getting the LNAV/VNAV? The IFD happily provides G/S guidance with the LNAV only approach. Thanks https://www.flightsafetyaustralia.com/2017/09/when-advice-isnt-the-same-as-guidance/ |
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AviSteve
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Posted: 30 Mar 2021 at 12:07pm |
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The Avidyne system is not approved for baro-VNAV operation. FMS guidance during an LNAV/VNAV approach is provided by GPS. When the "L/VNAV Unavail. Use LNAV MDA" message is issued, the IFD is saying that the integrity provided by the GPS is not sufficient to meet the integrity required for the approach. It's almost certainly because you don't have SBAS. On an LNAV approach, it is possible to get vertical guidance, but it is advisory only and will be annunciated as "LNAV+V". Advisory vertical guidance provides no guarantee of compliance with step down altitudes, so you should always be monitoring the descent to be sure you don't violate any restrictions.
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Steve Lindsley
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raybo412
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Posted: 31 Mar 2021 at 5:55am |
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Hi Steve
Thanks for your reply. I see in the Installation manual, Para 2.4.2, that if the unit is installed in accordance with that section, which my unit is, then LNAV/VNAV, LPV minima and so on. I'm aware that LPV and LP minima will require SBAS, which we may get in Australia sometime in the future. So I'm a bit confused as to your reply with regard the IFD not being Baro-VNAV approved. Unless of course I've misunderstood, but my reading is that LNAV/VNAV is a Baro-VNAV approach. Look forward to your sound advice. Regards Ray |
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AviSteve
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Posted: 31 Mar 2021 at 10:55am |
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LNAV/VNAV is not a synonym for baro-VNAV. The vertical part of LNAV/VNAV approaches can be conducted by reference to either barometric altimeter or SBAS GPS altitude, depending upon the capabilities of the navigation equipage. Since the IFD is not approved for baro-VNAV, it must have SBAS in order to fully perform the LNAV/VNAV approach.
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Steve Lindsley
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