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As written before, is baro-VNAV the only possible source of vertical guidance on LNAV/VNAV approach? I read somewhere that SBAS is also a possible source, but in that case wouldn’t it be a LPV approach then? As far as I know when you load a RNP approach into your avionics you don’t have a choice which one you fly (e.g. LPV or LNAV/VNAV - assuming we are certified for both). I would be grateful for any official acts references.

To clear it out - how it is possible to shoot LNAV/VNAV approach with SBAS augmentation, because I thought that every SBAS augmented RNP approach must be LPV.

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  • $\begingroup$ Hi. RE "I read somewhere that SBAS is also a possible source" -- where? a quotation plus link/citation will help us help you clear the confusion. $\endgroup$
    – user14897
    Apr 7 at 11:07
  • $\begingroup$ Sadly I don't remeber where :( $\endgroup$
    – Konrad
    Apr 7 at 15:53

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I believe @busdriver may have overlooked something. Your TSO 145 or 146 avionics will "fail down" from LPV (a more constrained lateral and lower minima approach) to LNAV/VNAV (wider horizontal guidance through final - higher vertical minima) if WAAS accuracy is degraded, so yes, you can fly LNAV/VNAV to a DA with an SBAS GPS even if not baro-aided.

Your box will always choose the "best" approach available - i.e., the one with the lowest minima. If there's an LPV approach available and the signal is good, you'll be offered that by your box. If there's only an LNAV/VNAV approach for the airport or your box or the WAAS signal is degraded, you'll be served up an LNAV/VNAV, or if the signal/box has major problems, just an LNAV approach.

In the AIM 1-2-2 b(1)(a)(1) it states:

LNAV/VNAV incorporates LNAV lateral with vertical path guidance for systems and operators capable of either barometric or SBAS vertical.

See boldmethod.com for a great comparison of the RNAV GPS approaches.

As GPS is more reliable (if the system is up...) than temperature sensitive baro-aiding, that does make sense.

What I'm not clear on is if the LNAV/VNAV step down fix altitudes need to be honored even if flying the glideslope with WAAS. One's avionics manual may dictate that - I haven't dug through mine to find out. It seems like an overabundance of caution and a relic of when these were flown only baro-aided, but something I'd be willing to do for safety sake. If anyone has a quotable source for an FAA opinion on that, I'd be grateful.

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    $\begingroup$ Welcome to the site. Nice answer. Also feel free asking the last paragraph as a separate question. $\endgroup$
    – user14897
    May 12 at 20:35
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Yes. While APV Baro-VNAV and APV SBAS share similar lateral design criteria, the vertical obstacle clearance surfaces are different. In APV Baro-VNAV procedure design is based on barometric reference as vertical guidance so that is to be followed. If LPV minima is published, also APV SBAS is then considered. In practice, the approach plate is the same with two (or three with LNAV) set of minimums.

At least in Airbus 350, the only equipment I've flown APV SBAS approaches with, there is an option to deselect the "Satellite Landing System" (or SLS) as Airbus calls it. That will leave you with "FMS Landing System" (FLS) which has barometric vertical guidance.

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  • $\begingroup$ Thank you, I understand that, but one question - the "true" LNAV/VNAV approach (with linear guidance all the time, etc.) can only be flown with baro-VNAV yes? If we chose SBAS as our augmentation we will then fly LPV, not LNAV/VNAV right? $\endgroup$
    – Konrad
    Apr 6 at 15:53
  • $\begingroup$ That is correct $\endgroup$
    – busdriver
    Apr 6 at 21:00

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