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4 Answers

Why RNP approaches use Baro VNAV?

Asked by: 947 views General Aviation

I have been studying questions on this site, and just found this explanation from John in a post: "At this point, the WAAS GNS and G1000 do not support approaches that are RNAV (RNP) and they are not in the database. These approaches do not use WAAS for vertical guidance, but use Baro VNAV (Link: http://www.askacfi.com/25241/challenging-approach-plate-for-me-kmso.htm)."

This got me curious. Why RNP approaches use Baro VNAV when they can use WAAS, which is more reliable and accurate and thus provides lower minimums. As I see it, RNP approaches offers greater lateral accuracies than WAAS approaches, and that might be the reason why the FAA requires additional instrument and special pilot training to shoot RNP approaches. But if you look at RNP charts, you easily find that RNP charts provide higher minimums compared to those of WAAS, and I kind of feel it betrays the purpose of the development of RNP approaches, which I assume is to provide greater accuracy than WAAS approaches.

So is there any good reasons that RNP approaches use Baro NAV instead of WAAS's electrical signal for their vertical guidance?  

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4 Answers

  1. John D Collins on Jan 19, 2017

    The airlines are for the most part not equipped with WAAS, so vertical guidance using WAAS is not available as an option for many air carrier aircraft. Pretty much all air carriers have Baro VNAV. Most airlines are planning on skipping WAAS and going to the next generation of dual frequency GPS using L1 and L5. Note that the WAAS or its generic capability SBAS is not available thru out the world and it would limit the usability of the equipment.

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  2. Lemontree on Jan 19, 2017

    Thank you John. Let me ask a follow-up question. I know some special equipment and pilot training are required to fly RNP approaches, but what is that special equiment? I know one of them is a FMS but what else? Can a Baro NAV be replaced with WAAS for vertical guidance in airports where the WAAS system is accommodated?

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  3. Best Answer

    John D Collins on Jan 20, 2017

    No, WAAS is approved as an alternate vertical guidance for RNAV (GPS) approaches with LNAV/VNAV minimums. WAAS vertical Nav is not approved for vertical guidance on RNAV (RNP) approach procedures.

    The special equipment is certified for the RNP specification. See AC 90-101A for details. The RNAV (RNP) procedures need to be in the database and both the aircraft and crew need to be authorized to fly these procedures. The aircraft has to have the approved equipment. Only the airlines and other certificated carriers, possibly some business jets have this type of equipment.

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  4. Lemontree on Jan 20, 2017

    Thanks John but I’m still a little confused. If you look at the AIM 1-2-2, b-(a)-(2) RNP AP APCH, it states “RNP AR APCH vertical navigation performance is based upon barometric VNAV or WAAS.” I found this after I asked you this question. Maybe there’s a way to use WAAS for RNP but it’s rarely used.

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