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WAAS GPS Fail Over

Asked by: 12623 views Flight Instructor

I'm working on my CFI-I, trying to sort out the various information on WAAS approaches, cross-walking references published over the last five years that may not have kept up with the most current changes.

Are there "fail down" criteria for a WAAS Approach with Vertical Guidance? 

I believe that if vertical guidance precision degrades on an LPV approach but LNAV/VNAV criteria are still met, the approach can be continued to DA. 

However, I also think that if vertical guidance degrades on an APV approach to the point where there is no VNAV guidance, an immediate missed approach is required, even if the airplane is above LNAV MDA. 

In other words, are there "fail down" criteria for VNAV that permit the approach to continue as an LNAV only approach? 

My guess is that an LPV can "fail down" to LNAV/VNAV, but neither of these WAAS APV approaches can "fail down" to an LNAV approach even above MDA. 

Is there an AIM or other authority that explains this?  Thanks!

2 Answers



  1. Mark Kolber on Jul 12, 2014

    There’s an old saying: “The more things change the more they remain the same.”

    Now, in the world of GPS you are asking the exact same question that has been discussed for decades about an ILS/LOC approach when the glideslope fails.

    In fact, righr here, on this very forum, is a discussion on the subject about 2 years ago: http://www.askacfi.com/5485/loss-of-glide-slope-on-ils-approach.htm

    The same considerations apply, There’s nothing that requires one action or the other. If the approach was simple with no stepdowns, weather indicated I would break out well above LNAV minimums and the failure came early, I’d consider continuing the approach. In any other situation, I’d prefer to regroup while on a missed approach delay vector or hold.

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  2. John D Collins on Jul 12, 2014

    The reference is in your AFM for the WAAS GPS. There is only downgrade to LNAV without vertical guidance from an LPV or LNAV/VNAV, or advisory vertical guidance on an LNAV or LP. The WAAS GPS will fail down to LNAV on a vertically guided approach at the point where the aircraft is one minute from the FAF. Prior to that point, if the integrity does not support vertical guidance, the LPV will annunciate with a yellow (amber) color rather than the normal green color.

    Here is the relevant portion of my GNS530W AFMS:

    3.2.2 GPS APPROACH DOWNGRADE
    During a GPS LPV, LNAV/VNAV, LP +V, or LNAV+V approach, if GPS accuracy requirements cannot be met by the GPS receiver prior to the Final Approach Fix, the GNS will downgrade the approach. The downgrade will remove vertical deviation indication from the VDI and change the approach annunciation accordingly from LPV, L/VNAV, LP +V, or LNAV+V to LNAV. The approach may be continued using the LNAV only minimums. After the Final Approach Fix has been passed, the approach will be aborted using the indications described below.

    During a GPS approach in which GPS accuracy requirements cannot be met by the GPS receiver for any GPS approach type, the GNS will flag all CDI guidance and display a system message “ABORT APPROACH – Loss of Navigation”. Immediately upon viewing the message, the unit will revert to Terminal navigation mode alarm limits. If the position integrity is within these limits lateral guidance will be restored and the GPS may be used to execute the missed approach, otherwise alternate means of navigation must be utilized.

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