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7 Answers

Monitoring the type of navigation you will use on final.

Asked by: 1052 views Aircraft Systems, Commercial Pilot, FAA Regulations, Flight Instructor, General Aviation, Instrument Rating

As far as I remember, and understand from AIM,  those aircraft with TSO-145, 146 certified WAAS can actually fly the Victor airways using GPS all the way to the destination, fly the approach but monitor the type of navigation to be used on final segment and cannot use the GPS.

On the sim, I flew the VOR approach with GPS overlay on MFD, and we were much offset from the GPS course loaded on MFD.

Yet, I am not sure if my understanding from the text is correct, could you chime in, I am willing to learn more about the advantages of having WAAS on board.


7 Answers

  1. connor on Nov 25, 2016

    Also, if the GPS database contains the approach overlay(G1000 always shows overlay on MFD in white and magenta color), I am understanding from the text that we still cannot use the GPS as providing lateral navigation.

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  2. Mark Kolber on Nov 25, 2016

    As has been mentioned more than a few time in this forum, including in an answer to a question you asked within the past week, there will be discrepancies based on the source of the data and the way it is handled. I think someone even pointed you to the specific AIM section that talks about it.

    In the case of VOR, the discrepancies can get pretty large. VORs require periodic alignment to account for changes in magnetic variation over time. Some VORs have not been realigned in years.

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  3. Best Answer

    John D Collins on Nov 25, 2016


    VOR alignment does not affect the true course, it affects the radial that must be set on the OBS to compute deviations from the true course. MY experience is that the US rarely re-aligns a VOR to match the current variation as many are still using the original variation.


    There are very few overlays of VOR or NDB approaches still available in the US inventory of approach procedures. Only approaches that have “or GPS” are part of the original overlay program and they may be flown with GPS on the final approach segment. The vast majority of VOR/NDB procedures don’t have a GPS option and the GPS map display of such an approach is only available for situational awareness. The VOR or ADF indicator is the primary navigation information and must be followed on the final approach segment. If there is a large discrepancy between what you see on the map and the VOR/ADF indicator, it may make sense to miss the approach and figure out what is going on, A discrepancy of a half a mile is not significant as the primary area for obstacle protection is +/- 1 NM at the VOR and grows to +/- 2.5 NM when 30 NM from the VOR. IOW, there is a reason it is called a non precision approach.

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  4. connor on Nov 25, 2016

    Thank you,and I wish happy thanks giving to all of you.

    Sir, so for the approaches without the “or GPS,” am I still legally allowed to fly the initial and intermediate segment with GPS and switch to primary navigation before flying final segment?


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  5. John D Collins on Nov 25, 2016

    I will give you a hint, the answer to your question is in AC 90-108 and in the AIM in section 1-2-3. Your assignment is to report back what it says.

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  6. Mark Kolber on Nov 26, 2016

    Of course it doesn’t affect true course, John.

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  7. Joshua Clark on Nov 28, 2016

    The AIM changed guidance on this matter in May of this year. There is now a note located in the section which does allow for GPS guidance past the final approach fix for non-overlay appraoches as long as the underlying navaid is operational and can be monitored during the approach.

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