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17 Answers

GPS Overlay Approach -Database Question

Asked by: 12779 views Airspace, FAA Regulations

I know that overlay approaches are almost nonexistent.  However, I am curious about the excerpt below from the AIM.   AIM Excerpt: “On overlay approaches, if no pronounceable five−character name is published for an approach waypoint or fix, it was given a database identifier consisting of letters and numbers. These points will appear in the list of waypoints in the approach procedure database, but may not appear on the approach chart.”

  • Assume for example that the database lists waypoint ABCDE but the paper approach chart does not have waypoint ABCDE
 

Question :  Why would the GPS database have the waypoint ABCDE but the published paper approach plate not have the ABCDE waypoint?

 

 

My thought is that it would make sense to have the waypoints match what is the database to what also on the published paper approach plate. I would be hesitant to fly a GPS approach if the waypoints do not match (for example database has ABCDE waypoint but the paper chart does not have waypoint ABCDE).

17 Answers



  1. Wes Beard on Aug 20, 2013

    This is used not often with with the MAP and/or runway approach end. You will see in the approach database RW01L for the approach end. Every GPS route needs a termination point so to accommodate these GPS overlay approaches the AIM excerpt applies. You won’t see the termination point on most GPS overlay approaches.

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  2. Mark Kolber on Aug 20, 2013

    Dan, I think you’;re missing the point of what these waypoints are for.

    Notice first that this applies to “overlay approaches.” These are almost all gone at this point, but they involve situations where where a GPS approach is overlayed on an existing NDB or VOR approach. In an overlay, nothing is changed about the underlying approach – not any of the courses, distances or waypoints – it’s just that its is permitted to be flown by GPS as well as VOR or NDB.

    So, for an example, let’s take a basic VOR approach (no overlay since those can be hard to find, but bear with me) – the VOR RWY 5 at KLBL (Liberal, KS) – http://155.178.201.160/d-tpp/1308/00498V35.PDF

    Notice that the VOR is on the field. So it’s a basic procedure turn with no FAF. You cross the VOR, go outbound, remain within 10 NM, make the turn and start down.

    Now, keeping in mind that you can’t change anything about the approach, just allow it to be flown with GPS. See the problem? Your GPS database has no idea where you should start the PT. You do, because you can watch the distance, but there is no waypoint in the GPS database that says “start the PT here” or “start down here.” So basically you have an approach that, except for the FAC, has no guidance whatsoever.

    But the hallmark of GPS approach is that it always tells you when to start a PT and when you are crossing FAF. With moving map capability, It even paints the PT for you. Can’t do it here because something is missing – a waypoint.

    That’s what the ABCDE waypoint is for – so the GPS can give you the proper guidance. And it’s not on the approach chart because, well, ultimately it’s just a VOR approach.

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  3. John D Collins on Aug 20, 2013

    Dan,

    Just to add to what Mark and Wes posted, an IFR GPS works differently than a VOR or other ground based Navigation system in several respects. First, a GPS on an approach uses point to point navigation, in other words, it needs to fly a path defined by a beginning and ending waypoint. Second, several things are scheduled to happen at specified distances from the Final Approach waypoint: 1) the CDI changes scale; the approach integrity parameters are changed and tested to meet approach requirements. Finally the final approach is defined as a waypoint. Many existing VOR and other ground based approaches don’t have a Final Approach Fix, or for that matter a leg with a beginning and ending waypoint at the FAF or the MAP. In these cases, waypoints need to be invented to perform these functions using a GPS on an overlay approach. The approaches themselves are not modified, but waypoints are generated so that the GPS can perform them.

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  4. Dan Chitty on Aug 20, 2013

    Mark and Wes,

    Thank you for the information.

    In summary and using for example the VOR 5 KLBL (and let’s pretend this is a overlay approach), the database lists ABCDE to begin the procedure turn.

    In other words, ABCDE would begin over the VOR since the procedure turn is predicated on the distance from the VOR. The GPS would count the distance from ABCDE of which I would monitor the 10NM distance from for the procedure turn distance limit since this is not a stand alone GPS approach or RNAV (GPS) approach.

    Let me know your thoughts.I greatly appreciate your kind assistance.

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  5. Dan Chitty on Aug 20, 2013

    John,

    Thank you also for your comments.

    If I need to invent a FAF I guess that I would simply use the distances for the FAF and MAP of which the underlying approach) VOR or NDB for example) is based upon.

    Is my assumption correct?

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  6. John D Collins on Aug 21, 2013

    No, its not you that needs to invent a FAF, the GPS requires one for its proper functioning. Not all approaches have a FAF, but all approaches flown by a GPS need one or in lieu of one, need a waypoint that serves the same function. Without one, the GPS will not sequence from terminal mode to approach mode and all that goes with that transition.

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  7. Mark Kolber on Aug 21, 2013

    As John says, you don’t need to invent an FAF. It’s already been invented and is in the GPS database. Perhaps you can locate one of the remaining overlay approaches (the have names like “VOR OR GPS RWY…” where the VOR or NDB navaid is on the airport for a real illustration.

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  8. Dan Chitty on Aug 21, 2013

    I was able to locate an active overlay approach for our purposes: Reference (KGGE) NDB /GPS RWY 5. This approach has the NDB located on the airport.

    1. There is CNF (FIVPA). Does FIVPA serve as the FAF for the GPS database? I am assuming the GPS would link up with FIVPA for the correct CDI sensitivity changes (terminal and approach mode) as well as any other GPS parameters.

    2. If CNF (FIVPA) did not exist (published) on the approach plate, what does the pilot need to do to program the GPS for the proper CDI sensitivity changes and other parameters? In this case, is it possible to have FIVPA appear in the GPS database but not published on the approach plate of which is the reason the AIM excerpt I have listed at the beginning of this post?

    Also I was able to locate another active overlay approach for our purposes that does not have the navaid located on the airport: Reference (KSMO) VOR /GPS –A

    1. Is there any special GPS programming the pilot needs to do since the FAF waypoint (BEVEY) is published on the approach plate?

    Thank you John, Wes, and Mark for the help .

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  9. Wes Beard on Aug 21, 2013

    A good example is KCOS NDB 35L approach. On Jeppesen plates the MAP is called RW35L. On NOS charts, the verbiage is missing bit I would guarantee you if you loaded the approach into the GPS you would see PETEY or CO and then RW35L.

    You must use the NDB for lateral guidance but the GPS can be used as a backup yo the NDB.

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  10. Wes Beard on Aug 21, 2013

    Another approach. K62 VOR or GPS-A approach. The MAP is [MAJIB] on NOS charts. This approach you can use GPS as the sole lateral guidance since GPS is in the approach title. The VOR doesn’t even have to be operational.

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  11. Mark Kolber on Aug 21, 2013

    Perfect example, Dan.

    But…

    . Is there any special GPS programming the pilot needs to do since the FAF waypoint (BEVEY) is published on the approach plate?

    John and I already answered this question. There is no special GPS programming the pilot needs to do. Whether the way point is on the plate or not is completely irrelevant.

    Here’s two screen prints of the approach in a GNS530. One is the approach loaded; the other is the moving map with the approach shown.

    That’s right out of the box. No special programming involved. No entry of user waypoints. Doesn’t matter if the IFR GPS is Garmin, Avidyne, King or…. If FIVPA were not shown on the chart, the GNS depiction would be exactly the same.

    Next time yo have a chance, load the approach into your IFR GPS unity and see for yourself.

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  12. Mark Kolber on Aug 21, 2013

    This forum doesn’t accept embedded images, so here are the links

    http://midlifeflight.us/posted/GNS530_FIVPA1.jpg

    http://midlifeflight.us/posted/GNS530_FIVPA2.jpg

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  13. Dan Chitty on Aug 21, 2013

    Thank you Mark.

    Can you send the screen shots to my personal email: danchitty@yahoo.com.

    I do not see the screen shots on this post.

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  14. Mark Kolber on Aug 21, 2013

    Click of the links.

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  15. Dan Chitty on Aug 21, 2013

    Excellent post. Thank you all.

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  16. Dan Chitty on Aug 21, 2013

    One additional question that I did not specifically ask:

    I know FIVPA is a CNF, but is its only fuction to act as point to activate the GPS approach mode? Also, any reason FIVPA is 4NM from the NDB?

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  17. Dan Chitty on Aug 23, 2013

    Additional questions that I did not specifically ask:

    I know FIVPA is a CNF, but is its only fuction to act as point to activate the GPS approach mode?

    Is FIVPA actually defined a “sensor FAF”?

    Also, any reason FIVPA is 4NM from the NDB?

    Since the GPS approach mode is supposed to scale the CDI to the proper approach mode parameters 2NM from the FAF, I guess the GPS approach mode would activate 6NM from the NDB in this case.

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