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Old 9th Oct 2014, 15:09
  #6 (permalink)  
Join Date: Jun 2010
Location: Canada
Age: 34
Posts: 382
I don't think we can, but then I'd like someone with a little more experience to comment on this, especially with regards the reality of commercial aviation vs. what the book says.
Reality should match the book but it doesn't always.

I'll give an example of an NDB approach in Canada that is NOT approved for a GPS overlay.

Example approach is the NDB RWY 01 into CZTM. For final approach track guidance you're required to use the NDB raw data as your primary navigation source. This airport is located where there are magnetic disturbances in the area. If you relied entirely on your RMI, you could be way off your inbound track which is not a problem from an obstacle standpoint because they've taken that into account and given you a very high MDA. It can throw you off though when you're expecting the runway to be a certain way in the windscreen when you break out and it's not.

Anyways... I would fly that approach using GPS as my primary navigation and have the NDB as a back up. It's silly to think that the NDB is going to provide you better navigational accuracy... it will most certainly not!! I'll OBS the final approach course on the NDB and use that to fly the approach.

If the NDB is NOTAMd to be u/s then I won't do the approach. Following the rules makes your life a lot simpler. There is no decision making here - you're not doing the approach. Even though we all know that it's much safer to fly it based off the GPS anyways - even with en route RAIM and CDI sensitivity!

I should clarify that I do still monitor the morse code and do timing for approaches where an overlay is not approved.

As far as why you can't do an overlay approach without it being published has to do with the approach needing to be in the database.

To add to what aterpster said:

If a VOR or NDB approach is not coded in the database as an approach, the FMS will remain in terminal mode sensitivity, alerting, and monitoring. You can force the sensitivity to 0.30, but you will still be in terminal mode.
You can force the CDI to 0.3 but you can't force the RAIM to go to 0.3 which is what you require to descend to altitudes in the final approach segment.

When an overlay approach is made, sometimes they'll add a GPS waypoint to act as a FAF when there originally wasn't one.

Example approach is the NDB RWY 31 (GNSS) at CYAV. There is no intermediate segment and no FAF. MUVRI has been added so that the approach will work on the GPS as it requires a FAF. Within two miles of MUVRI, the approach will go ACTV when the automatic check of approach-level RAIM (0.3) at the FAF and MAP says approach-level RAIM will be available. This is where CDI will transition from 1.0 to 0.3 at the FAF so that you fly the entire final segment with 0.3.

Technically, if you're doing the overlay, you can't descend to the MDA until past MUVRI as you require the CDI and RAIM to be at 0.3 and the CDI won't reach 0.3 until at the FAF. In that case you could look at your RMI and see that you're within 5 degrees of your final approach course and descend to the MDA. It's a bit awkward when there isn't a FAF to begin with.

Specifically as far as your approach goes, it sounds like you should have discontinued the approach and flown a different one. Possibly request vectors to intercept the ILS.

Didn't we just have a nice, exhaustive thread on overlay approaches? What happened to it?
There was one. Dunno what happened to it. There was a lot of good discussion in it.
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