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VDA dangers.

The recent changes made to certain RNAV approaches by the FAA are shameful at best. Not only was a TERPS mistake made in the first place with approaches for which the VDA is not obstacle-free below MDA (which it should have been), but the correction was made virtually in secret, and takes away the very desirable vertical guidance in many cases. So, shame on them.

But, turning to the June, 2013 issue of IFR for clarity and understanding, I found little, if any, help at all. You guys are supposed to make it all clear. What I found was an acronym jungle with unexplained visual aids that left me as confused as before.

Why not simply raise the VDA a little so that obstacles below MDA are safely cleared. Maybe a little higher minimums, but still having the CDFA (acronym backatcha) option in play?

Charles Clarkson

St Petersburg, Florida

You may have a basic misunderstanding of how this all works.

If an approach provides guidance to ... somewhere, the path to that somewhere is guaranteed obstacle-free within certain criteria. So, there are not going to be any obstacles between the FAF and the MDA along the prescribed path and altitudes. You're safe.

However, the point we made across two articles was that if you choose to descend below MDA, the glideslope-like path that is a mathematical construct of your navigator is not necessarily going to be clear. It never has been.

The FAA evaluates each approach according to strict rules. In a nonprecision approach (to an MDA), the path is clear to the MDA. But, when you make it to MDA per [section] 91.175 (b) (3) (c) you can only descend below MDA visually, without navigation guidance. For that reason, the path below MDA to the runway may or may not have the same obstacle protection as an approach that has a surveyed physical glidepath down to the runway.

We disagree with your premise that a VDA should have guaranteed clearance below MDA. Remember, these are non precision approaches that have no glidepath. The VDA (or a CDFA, if you're certified to use one) is a mathematical construct to provide a constant rate of descent from the FAF to the MDA, not the runway.

Raising the VDA wouldn't solve the problem of obstacles existing along the projected VDA between the MDA and the runway because there is no obstacle clearance surface associated with the VDA--remember, it's a mathematical construct, not a surveyed slope.

Increasing the VDA on approaches with visual area penetrations would affect multiple aspects of the approaches, such as limiting approach categories and increasing many other altitudes along the approach. Actually, if there were clearance surfaces associated with the VDA, considering how many approaches have obstacle penetrations in their visual area, many nonprecision approaches would probably cease to exist.

We're sorry you found the acronyms confusing. Unfortunately, those are the acronyms the FAA is using and we defined each of the new/uncommon ones. We certainly didn't create any of our own.

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Title Annotation:READBACK
Article Type:Letter to the editor
Date:Oct 1, 2013
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