Minimum Descent Altitude and How to Get There

by Wally Moran on May 26, 2010

In the Pilotworkshops.com IFR Proficiency series, titled “Descending on a non precision approach” respected flight instructor Rod Machado explains the benefit of descending to MDA early on non precision approaches. Rod makes the point that getting to the minimum descent altitude (MDA) prior to the visual descent point (VDP) allows the pilot a greater opportunity to see the required visual references during low visibility and therefore a better chance of landing. Rod advocates a dive and drive technique rather than a stabilized approach that plans to reach MDA at or near the VDP. He further advocates, in some cases, descending below the GPS advisory vertical guidance where available to get to MDA sooner.

While I realize disagreeing with Rod Machado about flying is like arguing with the Pope about religion, I would like to present a case for making all of our approaches as stabilized as possible and never descending below the GPS advisory vertical guidance when executing a non precision approach.

It is, of course necessary, to arrive at MDA in time to make a normal approach and landing but, in my view, there is little advantage to being there well before the VDP. This simply puts the pilot in the most dangerous area of the approach for a longer time. Yes, charted minimums provide protection but, a mistake at MDA gives little time for correction while a mistake at a higher altitude leaves greater margin. The lower one goes, the greater chance of hitting things that are fastened to the ground, therefore why be in such a hurry to get down there.

I prefer to begin my descent with a good solid 750 FPM. Since that is typically more than needed, I can shallow the descent as I approach MDA and the VDP if one is available. Then as I break out, I merely adjust my descent for landing as I would on any approach. In my view, this makes a much more stabilized approach. While the dive and drive technique may get you in one day when the visibility is at the very minimum, I am happy to give up that small advantage for the benefit of a more stabilized approach. A dive and drive approach is by definition unstable at least until the VDP.

Should the approach have an advisory glide slope as many do now, as an instructor, I would not teach to deliberately fly below it. First of all teaching to fly below a glide slope is just encouraging a bad habit and not a good thing to teach. Further we know that approaches with vertical guidance have less risk than those without vertical guidance so why give up a margin of safety if you can avoid it.

History has shown that non precision approaches with step down procedures, that is, intermediate level offs prior to MDA, are the most dangerous. Following the advisory glide slope if available will guarantee that one complies with those restrictions. That, if nothing else, will keep me on that glide slope.

So with all due respect to the Pope of our church of flying things, I prefer to make a stabilized approach adjusting my descent to arrive at MDA as near to the VDP as possible and do  not advocate deliberately going below the advisory glide slope.

What say others?

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