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mparis

VDP, what is it?

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Since nobody else is throwing topics out there, I'll throw VDP into the mix and share a few nuggets of interest for those working on their IFR rating or just need to brush up on important topics.

So maybe you've seen the big bold V depicted on approach plates and read a bit about VDP, but don't really understand what the practical application is.  VDPs are for non-precision approaches and are used to provide an aid to pilots for establishing a stabilized approach from the MDA to the runway visually.  So does this mean if you are past the VDP at MDA you MUST go missed?  No.  It just means it might not be a bad idea because if you have leveled off at the MDA, the closer you get to the runway, the steeper your descent must be and/or the farther down the runway you're going to be at touchdown.

So before we get much farther into VDP, it's worth mentioning what a stabilized approach means.  An exact definition is a bit hard to come by, but in essence it means you should be making your approach with minimal changes to pitch, power, and course.  In other words the plane should almost be able to land itself with very little correction from the pilot.  Now the airlines and many commercial operators will have much more exact definitions for what constitutes a stabilized approach written into their SOPs, and variances require a go around.  So it's probably worth putting some thought into developing your own standard on what constitutes a stabilized approach and when you're going to go around if you don't have one established, but this is a topic for another day.

Most VDP are set at about a 3 degree glidepath and very roughly a 50' threshold crossing height. Sometimes they are depicted on the chart and sometimes they are not.  I'm not going to go into the reasons why they might not be, but either way you can estimate your own VDP pretty easily if you remember the magic number 300.  318' of height at 1nm equals a 3 degree gradient.  Rounding down to 300 makes the math a little easier as does forgetting about threshold crossing height.  So if at 1nm I'm about 300' above the ground I'm in a pretty good spot for a stabilized approach.  At 2nm and 600', 3nm at 900', and so on.  I'm not much for doing math in the cockpit, but 300' is one of those rules of thumb that comes in handy for all sorts of situations.  The tricky part here is knowing the distance to the runway, which isn't the same as the distance to the airport.  Sometimes the GPS Database gives this to you and sometimes you can figure it off the DME.  Inevitably if you calculate your own VDP when one is listed on the chart, it's going to be slightly farther away from the runway because of rounding errors and failure to consider threshold crossing height, but it gets you in the ballpark.

If you take a look at one of my favorite approach plates, the VOR/DME for GPM 35, you'll see the MDA is 451' above the ground. If we divide this number by 300, you get pretty close to 1.5nm.  So at MDA and 1.5nm from the runway is a pretty good spot to depart the MDA and descend to the runway.  The approach plate lists JIPID at 12.4 DME and 1.9nm to runway.  So using nice round numbers 12 DME is going to be pretty close to our calculated VDP and should be a good spot to depart the MDA for the runway.  Naturally if we have a VGSI (PAPI), this is going to be a better reference, but we don't always have one.

 

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I always thought that you couldn't leave MDA even if you had the runway in sight until you reached VDP based on the FAR definition.  I just re-read the section in the AIM about it.  It says that the VDP is "harmonized" with the lowest MDA and the visual glideslope indicator.  So, that's kind of true because you can't descend below the VASI.  Of course not all runways have a VASI which would make the VDP all that more useful to making a stabilized approach.

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If you have a VGSI in sight then you don’t have to worry about VDP much other than the realization that the useable range of the VGSI is only about 2.2nm from the runway IIRC. There are some airports where the VGSI will actually run you into or near obstructions if you intercept too far out. Still gotta pay attention to those stepdowns. 

Another good reason to pay attention to the VDP is if you start your descent late and don’t break out prior to the VDP. Might as well level off or start climbing, and proceed to the MAP for a missed because the chances of a stabilized approach to the runway are slim. 

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well, you could climb, but you shouldn't turn to follow the missed procedure until you are at the MAP because it might end up in unprotected airspace, right? 

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Yes, you can’t execute the missed approach until you reach the MAP. IIRC, I think you can go as far as 2.5 nm after the MAP and still be guaranteed obstacle clearance provided you can meet or exceed the minimum climb gradient. 

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