CFII Debate - GPS in lieu of DME

Sluggo63 said:
This isn't quite true either (if you're talking about the published VDP). If there is more than one line of minima on an IAP, the VDP that is published is based on the lowest MDA. If there are minima lines for Cat's A-D and you're in a Cat D airplane, the published VDP is for the Cat A minima. Waiting until you're at the VDP may put you late descending. You'd have to start down before the published VDP to have a normal descent to the runway.
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I don’t understand this. If you backdraw a 3° slope (like an ILS) the glide slope will be the same for every airplane.

ETA: That’s more for a homemade VDP. A published one I always thought was just to get you past an obstacle, not necessarily provide a 3° slope.
 
Kritchlow said:
I don’t understand this. If you backdraw a 3° slope (like an ILS) the glide slope will be the same for every airplane.

ETA: That’s more for a homemade VDP. A published one I always thought was just to get you past an obstacle, not necessarily provide a 3° slope.
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A VDP has nothing to do with clearing an obstacle. It is purely a math formula to determine how far away from the runway the published descent angle intersects the MDA. There are obstacle-related reasons why a VDP would NOT be published, but they have nothing to do with making it over an obstacle before descending. That is what a stepdown fix is for, but the confusion between the two is very common among my instrument students.

Since the location of the VDP is dependent on the MDA being used to calculate it, @Sluggo63 is correct. However, there are very few straight-in lines of minima where the MDA is different between categories. It can happen, but it is rare.

More common is the situation where both LP and LNAV lines of minima are published. LP, having a lower MDA, would control the VDP. However, LP lines of minima aren't really that common either.
 
RussR said:
A VDP has nothing to do with clearing an obstacle. It is purely a math formula to determine how far away from the runway the published descent angle intersects the MDA. There are obstacle-related reasons why a VDP would NOT be published, but they have nothing to do with making it over an obstacle before descending. That is what a stepdown fix is for, but the confusion between the two is very common among my instrument students.

Since the location of the VDP is dependent on the MDA being used to calculate it, @Sluggo63 is correct. However, there are very few straight-in lines of minima where the MDA is different between categories. It can happen, but it is rare.

More common is the situation where both LP and LNAV lines of minima are published. LP, having a lower MDA, would control the VDP. However, LP lines of minima aren't really that common either.
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"A VDP has nothing to do with clearing an obstacle."??? The way I understand it is their reason for being invented in the first place was obstacle avoidance. The problem being solved was folk breaking out of the goo, seeing the airport, descending and getting lower than necessary/safe. It was the 'chop and drop' or 'dive and drive' scenario where this would happen. Stepdown fixes would of course give absolute clearance over an obstacle. But inside the last Stepdown fix, which may have been the FAF itself, seeing the airport quite aways out in good visibility made an 'illusion' that led to premature descents. The VDP was created to let you know that it's probably a good idea to hold your altitude at or above the MDA for awhile until you get to a point(VDP) where a normal final descent should begin.
 
Here's what the AIM says about a charted VDP:

"The Visual Descent Point (VDP), identified by the symbol (V), is a defined point on the final approach course of a nonprecision straight−in approach procedure from which a stabilized visual descent from the MDA to the runway touchdown point may be commenced. The pilot should not descend below the MDA prior to reaching the VDP. The VDP will be identified by DME or RNAV along−track distance to the MAP. The VDP distance is based on the lowest MDA published on the IAP and harmonized with the angle of the visual glide slope indicator (VGSI) (if installed) or the procedure VDA (if no VGSI is installed). A VDP may not be published under certain circumstances which may result in a destabilized descent between the MDA and the runway touchdown point. Such circumstances include an obstacle penetrating the visual surface between the MDA and runway threshold, lack of distance measuring capability, or the procedure design prevents a VDP to be identified." (emphasis added)
 
luvflyin said:
"A VDP has nothing to do with clearing an obstacle."???
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I apologize for the confusion. I spoke loosely, focusing on @Kritchlow 's statement about "getting you past an obstacle". A stepdown fix is what does that. A VDP is not moved around depending on where the obstacles are. That was my point, it's a simple calculation. Now, whether or not a VDP is actually published is a separate issue, which Wally describes above, and does involve obstacles. But once an MDA is determined, the placement of the VDP has nothing to do with obstacles.
 
RussR said:
I apologize for the confusion. I spoke loosely, focusing on @Kritchlow 's statement about "getting you past an obstacle". A stepdown fix is what does that. A VDP is not moved around depending on where the obstacles are. That was my point, it's a simple calculation. Now, whether or not a VDP is actually published is a separate issue, which Wally describes above, and does involve obstacles. But once an MDA is determined, the placement of the VDP has nothing to do with obstacles.
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“Getting past an obstacle” was too loosely worded on my part. What I meant was it’s a normal final approach path, which as far as I know will keep you clear of obstacles.
Point being the slope to the runway will be the same for all aircraft regardless of type.
 
Kritchlow said:
“Getting past an obstacle” was too loosely worded on my part. What I meant was it’s a normal final approach path, which as far as I know will keep you clear of obstacles.
Point being the slope to the runway will be the same for all aircraft regardless of type.
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And, if you have a Garmin, or such, with the vector-path green donut, nosing down at a charted VDP to the runway will assure you of not hitting anything.
 
Kritchlow said:
“Getting past an obstacle” was too loosely worded on my part. What I meant was it’s a normal final approach path, which as far as I know will keep you clear of obstacles.
Point being the slope to the runway will be the same for all aircraft regardless of type.
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Gotcha. I hadn't read the recent posts before replying to yours. The "..nothing to do with clearing an obstacle" set me off. I get it now it was about comparing a VDP to a 'hard five letter word TERP'd out step down fix' for lack of a better word.
 
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