Why is this VDP not authorized?

Question

I have to admit that I’ve always kind of struggled with the Visual Descent Point (VDP) concept. Looking at the Terminal Instrument Procedures (TERPS) definition:

“The VDP is a defined point on the final approach course of a non-precision straight-in approach procedure from which normal descent from the MDA to the runway touchdown point may be commenced, provided visual reference is established.”

Fine, I take that to mean that if I start down from Minimum Descent Altitude (MDA) too far after passing the VDP then my descent won’t be “normal”. What I don’t get is when the VDP is “Not Authorized”. Take a look at the RNAV (GPS) Z RWY 24 approach for KTTN. Included in the notes is the statement: “VDP NA with Northeast Philadelphia altimeter setting”.

What does that mean? Why is the VDP not authorized now that the MDA is 60 feet higher? If I didn’t want to descend too late before, I certainly don’t want to go past the VDP when I’m 60 feet higher! Is the note just telling me I better have the required visual references in sight so I can start down BEFORE the VDP? That might not be possible if visibility is at the 1 mile minimum.

@voretaq7:

I understand the implications of altimeter setting from a different airport and how your actual altitude could be higher than indicated.

I guess I'm hung up on what it means to "use" the VDP. Assuming that you use the Dive-n-Drive technique (rather than a constant descent rate), then your statement "Fly the approach until you have the runway environment in sight and then figure out your own glide path intercept based on the visual references you have" is what you are going to do regardless of what altimeter setting you use. In that case, what does it mean to "use" the VDP? In a light piston aircraft with airspeed under control, I may prefer to wait until the VDP and make a steeper than normal descent even if my actual altitude if higher than indicated. That way, in poor visibility, I avoid obstacles that may exist below the MDA prior to the VDP. So exactly what is the FAA saying I'm not authorized to do while using the PNE altimeter setting?

Answer 1

The key is in the part of the procedure description I've highlighted:
When local altimeter setting not received use Northeast Philadelphia altimeter setting -- you're using the altimeter setting for an airport 15 miles away.

15 miles may not seem like a lot of distance, but it's enough for there to be a notceable variation in altimeter settings. The FAA assumes this error will be in the least favorable direction (i.e. your altimeter will indicate you're higher than you really are), and accordingly they take some precautions when you're using another field's altimeter setting.
In this particular case:

1. The decision altitudes (DAs) are higher.
2. Baro-VNAV (vertical guidance for your descent based on your altimeter) is not authorized.
3. The use of the charted Visual Descent Point (VDP) is not authorized.

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The VDP is telling you "If you are on the approach path and remain at the MDA until over this point on the ground you'll be on the glide path to the runway" (or alternatively "You can plan a constant-rate descent that intersects the MDA over this point and you'll be on the glide path to the runway").

Using the charted VDP is specifically not authorized in this case because the nominal descent angle you will have passing through it counts on your altimeter being accurate and your aircraft being at the MDA. Since the MDA has changed and we're using a remote altimeter setting the location of the VDP isn't going to be what's shown on the chart. (For example, if you're 60 feet higher than you "should" be & everything else is perfect the VDP moves further away from the airport to provide the same glide path at a higher altitude).

Rather than charting several visual descent points (which would be futile anyway as you can't know the altimeter error portion of the equation) the FAA just says "Don't use it: Fly the approach until you have the runway environment in sight and then figure out your own glide path intercept based on the visual references you have."