# Why is this VDP not authorized?

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?

• You can always calculate (and then use) your own VDP; with the MDA being 60' higher than before, the new VDP would be about 0.2 NM further out. The chart is just saying that you can't use the published VDP if your MDA doesn't match the published MDA. – Ralph J Jul 28 '15 at 22:44
• But, what does "use" the VDP mean? Is the FAA not saying I'm not authorized to wait until the published VDP to descend below MDA if I have the PNE altimeter setting? So I MUST descend before the published VDP? Exactly what action is "Not Authorized"? – Greg Valvo Jul 28 '15 at 22:46
• It just means, don't use what's printed on the chart as authoritative. It isn't telling you that you must descend before or that you can only descend after or anything like that. It's just saying, don't blindly rely on the printed "1.2 DME" value, because that only works for the 620' MDA. If you back up your visual references with a VDP that you calculated (probably about 1.4 DME at the higher MDA), that's fine, and it's always your option. What you aren't using is their # -- not that you're disallowed from using an appropriately calculated VDP. – Ralph J Jul 28 '15 at 22:53
• That makes perfect sense and I'd be happy with your answer if "NA" in the notes meant "Not Applicable". But, as far as I know, it means "Not Authorized". The FAA is usually not wishy-washy when it comes to what's Authorized or not when it comes to IAPs. Hence, my confusion about why I wouldn't be Authorized to "use" (descend from) the VDP in this case. – Greg Valvo Jul 28 '15 at 23:02
• I think "how do I use a VDP?" would make a good separate question, since the original one has an answer now :) – egid Jul 29 '15 at 6:05

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.

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."

• Please see edit to my original post. It was too long to add as a comment to your answer. – Greg Valvo Jul 28 '15 at 22:37
• Even with the local altimeter setting, an altimeter can still be 75' off & still be with tolerance, so altimetery errors are always a bit of an unknown. A VDP ensures that you're in the right ballpark to start your descent, and not biting off on illusory visual cues too soon or waiting until unsafely steep to start the descent. The salient point is the effect of the higher MDA on the VDP, which was nicely explained. Pilots always have the option to back up visual references with a calculated VDP (even if none is published) -- just don't rely on one calculated for a different MDA! – Ralph J Jul 28 '15 at 22:48
• @GregValvo I tweaked the answer a little to clarify some bits, but like egid said you may want to ask a separate "How do you use a VDP?" question where we can go into a lot more detail (calculated vs. charted for starters). The short answer is you're kind of always "using a VDP" to some extent: when you have the runway in sight at the MDA & fly along until the PAPI lights say you've intercepted the glide path that's "the visual descent point". You just might not be using the one on the cart that the FAA calculated for you. – voretaq7 Jul 29 '15 at 16:41