Please explain VDP

[daysleeper2621]

I know that VDP stands for "Vertical Descent Profile" or something along those lines, but I don't understand how you're able to calculate it with only a DME distance and a time. I keep running across this question being asked during interviews and I have never in my life had to calculate it since I've never flown an aircraft that had the ability to fly VNAV/LNAV GPS approaches. I thought only aircraft equipped with an FMS or WAAS capable GPS could fly these approaches, but regardless it seems that you have to know how to calculate it even if you can't legally use it. Thanks for any help guys.

 

[Andy Neill]

At one operator, the calculation is HAT/300 to get an adjustment in miles or 10% of HAT to get an adjustment in seconds. For instance, if the HAT is 600 feet, the VDP would be adjusted 2.0 miles from the runway or the MAP time would be adjusted 60 seconds. The intent is to give you an approximate 3 degree angle from the VDP to the runway.

 

[smfav8r]

Visual Descent Point...you can use it on any non-precision appr....some of the newer jepp plates have the VDP already calculated for you....less mental math. It's the point on a non-precision appr where you are in a stabalized position to land. 3 to 1, etc.

In other words, (if the MDA is 600ft HATand the missed approach point is the runway),if you are at 600 ft HAT at 2 miles from the end of the runway and you have the lights in sight, you are in a position to land.....if at 1 mile from the end of the runway, you are at 600 ft HAT and still nothing, your not in a position to land! It's just a way to stay ahead of the aircraft on a non-precision appr to be ready to go around at the missed approach point.

 

[daysleeper2621]

What if you're only given a DME distance and the MAP is at the end of the runway, does that make a difference in how to calculate VDP?

 

[Andy Neill]

Again, if the MDA is 600 above touchdown and the MAP at the end of the runway (say 4.5 from the Final Approach Fix) then the VDP would be at 2.5 from the FAF putting you 600 feet above touchdown and 2.0 miles from the end of the runway.

 

[Draginass]

You mean "Visual Descent Point". It's the point at which you can begin a normal descent (usually 3 deg gradient, which is the same as a typical ILS) out of the Minimum Descent Altitude (MDA). The VDP has nothing to do with the missed approach point.

If, say, the MDA is 500' HAT, then divide 3 (deg descent gradient) into 500 (HAT), which equals 1.7 miles. Your VDP is 1.7 miles from the approach end of the runway. If there is a VOR/DME associated with the approach you can use that and figure out what DME readout corresponds to your VDP. If, for instance the VOR was mid-runway on a 12,000' runway, then that's about a mile from the approach end. So, add 1 mile to the approach end + 1.7 miles from the approach end to the VDP = 2.7 miles. So your VDP is at 2.7 miles on the DME readout. Of course, due to the location of the VOR and the "real" touchdown point about 1000' down the runway, the VDP calculation is not truely exact, but gives you a good approximation of a desired descent path out of the MDA.

 

[smfav8r]

Maybe I'm wrong, but I thought I said the same thing...

 

[Doc Holiday]

Technically, a VDP cannot be calculated by a pilot. It is published on certain IAP charts. While this can be considered a case of symantics, what many of you are referring to some operators call a PDP (planned descent point) or TTS (time to see).

According to the AIM 5-4-5(i), when a VDP is not published on the approach chart, terrain clearance has not been assessed. Therefore if you calculate your own "VDP", it may not necessarily be safe to desend below MDA at that point.

I'm not saying that PDPs/TTSs are a bad idea, not at all. They are excellent for situation awareness and thinking ahead to ensure a safe rate of descent to the runway will be used. Just don't let your calculated "VDP" lure you into a false sense of security when doing nonprecision approaches, especially if the weather is anything less than stellar. The PDP/TTS is generally used as decision point for if you can safely complete the approach straight-in, or if circling would be necessary to legally complete the approach.

 

[Draginass]

Doc - You are correct. Technically, a self-calculated VDP is a PDP. My bad for not making that clear. If there is a VASI or PAPI (visual guidance indicator), etc, an electronic glideslope, or an RNAV approach with a published decision altitude, an obstacle assessment has been done.

 

[LR25]

VDP's are great, I actually used one last month on a VOR APP. But that was only the second time in almost 2 years I actually used one other than the sim.

 

[FL420]

Concur w/Doc & Draginass

Technically, a VDP cannot be calculated by a pilot. It is published on certain IAP charts. While this can be considered a case of symantics, what many of you are referring to some operators call a PDP (planned descent point) or TTS (time to see).

According to the AIM 5-4-5(i), when a VDP is not published on the approach chart, terrain clearance has not been assessed. Therefore if you calculate your own "VDP", it may not necessarily be safe to desend below MDA at that point.

I'm not saying that PDPs/TTSs are a bad idea, not at all. They are excellent for situation awareness and thinking ahead to ensure a safe rate of descent to the runway will be used. Just don't let your calculated "VDP" lure you into a false sense of security when doing nonprecision approaches, especially if the weather is anything less than stellar. The PDP/TTS is generally used as decision point for if you can safely complete the approach straight-in, or if circling would be necessary to legally complete the approach.

Concur w/Doc & Draginass. Hope everyone reads your posts and thoroughly understands the difference between a VDP and a PDP before trying to use a PDP in marginal weather. Get some practice in the sim or in VMC before using a PDP for real.

 

[Draginass]

Having a VDP/PDP is critical for jets. Delaying descent past the VDP/PDP will result in the need for a high descent rate and perhaps un-spooled engines to make the touchdown zone, which is exceedingly dangerous. Light aircraft are much slower and more manueverable.

 

[Doc Holiday]

If there is a VASI or PAPI (visual guidance indicator), etc, an electronic glideslope, or an RNAV approach with a published decision altitude, an obstacle assessment has been done.

That is an excellent point.

 

[MauleSkinner]

Doc - If there is a VASI or PAPI (visual guidance indicator), etc, an electronic glideslope, or an RNAV approach with a published decision altitude, an obstacle assessment has been done.

Actually, if there is ANY instrument approach, an obstacle assessment has been done...no obstacles penetrate the 40:1 slope (unless otherwise published), which is roughly 1.5 degrees.

Or is there a separate assessment that you're referring to?

Fly safe!

David

 

[FL420]

Actually, if there is ANY instrument approach, an obstacle assessment has been done...no obstacles penetrate the 40:1 slope (unless otherwise published), which is roughly 1.5 degrees.

Or is there a separate assessment that you're referring to?

Fly safe!

David

Isn't an obstacle penetrating the "plane" the reason a VDP is charted IAW TERPS(i.e.-you can't leave MDA until passing the charted VDP?)

 

[Doc Holiday]

AIM 5-4-5(i)(2) "Approaches without VDPs have not been assessed for terrain clearance below the MDA, and may not provide a clear vertical path to the runway at the normally expected descent angle."

AIM 5-4-5(g) "...procedure designers perform a visual area obstruction evaluation.....Restrictions to instrument operations are imposed if penetrations of the obstruction clearance surfaces exist. These restrictions vary based on the severity of the penetrations, and may include increasing required visibility, denying VDPs and prohibiting night operations to the runway."

FL420, you may very well be right about the reason a VDP is charted. The way I see it, the key words in the above paragraph are "may include", as in it is not an all inclusive list. If anyone has more info on this please share it.