PDP vs. VDP

[blee256]

What is PDP? I've asked several people and most dont have a clue. I've tried to look it up, but still no luck. So i bring it to the geniuses at Jetcareers. haha. Is it profile descent procedure? Planned Descent point? and How does PDP differ from VDP. I already know VDP and how to calculate it, so I'll save you guys the time and trouble of explaining that.

[FOD]

Dont recall, but pretty sure its planned descent poing!

[cime_sp]

One stands for planned descent point and the other is visual descent point. A PDP can be calculated for anything. These are the "rules of thumb" you hear about alot....the whole altitude to lose in thousands * 3 for descent from altitude...or the HAT divided by 3 for approaches. The VDP uses the standard 3 degree glidepath (usually) and projects it up to where it meets minimums.

Now for my airline we don't really ever use the published VDP's because we round our minimums to the next highest 100 foot increment. also we usually fly appeoaches in white needles (FMS/GPS) which uses GPS or DME/DME based distance which can be different from VOR/DME.

[pscraig]

As previously said, VDP can either be charted or derived from a formula (HAT/300).

A PDP is a planned descent point, and isn't a specific angle. I use the formula that you should have 300' of altitude for every nautical mile from the runway, and it usually works unless there is a strong wind.

At XJT we can't do FMS approaches yet, but will very soon. Can't wait!

[Doug Taylor]

PS and CIME hit it "spot on".

Most schools teach the 'dive and drive' method of non-precision approaches. Dive down to MDA, level out and wait until the MAP to start the transition from approach to missed approach.

On the surface, the procedure is correct, but once you're not longer at a point where you can execute a normal landing, you're really supposed to be setting up and/or executing the missed.

I almost shot a LOC-BC into MLB a few days ago, but realized that I'm much better at visuals than I am with weird NPAP's (Non-Precision Approach Procedures).

[kellwolf]

Even though I've been taught the "dive bomb" approaches on non-precision approaches, I agree with Doug. It's far more beneficial (even if it is extra cranial exercise) to plan a descent to MDA to reach MDA when you hit the MAP. Tough to do on a lot of approaches in Orlando due to the overlying Class B airspace. Lots of mandatory alt (the kind with double lines) on a lot of approaches. But it's a lot better to hit the MAP at MDA of 800 on that VOR 7 into ORL and then letting the plane continue down than saying "MAP, runway in sight, chop and drop."

[moxiepilot]

[ QUOTE ]
What is PDP? I've asked several people and most dont have a clue. I've tried to look it up, but still no luck. So i bring it to the geniuses at Jetcareers. haha. Is it profile descent procedure? Planned Descent point? and How does PDP differ from VDP. I already know VDP and how to calculate it, so I'll save you guys the time and trouble of explaining that.

[/ QUOTE ]

If you have access to the book "Mental Math for Pilots" by Ronald McElroy, chapter 4 does a good job of explaining both and including how to calculate them.

[SteveC]

[ QUOTE ]
Even though I've been taught the "dive bomb" approaches on non-precision approaches, I agree with Doug. It's far more beneficial (even if it is extra cranial exercise) to plan a descent to MDA to reach MDA when you hit the MAP. Tough to do on a lot of approaches in Orlando due to the overlying Class B airspace. Lots of mandatory alt (the kind with double lines) on a lot of approaches. But it's a lot better to hit the MAP at MDA of 800 on that VOR 7 into ORL and then letting the plane continue down than saying "MAP, runway in sight, chop and drop."

[/ QUOTE ]
Just a small correction; sometimes you want to be hitting MDA well before MAP, or you won't be in a position to land. Example: MAP is at the runway threshold and MDA is 800 feet AGL. You need to work the VDP backwards from the runway, not the MAP.

[MikeD]

[ QUOTE ]
Just a small correction; sometimes you want to be hitting MDA well before MAP, or you won't be in a position to land. Example: MAP is at the runway threshold and MDA is 800 feet AGL. You need to work the VDP backwards from the runway, not the MAP.

[/ QUOTE ]

I was just about to roll in on the posts above......but I see you've already destroyed the target!

Agree. A non-precision approach is designed for a VDP if not published, and descending in order to be at MDA at MAP, might put you beyond a position to land from at least a straight-in, if at all. A non-precision approach final can be computed and flown to the VDP or prior (in order to be established, so you can shift your crosscheck to outside); it doesn't necessarily have to be a "dive bomb drop" to MDA.

[cime_sp]

Our airline procedures make us do the chop and drop. Our FMS won't give us guidance down to the MDA with a steady glidepath....So we do as we are told.

I hear you on the visuals though Doug...much easier than some sort of non precision. On those our FMS will even give us the imaginary glideslope all the way down to the runway and it will bug the VSI to tell you what to descend at. Now if we could only couple the autopilot to it we wouldn't have to think at all!!!!

[Doug Taylor]

You can't program a VNAV path in your FMS? Like arriving at the threshold at 50 feet above TCH?

[cime_sp]

We could...but we have the ability to load an FMS based visual with a centerline fix placed wherever we want. We then get the electronic "snowflake" (glideslope) and extended centerline. It will also bug the vsi on the fpm required to get from present position to the runway based on a 3 degree.

The other way would involve a lot more keystrokes. This way does the same thing but only takes 2 or three punches of the box.

Technically our vnav data is advisory only and you can't couple the autopilot to it, but it works pretty damn good.

[Doug Taylor]

We can do VNAV approaches as a CANPA procedure (constant angle non-precision approach) but we down hit the VNAV button, we just reference the VNAV profile diamond and try to follow it with vertical speed.

Epiphany! No one probably has any clue what we're talking about!

[cime_sp]

That's OK...I'm so new to the airplane that I'm not sure I do either!!!

I wish we could do the constant angle stuff. Sounds like fun.

[Doug Taylor]

Well, when you do your first CANPA, it doens't make sense why anyone ever does the 'dive and drive' with non-FMS aircraft. It actually works.

[cime_sp]

It sure makes a lot more sense than the way we do it.

[Doug Taylor]

It basically works like this. Set your baro bug to MDA +50 so you don't bust the MDA during the descent. Set, unless it's already in the box, a point to cross the threshold at 50 feet above the TCH (threshold crossing altitude) and the FAF at the prescribed altitude. As you hit the FAF dial in a standard 3:1 VSI -- probably around 700-800 FPM and reference your VNAV path.

Try it out one day in VFR conditions!

[MikeD]

[ QUOTE ]
We can do VNAV approaches as a CANPA procedure (constant angle non-precision approach) but we down hit the VNAV button, we just reference the VNAV profile diamond and try to follow it with vertical speed.

Epiphany! No one probably has any clue what we're talking about!

[/ QUOTE ]

117 has pretty much the same system as you describe it.

A-10 was.....well.........you as the VNAV computer and autopilot!

[Doug Taylor]

So you're saying automation in the A-10 is limited to a belt loaded Gatling?

[MikeD]

[ QUOTE ]
So you're saying automation in the A-10 is limited to a belt loaded Gatling?

[/ QUOTE ]

That's about it so far as automation. There is stuff like GPS/INS now for nav, but there's no coupling-up features, etc.