VDP's?

[NDPILOT]

What is the point of having VDP's? I know how they are supposed to be used - do they have anything to do with the electronic glideslope capability of some aircraft??

My hang-up on this issue is that a VDP allows a pilot to descend from the MDA at that point provided the "runway environment" is in sight. If you're at the MDA and you see the "runway environment" you can descend regardles if you're at the VDP or not.

What do you think about this?

Thanks

nd

 

[dkwid]

Having the runway in sight in some cases does not provide adequate safety to begin your decent from MDA.....per the TERPS, personal safety guidelines, company procedures....whatever and all those.

In these cases a VDP may be puplished. So you have the runway in sight you are at MDA but still you cann not descend until passing the VDP.

 

[601Pilot]

re: VDPs

A VDP (either depicted on the approach chart or calculated by the
pilot) provides you with a point at which you can descend with
a typical 3.0 degree approach. It's a point at which you can
descend to the runway without any "excessive maneuvering"
and on a stable approach. A typical 3.0 degree glideslope will
give you a descent rate of 5 times you groundspeed (not your
indicated speed!). So at 120 knots GS, you need about 600 feet
per minute descent.

Hope that helps!

601Pilot

 

[Illini Pilot]

around CMI (champaign, IL) where i fly often there is one for the VOR 4 approach. There are no stepdown fixes on the entire approach, so you cant technically be at the MDA for the entire 10 mi length of the approach inside the 10 mi. ring, and therefore be able to descend at pilots descretion when you have the rwy enviro. in sight. they dont want you doing this due to obs. clearance, noise abatement, so the IAP has hte VDP at 1.3 dme from the VOR(on field) which will give you a nice 3* GS to the rwy from the MDA.

 

[Caveman]

All a VDP does is identify the point where if you are at MDA an approx 3 degree descent path will take you to the runway. Descend prior to that and you will have a shallower approach. Descend after that and you will have a steeeper approach. If you are flying a high performance a/c any attempt to descend to the runway after passing the VDP will probably not be in compliance with your airlines definition of a stable approach and would also probably be construed by the feds as excessive maneuvering. You might pull it off just fine in a Cherokee at 70 KIAS but in a CRJ at 140 KIAS it would probably be excessive. Pilots that fly the bigger faster airplanes usually compute a rough VDP and decide ahead of time that if we don't have the runway in sight by then we are going missed once we get to the MAP even if we have the runway in sight at the MAP.

 

[enigma]

I prefer to view the VDP as an asset in determining when I'm not going to make a successful approach. If you don't have the runway in sight by the VDP, you will have to descend at excessive rates to reach the runway once you actually gain runway contact and that is the quickest way to become a CFIT statistic that I know of. One may be able to descend from past the VDP when flying a SE airplane with a 60kt approach speed, but doing so in something with double the speed will bite you in the behind.

regards
8N

 

[dkwid]

That is a very good point, enigma, I was thinking of that after I finished my post.

 

[NDPILOT]

I guess it helps to think beyond the speeds of the aircraft that I fly.

I appreciate everyone's input - makes a lot more sense now.

Muchas Gracias

nd

 

[DCAV8R]

We use VDPs on all non-precision approaches at the company which I fly for. We also use PDPs (planned descent point). If there is a published VDP on the approach, we would add .3 nm to the VDP. The reasoning behind this is simple. If you are doing 140 kts accross the ground and you wait until you get to the VDP to begin your descent to the runway, you're already unstabilized (and at this speed "behind the airplane"). If you initiate the descent at the PDP, by the time you reach your VDP the aircraft is set-up perfectly on a 3 degree glideslope.

Just my .02 cents.

 

[tk855]

What are some of the rules of thumb used in calculating a vdp? I have a formula to do it, but it isn't very friendly to use in the airplane.

 

[Saabslime]

What are some of the rules of thumb used in calculating a vdp?

One of the most common is to take 10% of HAT and subtract from your calculated time inbound from FAF.

 

[regionalcap]

or divide the HAT by 300...this will give you a distance from the runway

 

[Toro]

What are some of the rules of thumb used in calculating a vdp?

Gus wears a hat - old FAIPism.

VDP = HAT/GS

Divide the Height Above Touchdown altitude from the approach by the desired Glide Slope. Same thing as regionalcap said above dividing HAT by 300, but this takes into consideration that you may want something other than a three degree glideslope.

 

[tk855]

Thanks

Thanks guys, those are easy enough to use.

tk