Descent out of MDA, visual or VDP?

[SennaP1]

So when is it "legal" to descend out of MDA, do I have to leave when visual (in a normal position to land, to land in the TDZ,....) or do I HAVE TO wait until VDP to descend. I have an answer, just got into a discussion about it...

 

[philo beddoe]

Let's talk 135/121.
You are required to maintain VASI until lower is necessary for landing, right?
A VDP is where a 3 degree slope intersects the MDA. Therefore, to descend before the VDP would be similar to descending before intercepting the VASI.
For some people this might sound like a reach. I don't know.

Anyway, I do not believe that there is any regulatory mention of VDP's other than the correlation that I just made. Of course, why anyone would want to descend before the VDP is an open question.

Your particular company may have the requirement spelled out in greater detail. None of my 121 or 135 employers had any mention of VDP's in their manuals.

What do I win?

 

[bafanguy]

So when is it "legal" to descend out of MDA

The only place I see where "legalities" and VDP are used in the same published context is: 121.651(c)(4). I believe it applies to those approaches which "incorporate..." a VDP. This seems to mean contain a published VDP. I know NOTHING about 135.

Of course, if one isn't published, you'd need to have a ballpark one in your mind anyway to avoid leaving MDA after establishing visual contact, satisfying the other legalities, and ending up too low before getting to the runway. Visual cues are pretty deceiving in the murk. Requiring a VASI on every runway without a precision approach is the answer, but this involves government........and we all know what that means.

The winged lawyers will now step in....

 

[flyer172r]

According to section 5-4-5 of the AIM, "The pilot should not descend below the MDA prior to reaching the VDP and acquiring the necessary visual reference."

That's the only mention of VDP I could find in the AIM or FARs

 

[MikeSF340]

How to calculate a VDP? Both time and distance?

Was just wondering how to calculate the VDP. Thanks for all the help.

Mike

 

[Mel Sharples]

Was just wondering how to calculate the VDP. Thanks for all the help.

Mike

Just base your descent on the profile 300 feet per nautical mile. If you have to, set up a ratio of 300:1 to your height over "n" (VDP distance).

For example, let's say your MDA is 550 feet:

300 x 550
..1..... n

300n = 550

n = 550/300

n = 1.83


So your VDP is 1.8 miles. So start your descent at 1.8 miles and you are fine. I'm sure there are other methods but this one is quick and easy.

Good luck...

 

[Rez O. Lewshun]

Keep in mind that many VDPs are greater than one mile from the runway, yet many non-precision approaches have one mile or 1.5 miles visibility minimums.

Therefore in order to shoot a non precsion approach you have to fly a steep glide path.. ie not stabilized... ouch.... what is a pro pilot to do?

Your VDP distance from the rwy is actually your visibility minimums....

 

[80/20]

Keep in mind that many VDPs are greater than one mile from the runway, yet many non-precision approaches have one mile or 1.5 miles visibility minimums

This raises another question – should we always be able to land if the weather is at minima? Procedures may often be designed at at desk far away by a person who is just looking at the numbers recorded by surveyors. I know several non precision approaches where the pilots who frequently fly there would say that you can forget to land there safely when the visibility is at minima.

 

[Mr. Cole]

Also keep in mind that not every non-precision approach has a VDP due to terrain and/or obstacles in the projected path.


Dave

 

[Illini Pilot]

Just base your descent on the profile 300 feet per nautical mile. If you have to, set up a ratio of 300:1 to your height over "n" (VDP distance).

For example, let's say your MDA is 550 feet:

300 x 550
..1..... n

300n = 550

n = 550/300

n = 1.83


So your VDP is 1.8 miles. So start your descent at 1.8 miles and you are fine. I'm sure there are other methods but this one is quick and easy.

Good luck...

just use the HAT (height abv tdz) /300...

therefore: MDA 800', TDZE 200' = HAT 600' and MAP is 1.0 DME

600/300=2 nm so 3.0 DME is your VDP

for a quick descent rate at 300 fpnm or a 3 deg GS, remember 120 kts will give 600fpm (2 nm/min), 90kts will give 450 (1.5 nm/min), 60 kts will give 300 (1nm/nmin) so you can estimate from there