Visual Descent Point?

fiveoboy01

fiveoboy01

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Machado's book seems to say that you may NOT descend below MDA if you are not at the VDP, even if you have the runway environment in sight.

This morning my II said that it basically tells you that if you're past the VDP and you don't have the runway despite being at MDA, you'd better think about going missed(even though you haven't yet reached the MAP) because your descent angle steepens the closer you get to the runway.

I guess I'm a little confused because of these two statements, anyone care to clarify things? Is it advisory, or mandatory to stay at MDA to the VDP even if you have the runway in sight prior to it? Maybe I'm overthinking it.
 
If you have the means to identify the VDP, you can't descend below MDA until reaching it. There's an obstacle down there somewhere.

If you don't have the means to identify the VDP, use your best estimate a normal glide path from MDA to the runway. The VDP a is generally located on a normal glide path, and there's enough protection built into the obstacle analysis to prevent hitting the obstacle if reasonable care is used.

What's the definition of reasonable care? :dunno: it probably doesn't include anything much flatter than 3 degrees, or 300 feet per mile.

As far as thinking about going missed, that's definitely good technique, especially if you transition into anything much over 100 knots across the fence. Required descent rates increase dramatically after you pass the VDP.
 
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From how I understand VDP, it does not make any guarantees for obstacle clearances. Instead, it guarantees that the pilot can make a standard descent to the runway without the need for excessive maneuvering.

Once you are at MDA and have the runway in sight, the VDP is pretty much pointless because you are in VMC at that time and provide your own obstacle clearance.
But I do agree that it is useful to know where the VDP is if you are at MDA and don't have the rwy in sight yet. Once you're past VDP, it might not be safe to try to land if you see the runway. So using the VDP as a MAP is not a bad idea (while in IMC).
 
As an instrument student, this is exactly the sort of ambiguity that continues to annoy me. This is -important-, and yet even experienced pilots don't understand it.

Imagine just learning about MDA, thinking you understand it ("stay at MDA until you have the runway environment (one of 10 things) in sight." Well, ok.

O HAY, here's a visual descent point. It.... allows you to descend if you have the runway in sight.

But. But... I thought...
 
So you break out at 2000' with a 800' MDA but you're not allowed to descend below 800' until after the VDP?
 
Hey, this is a pretty good article on VDPs: http://www.aviationchatter.com/2009/12/how-to-calculate-a-visual-descent-point/

To limit the temptation to proceed with a landing under unstabilized approach conditions and prevent collisions with obstacles along the final approach path, the FAA began publishing visual descent points (VDPs) on many GPS plates. Marked by a “V,” the VDP is the last point at which a descent from the MDA to to the touchdown zone can be made at a stable three degree glideslope. If the runway is not in sight by the VPD, a missed approach should be executed.
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Right. And if it's not in sight before the VDP and you're at MDA, you're probably not going to see the runway anyways, so you're going missed either way.

The AIM says this:

VISUAL DESCENT POINT. A defined point on the final approach course of a nonprecision straight-in approach procedure from which normal descent from the MDA to the runway touchdown point may be commenced, provided the approach threshold of that runway, or approach lights, or other markings identifiable with the approach end of that runway are clearly visible to the pilot.

To me it's not clear or I can't interpret it right to get my answer about descending below MDA prior to reaching the VDP, if I have the runway in sight. Machado's book states this(quoted):

When a VDP is established on an approach, and you have the equipment onboard to identify the VDP, you must wait until reaching the VDP before descending from the MDA. You should not descend below the MDA prior to reaching the VDP even if the runway is clearly in sight. If you don't have the equipment to identify the VDP, fly the approach as if no VDP has been provided.

So, first he says you "must wait", then he says you "should not" descend. Which is it? Am I prohibited from descending if I've got the runway? Or is it a discretionary thing?

That confuses me. Don't descend below MDA till this point, unless you can't identify it, then fly the approach a different way. If the VDP is due to an obstacle on the ground, this seems contradictory to me. Keep in mind I'm talking if you've got the runway, such as in the example given by Brian in post #6.

Perhaps it's not much of an issue. If I can't see the runway at the VDP I'm probably not going to be able to do anything but go missed anyways. But I always try to understand clearly how things work.
 
"A defined point on the final approach course of a nonprecision straight-in approach procedure from which normal descent from the MDA to the runway touchdown point may be commenced, provided the approach threshold of that runway, or approach lights, or other markings identifiable with the approach end of that runway are clearly visible to the pilot."
 
fiveoboy01 said:
That confuses me. Don't descend below MDA till this point, unless you can't identify it, then fly the approach a different way. If the VDP is due to an obstacle on the ground, this seems contradictory to me. Keep in mind I'm talking if you've got the runway, such as in the example given by Brian in post #6.
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I agree that it is confusing. Too bad that the confusion stems from Mr. Machado's book, he's a very bright (and funny) instructor.

The VDP is nothing but a convenience feature on the plate. It has nothing to do with obstacle protection.
 
The article I link to sort of makes the point that the FAA thinks a stablized approach is very important. The VDP appears to be the last point at which you can be on a stablized approach.

I'm not sure I'd want to start down earlier anyway. This is all still kind of new to me, but if I'm reading this right, LOC 36R at HSV (http://155.178.201.160/d-tpp/1509/05488IL36R.PDF) puts me 545ft AGL a mile away from the runway at the VDP. I'm pretty sure I don't fly a pattern that wide, why would I want to be lower then?
 
That part I get. For instance on the plate in question, the LOC/DME RWY 1 approach at DLL, the VDP is 1.4 miles from the threshold and the MDA is 1460 feet, or 481 AGL. I looked up at the VDP and thought yeah, no way I can do anything but dive bomb to get down now. So that part I get, I just wasn't sure if it was regulatory.

Seems now that it's not about obstacle clearance but avoiding an excessively steep approach. I agree that Machado is a nice guy, and I like his book, his wording in this instance was just a little confusing, to me anyways. I mainly wanted to know if the VDP was regulatory and apparently it isn't.
 
Another thing to consider, if you're flying at night the VDP may help you clear an obstacle that may be difficult to see. Just because you have visual on the runway lights doesn't necessarily mean you should descend below the MDA before the VDP.
 
fiveoboy01 said:
I agree that Machado is a nice guy, and I like his book
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You should try to meet him. He travels with the AOPA Fly-Ins and gives presentations (mostly on safety). He is a very nice gentleman in person as well. I liked talking to him (got a minute or two of his time). He was also baffled when I told him that it was his voice that got me into aviation. :)

kevmor99 said:
Another thing to consider, if you're flying at night the VDP may help you clear an obstacle that may be difficult to see. Just because you have visual on the runway lights doesn't necessarily mean you should descend below the MDA before the VDP.
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Again, the VDP is not meant as a guarantee of obstacle clearance. It is just a hint that past the VDP, excessive maneuvering (as in: steep descents etc) might be required to land on the runway.
PAPI is for obstacle clearance (if you don't see more than 2 reds, of course).
 
BigBadLou said:
You should try to meet him. He travels with the AOPA Fly-Ins and gives presentations (mostly on safety). He is a very nice gentleman in person as well. I liked talking to him (got a minute or two of his time). He was also baffled when I told him that it was his voice that got me into aviation. :)
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I was able to be at his presentation at the FAA Safety building at Oshkosh. It was a great presentation. He's a good ambassador for aviation, no doubt:)
 
From one of Wally's TERPSICHORE articles:
Keep in mind only FAA-charted VDPs assure obstacle clearance be- low the MDA.
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The "only" is in there because he's been referring to what he calls "impromptu VDPs" that we calculate ourselves to determine good descent/missed approach points. Bottom line, though, is that published VDPs are there for obstacle protection.

Full article here.
 
Good old Rod led you astray with his wording.
FAR 91.175(c) tells you what you need to operate below the MDA.
  • Need to be in a position to be able to land with a normal rate of descent and maneuvering
  • Have the flight visibility for the approach
  • and need to have one of the acceptable visual references in sight.

So yes, you can go below the MDA before the VDP if you have all three of those. This is one of those: Is it legal? Yes. Is it smart? Maybe not depending on what type of aircraft you are flying. For instance the VDP is calculated using the lowest category minimums. If the minimums are higher for a Cat D airplane flying the same approach. The charted VDP will not work for him. Going below the MDA before the VDP and then inadvertently going back in to IMC could put you in a dangerous situation. So make sure you know you can make it to the runway if you are going to do it.

I still go to the MAP even if I don't have visual at the VDP. While I am more than likely not going to make the landing happen in a Cat D airplane. A Cessna 150 with 40 degrees of flaps may think he is in a perfectly acceptable situation to make a normal landing. Also depends on the airports VGSI/descent angle. When I go into a fighter/bomber base like Barksdale, I know the VGSI and approach angle is much lower than the normal 3 degrees. So I know I can go past the VDP and still make a normal descent to landing.
 
Only situation I can think of where it would be a good idea to descend below MDA before the VDP would be if you're right at the cloud bases, have the runway and obstacles ahead in sight, and are afraid that you might stray back into the clouds at present altitude before reaching the VDP. In that (admittedly narrow) case, it might be smart to descend another 100 feet (or whatever) and then cruise along at that sub-MDA altitude before reaching the VDP.
 
Mistake Not... said:
The article I link to sort of makes the point that the FAA thinks a stablized approach is very important. The VDP appears to be the last point at which you can be on a stablized approach.
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The last point of a stabilized 3 DEGREE (or so) glide slope to hit the TDZ of the runway.

Of course, most light aircraft have no problem doing stabilized descents sharper than than three degrees. Further, most instrument approaches are to much longer runways than the 850' I ned in my plane.

It's purely advisory and of dubious use in light singles.
 
Problem is, most approaches don't have a VDP published. Yet, you're supposed to know by calculating backwards the 3 degree. Hands up now - how many of you do that on a non precision?
 
mcmanigle said:
Only situation I can think of where it would be a good idea to descend below MDA before the VDP would be if you're right at the cloud bases, have the runway and obstacles ahead in sight, and are afraid that you might stray back into the clouds at present altitude before reaching the VDP. In that (admittedly narrow) case, it might be smart to descend another 100 feet (or whatever) and then cruise along at that sub-MDA altitude before reaching the VDP.
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Caution, Will Robinson! That didn't work out for this crew:
REMARKS- DSCNDD BLO MDA HIT TREES APRX 4610FT NE OF RWY.ACFT,LOW ALT APRX LVL,BOTH LDG LTS ON IN HVY SNOW.​
Of course, you did say "have the runway and obstacles ahead in sight", but you don't know what you don't know.

dtuuri
 
stratobee said:
Problem is, most approaches don't have a VDP published. Yet, you're supposed to know by calculating backwards the 3 degree. Hands up now - how many of you do that on a non precision?
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Simple, use the HAT / 10 method where you get the number of seconds to subtract from the time box on the approach for your speed.

Example: If your HAT is 469', use 469’ / 10 = 47 sec. If the time for 120 kts groundspeed is 2:51, subtracting 47 sec gets you a VDP time of 2:04 sec.
 
okav8r said:
Don't know who wrote that, but VDPs were being used BEFORE any GPS approaches were being developed. And, the VDP crossing altitude is NOT mandatory, however, it may coincide with a visual glide slope indicator, and there is a rule that addresses use of a VGSI.
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Correct. Except for a few years VDPs, where charted, were mandatory for Part 121 operations. Didn't last very long, though.
 
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