Why is the VDP "Not Authorized"

[ruthsindelar]

Why is the VDP not authorized on KBMC GPS 34 approach when using Ogden's altimeter?

http://204.108.4.16/d-tpp/1004/05477R34.PDF

I understand my MDA is 100 feet higher and thus my decent from the published VDP would be a bit steeper. But I would think the FAA would still want pilots waiting to the VDP to descend. Also, my understanding is the VDPs are "highly encouraged" but not regulatory. Is that correct?

Thanks

 

[DC34ME]

A VDP is just a point for a pilot to go missed, like an MDA. VDP's are not regulatory, they are the same altitude as the MDA. So at MDA you would either see the airport environment and descend (at the VDP) or level off at the MDA until XEXXA and go missed. On this approach if you use the non local altimeter, you reach MDA above the VDP altitude.

But you would think the VDP would move up 100 feet too, right? Now I'm confused about all this!

 

[Danos]

The VDP would only work at the altitude you would use with the BMC altimeter setting. Add 100 feet to that and you would have to push the VDP back about a 3rd of a mile.

 

[mantakos]

But you would think the VDP would move up 100 feet too, right? Now I'm confused about all this!

VDP isn't an altitude, it's a "fix" that you cross, a distance from the MAP.

You cannot descend below the MDA until you have the runway in sight. That means that if you do have the runway in sight, then you can descend below the MDA. What a VDP says is "even if you have the runway in sight, don't descend below the MDA until after you've passed me". The intent is to keep you from making too shallow a descent, in conditions that are technically "visual" (you have the runway environment in sight), but in which a too-shallow approach to the runway might still be hazardous, particularly at night or in low visibility.

Change the MDA, and you have to move the VDP horizontally, either closer or farther from the MAP. This chart doesn't plot a new VDP in the rare case where the MDA needs to be adjusted due to getting the altimeter from a remote weather site.

BTW, I've always read "VDP NA" as "VDP not applicable".
-harry

 

[poadeleted20]

A VDP is just a point for a pilot to go missed, like an MDA.

Not quite. If you look at the definition of the VDP, it is merely the point at which the normal glide angle to the runway (3.08 degrees in this case) intersects the MDA. The missed approach point is always farther along, usually at the end of the runway, although it's actually 0.5 miles short of that on this approach (probably because the terrain beyond the runway would otherwise compromise the missed approach procedure or require a higher MDA). Executing a missed approach turn before the published MAP can compromise obstacle clearance or airspace, so while it's OK to climb if you don't see the runway by the VDP, don't turn until you reach the published MAP.

VDP's are not regulatory, they are the same altitude as the MDA.

While the VDP is reached at MDA, the MDA is a line along the final approach course while the VDP is only one a point along that line.

So at MDA you would either see the airport environment and descend (at the VDP) or level off at the MDA until XEXXA and go missed.

Most instructors teach instrument trainees to descend to the MDA expeditiously, resulting in reaching and leveling at the MDA before reaching the VDP. This gives you time to stabilize the aircraft at MDA, and look for and find the runway before you reach the VDP (published or self-computed). You would not want to descend below the MDA before the VDP, as this shallow approach could compromise obstruction clearance before reaching the runway. OTOH, once you pass the VDP without seeing the runway, you know that the chances of completing the landing "at a normal rate of descent using normal maneuvers" are decreasing rapidly, and a missed approach is increasingly likely even if you eventually see the runway before the MAP.

On this approach if you use the non local altimeter, you reach MDA above the VDP altitude.

You could look at it that way, or you could say you reach the true VDP for that MDA (as opposed to the published MDA) further from the runway.

But you would think the VDP would move up 100 feet too, right?

The MDA moves up 100 feet, but the VDP moves out by the distance required to descend 100 feet on (in this case) a 3.08 degree descent angle, which computes out to 1858 feet, or .3 miles. Thus, when using the alternate altimeter setting, you'd want to consider your VDP on this approach to be 1.0 rather than the published 0.7 miles from XEXXA.

Remember -- VDP is a point, not an altitude!

 

[ghogue]

.........

BTW, I've always read "VDP NA" as "VDP not applicable".
-harry

Harry,

NA means Not Authorized
N/A means Not Applicable

From FAA Order 7340.2 Contractions

 

[DC34ME]

I'm sure Ruth will get an answer in all this, I know I understand it better now. BTW I know that a VDP is a point not an altitude, how I worded my post was rather scattered. I was just saying it is another point for a pilot to decide to go missed, but you don't have to at VDP. And if you draw a 3.08 degree line to an altitude 100ft higher than MDA, would that not also be 100ft higher than the old VDP? It doesn't matter, VDP is not authorized with Ogden altimeter, and VDP's are not regulatory.

 

[dmccormack]

Remember -- VDP is a point, not an altitude!

Good discussion!


VDPs locally are on non-precision approaches with intervening ridges or hills. Fly them VMC and the VDP makes lots of sense.

 

[poadeleted20]

And if you draw a 3.08 degree line to an altitude 100ft higher than MDA, would that not also be 100ft higher than the old VDP?

Well, yes it would, but it would also be a third of a mile farther out, and if you're driving along at MDA, the bigger issue is not the height of the VDP, but rather how far from the runway you are. My concern with your discussion was that is sounded like one would treat the VDP like a DH, i.e., upon reaching, if field not in sight, go missed immediately. I wanted to make sure nobody got the wrong idea from that.

 

[John Collins]

Most of our GA airplanes are capable of a descent angle much greater than 3 degrees. In my Bonanza I still have power on with a six degree descent angle. If you decide to use the VDP as the point where you initiate your climb (not the turn) and miss the approach, you will typically need greater visibility than the minimum specified on the approach.

In this case, the VDP is 1.2 NM (1.4 statute miles) from the threshold and the visibility minimum is 1 statute mile. This means that if the visibility is at the minimums, and you choose to miss at the VDP, you won't see the runway. Whereas if you continue at the MDA all the way to the MAP, you have a reasonable chance of completing the approach. With a 7500 foot runway, even at the high altitude, I would have no problem descending from the MAP to the runway with loads of runway to spare. For a GA airplane there is no requirement to touch down in the first 1000 feet of the runway and I am very comfortable in my aircraft so long as I can touchdown with at least 2000 feet of runway remaining.


One other option that provides the pilot more time on this approach is the circling option, as it is at the same MDA. If the aircraft is too high, circling provides the pilot the time to circle to either runway at the safety of the circling altitude. One should practice the circling maneuver and understand the techniques used to take advantage of the maneuver. In particular, one should understand how to offset the aircraft appropriately so that a base to final is not overshot, while remaining within the protected airspace.

 

[taters]

In particular, one should understand how to offset the aircraft appropriately so that a base to final is not overshot, while remaining within the protected airspace.

+1..

IMO this is the most overlooked aspect of approaches in IFR training. This is not something that will be just like "flying in the patch at home" Many have burned in using an unsafe/uninformed circling maneuver.



I think of the VDP as the point where I would like to see my normal decent to landing using normal maneuvers begin.

 

[dmccormack]

With a 7500 foot runway, even at the high altitude, I would have no problem descending from the MAP to the runway with loads of runway to spare. For a GA airplane there is no requirement to touch down in the first 1000 feet of the runway and I am very comfortable in my aircraft so long as I can touchdown with at least 2000 feet of runway remaining.

Exactly -- GA Singles are NOT Jets. We can slip, S turn, and get very slow in order to lose altitude quickly.

 

[poadeleted20]

Exactly -- GA Singles are NOT Jets. We can slip, S turn, and get very slow in order to lose altitude quickly.

Those maneuvers are easy on VFR days, but much tougher in extremely low vis when you have no external horizon reference (and if you didn't see the field from the VDP, it's "extremely low vis" in my book). It's way too easy to lose external attitude references and either run out of speed or get an excessive descent rate going. For those reasons, I teach that late descent in low vis is a trap to be avoided.

 

[John Collins]

+1..

IMO this is the most overlooked aspect of approaches in IFR training. This is not something that will be just like "flying in the patch at home" Many have burned in using an unsafe/uninformed circling maneuver.

I teach three main ways of spacing.

1) overfly the runway and at the end conduct an 80/260 turn to final
2) at the departure end, turn 90 degrees and time 15 seconds, then turn down wind.
3) at the departure end, turn 45 degrees for 20 seconds, then turn downwind.

The last two of these methods space the airplane out from the landing runway so that the base leg is approximately 15 seconds, no wind. The first method requires continuous turning, but spaces one out appropriately for completing the turn to final with the same bank angle.

If you use your normal sight picture for pattern altitude spacing for down wind, the turn to final will be overshot.

 

[dmccormack]

Those maneuvers are easy on VFR days, but much tougher in extremely low vis when you have no external horizon reference (and if you didn't see the field from the VDP, it's "extremely low vis" in my book). It's way too easy to lose external attitude references and either run out of speed or get an excessive descent rate going. For those reasons, I teach that late descent in low vis is a trap to be avoided.

True -- S turns in vis that was so low that you can't see the airport at the VDP could cause disorientation.

S turns could be used if you have a cloud at the VDP, then break out 1/4 miles after the VDP (often in NPA min weather the cloud bottoms aren't perfectly smooth)

But full flap slips? As soon as the runway's in sight, it's a tool and a "normal landing maneuver."

 

[John Collins]

Those maneuvers are easy on VFR days, but much tougher in extremely low vis when you have no external horizon reference (and if you didn't see the field from the VDP, it's "extremely low vis" in my book). It's way too easy to lose external attitude references and either run out of speed or get an excessive descent rate going. For those reasons, I teach that late descent in low vis is a trap to be avoided.

A counter point is that when visibility is low, the extra time to acquire the runway requires that you either circle or start your descent later than the VDP. I agree with your point that the maneuver is more difficult with low visibility, but even with visibility of 2 SM, it can take some time past the VDP to acquire the runway, particularly if there are no approach lights. If you always miss at the VDP, you will not complete many of these type approaches in lower visibility with a landing , that can be safely accomplished. Each pilot should have there own level of comfort and personal minimums, but one should not expect to complete the approach at the VDP when visibility is low, and in many cases well above minimums.