calculating VDP-why?

[Will Kumley]

I have my instrument checkride scheduled for this week and as a member of overthinkers anonymous I'm racking my brain on all the things the examiner could ask me. I talked to a person that had their checkride with a different DPE recently and he said he was asked to calculate a non published after he had already crossed the IAF. My question is, why would this be a skill I should focus on? From everything I've read, if a VDP isn't published its because there is an obstruction in the way. If the DPE asks me to calculate a rough VDP during preflight I know the formula but will also mention that with a GPS navigator my plan is to fly to the MDA, and fly that altitude until the missed approach point. If I see the runway I'll determine if I can make a stabilized approach and land with the space remaining, if not, I'll go missed. I also think trying to calculate something after the IAF is asking for problems as I should be focused on staying on centerline and glide path.

 

[Larry in TN]

Know if you've passed the VDP, and by how much, can help you to decide if a stabilized decent can be made or help you descend when to initiate your descent from MDA.

With low visibility, it can be more difficult to judge your descent from MDA visually. A VDP gives you additional information to help make better decisions.

 

[midlifeflyer]

I would definitely argue with the DPE who insisted I calculate the VDP once on the approach. If one calculates a VDP it's calulated as part of the approach briefing.

my plan is to fly to the MDA, and fly that altitude until the missed approach point. If I see the runway I'll determine if I can make a stabilized approach

The VDP is telling us that if we can see the runway environment at a certain point, we can make a 3° descent to the runway and that if we go beyond that point, our required descent rate will be higher and potentially unstable.

We calculate (or look for) a VDP so that we know in advance- before we begin the approach whether or not we will be in position to make a stabilized approach. If we are still in the clouds at the VDP there no real value in continuing at low altitude from MDA to the MAP at the runway threshold instead of beginning our climb. It's just another way to stay ahead of the airplane.

 

[RyanB]

I actually had a similar conversation with a student recently. I asked, “what should we consider if we’re at MDA and still IMC past the VDP?” “Can we still land?” The answer is, it depends how far past we are.

Take HAT/300. If HAT is 200ft, we need to have the runway environment in-sight by .7nm to the MAP. If not, we need to consider a go-around shortly thereafter.

I disagree that a VDP should be calculated while at the IAF. That needs to be done well prior to commencing the approach, but it seems the intention of the DPE is to simply test your skills.

 

[MauleSkinner]

I would definitely argue with the DPE who insisted I calculate the VDP once on the approach. If one calculates a VDP it's calulated as part of the approach briefing.

What we don’t know is how the conversation went. I doubt that it was, “calculate a VDP right now,” Probably something more along the lines of, “Did you calculate a VDP for this approach?”
“No.”
“Do you think it would be good to have one?”
“Yes.”

Either way, I can easily see it being something the examiner sees as a realistic distraction, and an appropriate answer might be, “Make a note, and we’ll talk about it on the ground.”

The VDP is telling us that if we can see the runway environment at a certain point, we can make a 3° descent to the runway and that if we go beyond that point, our required descent rate will be higher and potentially unstable.
We calculate (or look for) a VDP so that we know in advance- before we begin the approach whether or not we will be in position to make a stabilized approach. If we are still in the clouds at the VDP there no real value in continuing at low altitude from MDA to the MAP at the runway threshold instead of beginning our climb. It's just another way to stay ahead of the airplane.

Put another way, it’s the difference between “I made a stabilized approach. Imagine that!” and, “What do I need to do to ensure that this approach will be stable?”

 

[Will Kumley]

Good points, thanks! For the other student, he explained it as "can you calculate the VDP before we land?" This was after he was on the approach and I'm curious if the DPE was attempting to distract him with a math problem. Luckily for him it was an easy calculation at the airport he was at. I can see the value of it if done prior to flying the approach, but once on the approach my focus goes to that. As an example, the airport I'm based at doesn't have a VDP listed. If I use the simple calculation it comes to 2.8 miles, but the approach is a circle to land. If I'm still in the clouds at 2.8 miles I'm likely not landing straight in, but a circle to land could still work at which point a VDP doesn't matter as much. I also understand VDP's don't apply to circle to land but this approach could provide a straight in as well even though its only listed as a circle to land.

 

[midlifeflyer]

If I use the simple calculation it comes to 2.8 miles

Fortunately it’s an easy calculation.

And yes, a circling-only approach would not be expected to have a published VDP, which assumes a straight in landing. (Not all straight in approaches have them either. There are several reasons.)

 

[Albany Tom]

I'll just toss out a question - are you sure you'll be able to judge if you have enough distance to land if you are at MDA and close to the runway? I say that because MDA could be 700' AGL, and that puts the 3 degree descent at a bit over 2 miles, or over 13000 ft, if my math is right. Now you can make an approach a lot steeper than that, but can you visually tell if you have enough room?

To me the concern is at night, or where there's low horizontal visibility. There's a defined ceiling, and you're below it at MDA, but you only have 1.5 - 2 miles forward visibility. You're at 700', you see the threshold at 2 miles, you can come in at about 3.3 degrees and you'll be fine. I think that's 3 of 4 lights white on papi. If you see it at 1.5 miles, then I think that's about 4.4 degrees. 1 mile? Again dependent on my math, but sounds like 6+ degrees, or 5 and landing 2000' long. Neither sounds ideal.

I think the exercise is at least partly to bring awareness of the potentially non-obvious risks of flying MDA to missed point and then trying to land.

 

[John Collins]

Calculating a VDP is a skill that one should be able to accomplish as part of a briefing, but the standard VDP does not always apply to all situations or aircraft. In my Bonanza, I need to carry power at 6 degree descent angle which is twice the typical VDA of 3 degrees, power off is closer to 8 degrees. Conservatively, I need at most 2500 feet of runway to get stopped, so a runway that is 5500 feet, like my home base, where the VDP is 1.2 NM prior to the threshold. At the VDP, I am still almost 1.4 SM from the threshold and the LNAV MDH is 474 feet above the TDZE elevation with a visibility required minimum visibility of 0.5 SM. If the visibility is at minimums, I should be able to use the dive and drive method to the MDA, see the runway by 0.4 NM, leave the MDA for descent to the runway, without overshoot. So flying the approach using the CDFA method requires substantially more visibility in order to complete the approach. I would not try this in a jet.

If a VDP is not charted on a straight-in approach, then a the visual segment is not clear on a 20:1 and of note, a 3 degree descent angle is only 19:1. If the note in the profile view says "Visual Segment - Obstacles", then flight test has further determined that descent on the VDA has encountered obstacles and that continued descent on that angle can be hazardous to your health. Below the MDA, there is no guarantee that the path to the runway is clear using the VDA.

 

[midlifeflyer]

I'll just toss out a question - are you sure you'll be able to judge if you have enough distance to land if you are at MDA and close to the runway? I say that because MDA could be 700' AGL, and that puts the 3 degree descent at a bit over 2 miles, or over 13000 ft, if my math is right. Now you can make an approach a lot steeper than that, but can you visually tell if you have enough room?

To me the concern is at night, or where there's low horizontal visibility. There's a defined ceiling, and you're below it at MDA, but you only have 1.5 - 2 miles forward visibility. You're at 700', you see the threshold at 2 miles, you can come in at about 3.3 degrees and you'll be fine. I think that's 3 of 4 lights white on papi. If you see it at 1.5 miles, then I think that's about 4.4 degrees. 1 mile? Again dependent on my math, but sounds like 6+ degrees, or 5 and landing 2000' long. Neither sounds ideal.

I think the exercise is at least partly to bring awareness of the potentially non-obvious risks of flying MDA to missed point and then trying to land.

Good question. Not everyone needs that 3°. That short field over obstacle is about 4.5° in a number of aircraft and I use a similar profile as standard for airports where there are issues at the threshold (like many in mountain areas). The standard is probably 3° for no reason other than that's it's the "standard" for a stabilized approach from the IF onward (absent stepdowns between the IF and FAF that don't fit the 3° profile).