Visual Descent Point (VDP) Calculations

Posted by wbeard52 in Approaches, VDP on July 19, 2011

In the previous post, I talked a lot about the theory behind VDPs; why they exist, what the purpose is and the reason why we should figure out the VDP for every nonprecision approach we fly.  In this post, I am going to discuss some methods to figure out your VDP point and provide an analysis of the data.

There are basically two methods to calculating the VDP point.  The first method is the HAT / 10 method where we get the number of seconds to subtract from the time box on the approach for our speed.  The second method is to calculate the distance the VDP point is from the threshold and figure out a way to identify that point either by a DME distance, GPS along track distance, crossing VOR, etc.  Obviously, the first method is the easiest in the airplane to calculate unless or course you have a GPS then the second method is more accurate.

Let’s take a look at an NDB approach into Salina, KS.

The first method is 469’ / 10 = 47 sec. The time for 120 kts groundspeed is 2:51.  Subtracting  47 sec  get us a VDP time of 2:04 sec.  The second method is 469 / 300 = 1.56 NM or 4.14 NM from the FAF.

As you cross the FAF, start the timer and 2:04 sec later either see the runway or perform a missed approach. Just a side note, TERPS do not account for obstacle clearance with the missed approach turn is initiated before the missed approach point. So if you decide to go missed climb on course and pass the MAP before turning.

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There are inherent errors with the two methods described above but are close enough for government work. I am going to explain these errors so you can make a better decision for the aircraft speeds you actually fly.

The first method is only accurate for aircraft that have a groundspeed of 113kts. If there is a headwind or a tailwind the time will obviously be different.  In the picture listed below, I have created a formula that will give you a divisor for any groundspeed you wish.  The most common ground speeds are listed below and you can see that a divisor or 10 is really only good for speeds between 110 kts and 120 kts.  For slower aircraft the divisor is lower meaning there is more time after the VDP point to the threshold than for faster aircraft.  In afterthought, that makes perfect sense… it passes the reasonableness test.  Since we are talking knots which is NM / hour, I needed to covert knots to feet per second hence the 6076.1 / 3600 in the equation.  Notice the equation does not include time or distance.  This means that these variables are factored out of the final equation.

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Have you ever wondered what your glide angle is if you descend before or after the VDP point? I think the analysis will convince the pilot to treat the VDP point as a decision point on any non-precision approach. This simple change in behavior will increase the safety of the flight. The descent angle to the landing threshold is hugely dependent on the ground speed of the aircraft is flying and the distance the VDP is from the threshold.. A slower aircraft has more leeway on descending at the VDP than a faster aircraft. In the example below, the VDP is 1NM away from the threshold and is 318’ high.  The chart below shows that a faster aircraft needs to descend on time at the VDP point in order to land in the touchdown zone of the runway otherwise non normal maneuvering may be required. 

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A put together one last example of how we can quickly put together something we can take into the aircraft for VDP calculations.   This spreadsheet requires information typically found right on the approach plate.  It will calculate the effects the wind has on the groundspeed and will adjust the time accordingly in the third table.  The VDP times are adjusted as well.  You may need to look in the A/FD for some of the information.  For the Salina Rwy 17 approach I have calculated the VDP information to the right.

Notice that a slower speed causes the VDP time to increase while a faster time causes the VDP time to decrease.

I used a glide slope of 3.00° because the A/FD shows the VGSI set to that glide angle.  The A/FD also lists the TCH as 52’ AGL.