The Visual Descent Point (VDP)
It’s the little black V on some non precision approach plates. It’s the visual descent point, or VDP, and it causes a good deal of controversy amongst instrument rated pilots.
Knowing your approach plates like the back of your hand is essential to the IFR student, and it is covered in depth, along with other important topics, in our IFR Ground School.
Take a look at the the profile view for the localizer approach to runway 17 in Toelle, Utah. It’s at the 2.1 DME on the localizer for the approach.
So what is a VDP? AIM 5-4-5 says it’s a defined point on the final approach course. It’s found on non precision, straight-in approaches. In other words you won’t find it on a circling only approach. It’s the point from which a stabilized visual descent can be made from the MDA, to the runway touchdown point.
The distance is based on the lowest MDA published on the plate, and on either the angle of the visual glide slope indicator, in other words the PAPI or VASI, or the VDA, the Vertical Descent Angle of the approach.
In plain English, it’s the procedure designers' attempt to show you when to begin a descent from the MDA on a non precision approach. Non precision approaches carry a risk of either hitting terrain or obstructions if a descent is started too early, or overrunning a runway if a descent is started too late.
How is the VDP designed?
Let’s see if we can figure out how the procedure designers chose where to put the VDP on this approach. First, there’s our defined VDP on the final approach course. We need to be able to identify it in the cockpit with something like DME or GPS. Next, the MDA: 4640 feet. The AIM says it’s based on the lowest MDA so keep this in mind if the MDA is higher for Cat C or D aircraft. It’s designed to descend to the touchdown point, so we’ll want to identify where that is. Finally, it’s based on the procedure vertical descent angle. Here it’s 3 degrees, but some approaches will use a different angle.
That's all the variables the procedure designers use to define a VDP.
Take a look at the graph on the left. There is an approach path using a 3 degree angle extending up from the runway touchdown point. (This one is not to scale). Then, we have the point where the descent should start. This is where our VDP is. It’s a lot easier to take height above touchdown which is the MDA minus the touchdown zone elevation, here it’s 343 feet AGL.
Now this looks like High School trigonometry right? We want to know the ground distance from the touchdown point, to the VDP. The math involves taking the cotangent of the angle and multiplying it by the height of 343. This gives us 6,545 feet between the VDP and the TD zone.
Ouch! Trigonometry? No thanks!
Definitely don’t worry about this math! We’ll see an easier way to approximate this further down, but this is how the distance on the approach plate is determined.
Now, 6,545 feet is from the VDP to the touchdown zone, but the approach plate shows distance from the runway threshold. So in order to get the distance to the threshold, we’ll need to subtract the distance to the touchdown zone.
On runway 17 The distance from the threshold to the beginning of the touchdown zone is a standard 500 feet. This is what we will subtract. This gives a ground distance from the VDP to the threshold of 6,045 feet, which is just barely short of 1 nautical mile. If we look at the approach profile view again, we see that this is indeed the distance charted from the VDP to the runway threshold, 1 nautical mile.
Of course we shouldn’t be going anywhere near trigonometry while flying an airplane, so we can use a rule of thumb to approximate the VDP: take the height above touchdown for the MDA, and divide it by 300. That gives us 343 divided by 300 which is 1.1 nautical miles. Not quite the same in part because it doesn’t take the touchdown zone into consideration, but this calculation will always err on the conservative side. Now we don’t really care about how to calculate the VDP on this approach because it’s done for us, but as we’ll see later, that’s not always the case.
Is it safe to leave the MDA after or before the VDP?
Since a non precision approach doesn’t offer vertical guidance, it’s typical for an aircraft to arrive at the MDA, level off, and search for the runway. If the visibility is decent, we might gain sight of the runway pretty early on, before reaching the VDP. If we start a descent from here, we’ll need less than that calculated 3 degree descent rate in order to make it to the touchdown zone and prevent landing short.
Recall what 91.175(c) says about leaving the MDA: three conditions need to be met before going below the MDA. We need to be continuously in a position to make a descent to land, we need flight visibility to be above minimums, and we need to be able to see one of a list of visual references. It doesn't say anything about going below the MDA before the VDP and it’s perfectly legal to do so. It does carry added risks of hitting terrain and obstructions starting an early descent though, and without sight of the PAPI or VASI at this point, it might not be possible to stay clear of them.
The same AIM chapter that defines the VDP also states clearly that the pilot should not descend below the MDA prior to reaching the VDP. So again, we may not be breaking 91.175(c) by descending early, but it’s strongly recommended we start that descent only at the VDP.
What if we gain sight of the runway after the VDP? We’ve delayed our descent as we’re required to, but if the descent is started after the VDP, it will require a steeper angle of descent to reach the same touchdown point. What does the regulations say about steep descent rates? Nothing except that the aircraft has to maintain a normal rate of descent. This is a bit subjective, but starting a descent slightly after the VDP doesn’t seem to break the rules requiring a normal rate. We could maintain that same 3 degree descent rate. What this would do is carry us further down the runway potentially past the touchdown zone. We’d be relying on our hopefully intimate knowledge of our aircraft performance, runway length and conditions, and density altitude to assure ourselves we won’t overrun the runway, which adds risk, but also doesn’t break any rules.
Commercial operations under part 121 and 135 require the descent rate to take the aircraft to the touchdown zone, which is the first 3,000 feet of runway. If we’re part 91, we don’t need to follow this, but an argument could be made that landing past the first 3,000 feet of the runway isn’t exactly a normal maneuver as the spirt of the regulations seems to call for.
For these reasons, many pilots tend to play it conservative and treat the VDP as a decision point. In poor visibility, if the pilot can’t spot the runway from the VDP, they will decide to execute a missed approach. This decision is unchanged, even if further along they do gain sight of the runway. The missed will still happen, and will be executed at the missed approach point.
There’s an interesting implication to this practice. Looking at this same approach, the distance from the VDP to the runway threshold is 1 nautical mile. The visibility minimums are 1 half statute mile. If the weather is at minimums meaning we can legally shoot the approach, we won’t have sight of the runway at the VDP, triggering our decision to go missed, even though once we arrive at the half statute mile point, we do gain sight of the runway and should be within our legal rights to land.
Why would the FAA design approaches where the VDP is out beyond the legal visibility minimum? It’s because one is not directly related to do the other. Visibility minimums are a legal definition, whereas VDP is designed to aid in your decision process. Remember that what keeps you legal doesn’t necessarily keep you safe.
Let’s look at an example where not using the VDP as a decision point might actually get us into legal trouble, as well as mortal danger. The Localizer approach to runway 28 at Baltimore. The visibility minimums are very low for a non precision approach, RVR 2400. Our aircraft could be well passed inside the VDP which is .9 nautical miles from runway threshold, and just gain sight of the runway at minimums. Let’s say we’re doing a typical category B approach speed of like 100 knots. If we want to make our touchdown zone, the first 3,000 feet of runway, from this point, at a height above touchdown listed at 477 feet, we would need a descent rate of 893 feet per minute. The aggressive rate might not satisfy the requirement in 91.175 of a normal rate of descent, and if we used a less aggressive descent rate, we’d sail considerably past our touchdown zone, also not ideal.
What if there is no VDP?
AIM 5-4-5 also cautions that a VDP may not always be published for a non precision approach. This could be because of an obstacle penetrating the visual surface area, no way of determining the VDP point from the cockpit, or the design itself preventing a means to identify a VDP.
Here’s one of those approaches, the RNAV into runway 10 at Bear Lake County. Notice on the profile view there’s no black V marking the visual descent point. We can use our rule of thumb to create our own: We take the height above touchdown for the non precision LNAV minimums of 329, divide it by 300 to get 1.1 nautical miles from the runway. This would put our made up VDP somewhere between ISELE and GLUTS.
We could use this as our decision point, but we need to ask ourselves why the FAA didn’t just put one here for us.
Is it because of obstacles in the approach zone? Well the presence of this light grey stipple emanating from the runway on the chart tells us that the visual path from the MDA to the runway is clear of obstacles, so that can’t be it.
What if we plot these points out on a map?
If we connect ISELE, our VDP, and the missed approach point GLUTS, and connect the approach course between them, we see that it is considerably offset from the runway centerline.
When we arrive at our calculated VDP on the approach, if we see the runway, we would need to make a right and a left to maneuver on to final as we descend. A VDP is calculated based off a straight line distance, not one where we’ll be making turns to get established. This maneuvering will increase our ground distance so we need to be more conservative when making these VDPs ourselves.
We can also see on the plan view of the chart that the final approach course is offset from the runway almost 20 degrees.
So while adhering to a VDP isn’t a hard legal requirement the way an MDA or a missed approach point is, there are many reasons why pilots play it conservative and make their decision point at this spot on a non precision approach. You can create your own on an approach that doesn’t have one using the divide by 300 rule of thumb, just remember to add in a bit of a buffer. If there isn’t a VDP shown, there’s a good reason for it. Also, since VDPs are derived from MDAs, if the MDA raises because of something like inop. equipment or an off field altimeter used, the VDP might not be relevant anymore.
Knowing your approach plates like the back of your hand is essential to the IFR student, and it is covered in depth, along with other important topics, in our IFR Ground School.