Visual Descent Point

[MarineGrunt]

I was studying an interview gouge, and I read that one interviewee wrote, "Also, he expected me to calculate the VDP even though there was none depicted on the approach."

How does one calculate the VDP? Does he mean Visual Descent Angle? (3 degrees)

While we're on the subject, is it usually advisable to go missed at the VDP if you do not have the rwy ennvironment?

 

[TR4A]

How does one calculate the VDP? Does he mean Visual Descent Angle? (3 degrees)

Normally use 300ft/nm on a 3 degree glidepath.

600' MDA = 2 miles from rwy
450' = 1.5

 

[Starcheck]

Try this works great.

Time for the appoach - HAT/10 = Time to see (or VDP)

Example;

Time = 2:12
HAT = 600 feet

2:12 - 60 = 1:12

1:12 is the estimated VDP and gives approximately 3 degree slope.

This also allows for different ground speeds.

Remember to check for obstacles (Cumulogranite) that could encroach on the 'normal' glidepath.

 

[avbug]

You could begin the missed at the published VDP, or your own ad-hoc "vdp," but you still cannot begin any turns until reaching the published MAP. You can climb, but that's it.

 

[flyifrvfr]

If you don't see the runway at the VDP you can still continue the approach until the MAP. If you go missed at the VDP you may never get the chance to land because normally the MAP is past the VDP.I post this in reference to a VOR approach at KDMW.This approach has a VDP on the plate,But I still use timing of the approach as my MAP when no other MAP fix (dme) is given.So as you can see if you didn't fly the approach as dipicted when the weather is near minimums,you might not see the runway from the VDP.Remember the rule is as long as you see the runway enviroment and are in position for a normal approach (no dive bomber) you can land.

 

[firstthird]

Apparently the FAA and many of the majors think that once you are past the VDP (depicted or calculated) regardles of whether you see the runway later you will not be a position to land using a 'normal approach.' I can't speak much to cessnas and small planes, but the theory with the big ones is that once you are past a 3 degree descent point it would then be a non-normal approach and thus more dangerous. This is still being figured out some though I think and the regulations haven't caught up with the theory necessarily. Meaning, unless your company says otherwise, it might be legal to try to land a 737 from the MAP at 600 feet AGL and a mile from the thresehold but it wouldn't be smart or good piloting technique. (unless of course you consider a 6 degree glideslope normal approach technique) IMHO.

Avbug is absolutely correct though about beginning the missed, go ahead and start a climb, but no turning until you get to the actual MAP.

 

[flyifrvfr]

My assumption is that of small planes like I fly now. I have no experience flying large aircraft so I don't want to speak of what I don't know.In a light twin I would do as I posted and I can only wait to learn what I must do when flying a Kingair or business jet.

 

[MarineGrunt]

Apparently the FAA and many of the majors think that once you are past the VDP (depicted or calculated) regardles of whether you see the runway later you will not be a position to land using a 'normal approach.' I can't speak much to cessnas and small planes, but the theory with the big ones is that once you are past a 3 degree descent point it would then be a non-normal approach and thus more dangerous. This is still being figured out some though I think and the regulations haven't caught up with the theory necessarily. Meaning, unless your company says otherwise, it might be legal to try to land a 737 from the MAP at 600 feet AGL and a mile from the thresehold but it wouldn't be smart or good piloting technique. (unless of course you consider a 6 degree glideslope normal approach technique) IMHO.

Thats what I was thinking.

So in an airline interview (sim portion) would it be the better option to go missed at the VDP?

 

[TonyC]

Thats what I was thinking.

So in an airline interview (sim portion) would it be the better option to go missed at the VDP?

Airline Interview suggests, in my mind at least, a Transport Category aircraft, and that in turn means you must land in the touchdown zone, that is, the first 1,500 feet of the runway. A properly designed VDP is designed to put you in that TD zone, so flying beyond the VDP at the MDA pretty much guarantees you will NOT be in a position from which to make a landing. (I realize I've made a few assumptions here, and I haven't quoted regs, but the assumptions are important, and I think the regs will back me up here.)

Interview... Once you've passed the VDP at MDA, you've made the decision to Execute the Missed Approach procedure. Two considerations - - 1) the Instrument Procedure and 2) the aircraft procedure. Nothing prevents you from beginning the aircraft procedure (power, configuration, maybe something else?) while beginning to climb and/or accelerate. (I'm trying to keep this aircraft generic.) You can climb at any time, but you can't turn before the MAP. If you've already started the configuration and acceleration/climb routine just beyond the VDP, all you should have left to do is a nice casual turn at the MAP.

Whatever you do - - do NOT give any indication that you are even REMOTELY considering the option of LANDING once you've passed the VDP. You're not in a C172 with tower clearance to land long. You WILL execute the missed approach procedure, right?!?!?

Good luck on the interview.

(Firstthird - - ironic that you have SWA in your AVATAR and talk about 737's on steep approaches - - I am PRETTY sure they frown on those! )

 

[TonyC]

How does one calculate the VDP? Does he mean Visual Descent Angle? (3 degrees)

Might I go out on a limb and offer my perspective on this question?

Without reference to AIM or any other "official document" I would tell you that a VDP (Visual Descent Point) is the point along the MDA where I need to begin a normal descent in order to land in the touchdown zone. In other words, it's the point where a normal glidepath intercepts the MDA.

Now, if I determine 3 degrees to be a normal glidepath, or descent angle (and it's a pretty standard number), and I apply the 60:1 Rule (approximation, granted, but it works great for this), then I can use information provided on the "Approach Plate" to help me determine this VDP. Take the HAT for a straight-in approach and divide by 300 to get a distance from the runway.

For Example: HAT = 450 - - - - > 450/300=1.5

So, now you have 1.5, what do you do with it? How do you apply it in the cockpit?

If you have a glass cockpit of some sort, you might be able to read the distance to the runway directly from the information displayed in your cockpit - - if that's the case, you're done. If you don't have glass, no problem. Usually, you have some sort of DME information available.

Let's say you're conducting a VOR approach where the VOR is located beyond the intended landing runway. You see that the approach end of the runway is 5 miles beyond the FAF, and the FAF was 12.0 DME. You can determine that the end of the runway (maybe the MAP, too) is 7.0 DME. Add in your VDP number (1.5) and you can locate the VDP in terms of DME - - 8.5 DME.

Obviously the math will be different if the NAVAID is behind you, and won't apply if it's not located along the approach course, or if it doesn't exist at all. Then you'll have to rely on the timing methods of computing a VDP mentioned above.

A VDP, when used wisely, can be an extremely useful tool. You can continually update your descent progress with reference to the VDP to determine if you're too high and need to increase your descent rate, or too low and need to decrease the rate. Obviously it can be useful in showing you where to begin a descent if you see the field (you should not depart MDA prior to the VDP), and it can help you in deciding to execute the Missed Approach (better to execute from level flight at the MDA rather than from a gaggle near the graound when you FINALLY realize you can't make a safe landing).

Hope that helps.

 

[firstthird]

Tony C,
I'm sure they do also. I haven't started at SWA yet, just in the pool. I've been flying 737 for the Navy but look forward to learning the LUV way soon. That is why my avatar is just a kid's ballon blow up 737 and not a real one, I'm still a wannabe who hopes to be on property soon.

 

[TR4A]

you must land in the touchdown zone, that is, the first 1,500 feet of the runway

I though the Touchdown Zone was 3000 feet.

 

[TonyC]

I though the Touchdown Zone was 3000 feet.

What you hear is the sound of my limb creaking....

I stand corrected.

 

[KickSave]

My understanding is that the VDP represents the point where a STRAIGHT IN landing can be made under normal maneuvers, and land in the TDZ. Being that the MAP will be well beyond the VDP, approaches to land can still be made if the airport is located after the VDP, but before the MAP, only it must be a circling approach. I can't speak for various airline policies, but my understanding is that, regardless of whether a published VDP exists, calculating one using that 300/1NM rule gives you the point for a normal straight in landing, but beyond that, circling approaches are still acceptible up to the MAP.

Still depends on what the pilot (or the FAA?) considers "normal maneuvers??? Any thoughts?

 

[avbug]

Kicksave,

Yes, you can always circle. However, the premise of the VDP is the straight-in approach; it's a method by which to see the runway and achieve a stabilized 3 degree descent to the runway, straight-in. At many airports, by the time you reach this point, you're already below the circling minimums, and a missed approach is in order if the required visual references aren't available at the VDP.

 

[SkyWestCRJPilot]

Here is another reason for a VDP. Let's say you come down to your MDA and you are there early. If you see the runway you shouldn't start a descent from the MDA till you are at your VDP. That is the point at which a continous descent to a landing using normal manuevers can be made. Otherwise you may end up coming down from you MDA too early and your last 1-2 miles of the approach may be at a 1-2 degree glideslope. You could end up taking out a few power lines or trees in that case. Ideally when you reach your VDP while at your MDA the VASI shows red over white. Incidentally I was reading in Aviation Week that a country in Europe has decommissioned all of thier nonprecision approaches. They believed that unstablized nonprecision approaches are a leading cause of CFIT.

 

[spanky2]

My previous airline started training (2001 or so) for CANPA...constant angle non-precision approach. No more "dive and drive" with logic being that diving to the MDA at low altitude was exceeding 1000 fpm which is not allowed. SOOO..result is a quasi-precision approach based on computing VDP (300' per mile) and backing that up to FAF altitude and then using the requried vertical descent rate ... not to exceed 1000 fpm...to arrive at VDP altitude. If rwy not insight at that point...TOGA, power, pitch, pos rate..."gear up"...etc.

Even with a depicted VDP...we still had to come up with the descent rate...ball park is always going to be 700 - 800 fpm for normal 737 speed. Not sure if this has changed.

 

[JJJ]

you must land in the touchdown zone, that is, the first 1,500 feet of the runway
--------------------------------------------------------------------------------

I though the Touchdown Zone was 3000 feet

or the first half of the runway which ever is shorter, right? For runways less than 6000 ft.


You can't start down before the VDP, but I think saying that leaving the MDA past the VDP is unsat or unsafe is not true. The regs say using normal manuvers, normal rate of decent, and land within the touchdown zone or first half of the runway. Starting down at the VDP normally allows for about a 3 degree decent at around 700-800 fpm at jet speeds. Usually a "stabilized approach" is 1000 fpm or less. This would allow you to decend from the MDA past the VDP and still make a stabilized approach to the runway.

If this were not true, you could only decend at exactly the VDP, not before or even .1 after.

 

[TonyC]

or the first half of the runway which ever is shorter, right? For runways less than 6000 ft.


You can't start down before the VDP, but I think saying that leaving the MDA past the VDP is unsat or unsafe is not true. The regs say using normal manuvers, normal rate of decent, and land within the touchdown zone or first half of the runway. Starting down at the VDP normally allows for about a 3 degree decent at around 700-800 fpm at jet speeds. Usually a "stabilized approach" is 1000 fpm or less. This would allow you to decend from the MDA past the VDP and still make a stabilized approach to the runway.

If this were not true, you could only decend at exactly the VDP, not before or even .1 after.

I'll defer the "first half of the runway" issue to someone who knows more about it than I. I have never heard of that being in the regs, but there's a lot that I don't know.

I will, however, address your last statement. If you compute a VDP based on a given rate of descent to a certain point on the runway, you must begin the descent from that point at that given rate of descent in order to arrive at that certain point on the runway. If you delay the descent, you must either increase the descent rate, or land beyond the planned point of landing, or both. How much deviation you allow in terms of descent rate and touchdown point are matters of judgment. Is 3.1 degrees acceptable? Is 500 feet long acceptable? You make the call, but it IS a judgment call. On an airline interview, I don't think you can go wrong being conservative.

 

[SkyWestCRJPilot]

I don't think that descending slightly past the VDP is a problem. What about headwinds or tailwinds. A 40 knot headwind vs. a 10 knot tailwind will affect your descent rate more than a few tenths of a nm. Most MDAs I've seen are around 350-600 feet above the TDZE. That creates a VDP of around 1.1 to 2 nm before the runway. At the higher MDA and a farther VDP (2 nm) you have a little more room to manuever and adjust your descent rate than with a lower MDA and closer VDP. It requires good judgement taking into account your familiarity with the airport, aicraft characteristics, runway length, and contamination. It would differ how strict I would be were I flying a DC-10 vs. an E-120. Nonprecision approaches are inherintly unstabilized approaches so you have to have good judgement when it comes to descending "slightly" past the VDP. That would be my longwinded explaination during an interview.