Timed VDP

[Tref]

I have heard recently that some pilots are being asked to calculate a timed visual descent point for a non-precision approach. While it's not difficult to do, I've never heard of anyone doing this in actual operations before and according to the AIM, "the VDP will normally be identified by DME." And, "pilots not equipped to receive the VDP should fly the approach procedure as though no VDP has been provided."

I think that timing is too inaccurate to be useful for VDP location identification and that it should not be used. Can anyone provide any references that state otherwise?

Thanks,
Tref

 

[Andy Neill]

I guess the question is, "Upon what will you base your decision to descend once you break out at MDA?" I agree that the timed version will probably be less accurate than using DME, but it still provides SOME useful info.

Let's say the MDA is 400' HAT and my VDP is now 10% of 400 (40) seconds short of the MAP. If you break out more than 40 seconds before MAP, will you begin your descent then or will you wait until your calculated VDP? If you don't break out until 20 seconds after VDP, will you attempt a landing or climb and fly your MA at the MAP?

A Jetstream crew in Missouri descended before the VDP on a DME calculation and hit trees 13 seconds later. All but two died.

 

[Serrano]

A Jetstream crew in Missouri descended before the VDP on a DME calculation and hit trees 13 seconds later. All but two died.

A Jetstream only requires two crew members. I speculate that what you mean to say is that of 15 souls on board there were 13 fatalities.

At the time of that acident Corporate Airlines did not train for or require the use of any VDP proceedures. The Captain called the runway in sight. I have never had a good answer to what he thought he saw, I do not believe that it
was RW 36 at KIRK.

If you do in fact have a runway in sight a timed VDP can cause you an unnessasary missed approach in the event that you screwed up the time or
encountered more tailwinds by forcing you to remain at the MDA untill
the clock runs out. IMHO a timed VDP should only be used as a test for reasonableness to provide some kind of WAG as to where you are on the approach.

The bottom line is that you have to be really careful when you are down
close to the ground in the clouds or the earth will rise and smite thee.

 

[rickair7777]

With no VDP published and no DME, it doesn't hurt to time a homemade VDP...you're going to time the FAF to MAP anyway right? Determining the expected VDP time in advance is a good SA tool...but you don't necessarily just chop & drop when you hit that estimated VDP time either.

 

[Tref]

I appreciate the thoughts, but does anyone have any official reference to timed VDPs? I've never seen one and I'm wondering if one exists.

BTW, the 10% of HAT for a timed VDP is only accurate if you're groundspeed for the entire Final Approach Segment is exactly 120kts. If it's not, then it's time to break out the calculator. Personally, I would prefer to look for a VASI/PAPA indication rather than use a less than reliable timed VDP.

Along the same lines, has anyone ever seen a step down point on a nonprecision approach that was based on time? I don't think one exists. I know that in order to use a VDP, at minimum the approach lights must be in sight, but the idea of obstruction clearance is the same in both cases.

 

[Tref]

With no VDP published and no DME, it doesn't hurt to time a homemade VDP...you're going to time the FAF to MAP anyway right? Determining the expected VDP time in advance is a good SA tool...but you don't necessarily just chop & drop when you hit that estimated VDP time either.

Would you stay at the MDA until your VDP time had passed, even if it looked like you were getting too high for a normal landing? If not and you would descend based on visual cues anyway, then what's the point?

Not being argumentative, and I agree that it might help with SA, but sometimes bad info is worse than no info.

 

[charter dog]

Aim 5-4-5

Here's the rest of what the AIM has to say about VDPs:

f. Visual Descent Points (VDPs) are being incorporated in nonprecision approach procedures. The VDP is a defined point on the final approach course of a nonprecision straight-in approach procedure from which normal descent from the MDA to the runway touchdown point may be commenced, provided visual reference required by 14 CFR Section 91.175(c)(3) is established. The VDP will normally be identified by DME on VOR and LOC procedures and by along-track distance to the next waypoint for RNAV procedures. The VDP is identified on the profile view of the approach chart by the symbol: V.

1. VDPs are intended to provide additional guidance where they are implemented. No special technique is required to fly a procedure with a VDP. The pilot should not descend below the MDA prior to reaching the VDP and acquiring the necessary visual reference.


2. Pilots not equipped to receive the VDP should fly the approach procedure as though no VDP had been provided.

If someone is requiring pilots to calculate "home-grown" VDPs on the basis of timing, I can't imagine where they found any regulatory or advisory guidance to do so. This may exist, but I just haven't seen it. So here's my take on it. I remember back when they first started publishing VDPs, there was some discussion among myself and some local pilots as to whether they should be calculated where not published. Among the questions raised were:

1) If no VDP is published, and we wish to create our own, what descent angle will be required in order to provide sufficient obstacle protection?​

2) What about NPAs where timing from the FAF is used to identify the MAP?​

The consensus was that:​

1) If a PAPI or VASI serves that runway, then obstacle clearance is provided if the calculated VDP is located at the intersection of the MDA and the PAPI/VASI path. If not, then there would be no known level of protection unless an obstacle analysis was available for that runway.​

2) The timed VDP would be of questionable benefit due to it's inherent potential for inaccuracy.​

Conclusions:​

A published VDP should be considered as though it were a stepdown fix, especially during low visibility and night operations. Descent should never be commenced prior to the VDP during limited visibility. Visual obstacle clearance is of limited value at best during these conditions.​

Any non-published (calculated) VDP should be considered as being for planning purposes only unless an airport obstacle analysis has been completed or a VASI/PAPI path will be followed. Quite useful if used solely to determine a descent point from MDA in conditions which allow good visual obstacle clearance or as a point at which you would expect to intercept the PAPI/VASI path. The timed VDP would have all the value of SWAG! Timing is fine for a MAP where there is nothing to hit at MDA anyway, but will be of little value in determining a descent point from MDA to the runway.​

My favorite use for the "home baked" VDP is on approaches with no straight-in mins published, but a straight-in landing is usually expected to be accomplished. ASE and SMO are good examples.​

MDA at SMO on the VOR or GPS-A is about 500 HAA. Trouble is, you can't descend from 940 HAA until 1.5 NM from the end of the rwy. (2.4 DME) The PAPI is 4.0 degrees and is located about 700' from the end of rwy 21. The PAPI path is at about 200' below you when you begin descent to MDA from 1120 msl. (940 HAA) You'll never make it in a jet without vastly exceeding stabilized approach criteria. If visibility is OK, you can circle if cat D mins exist. If not, see ya! So when must you see the airport by to have a realistic shot at a straight-in? The 4 degree PAPI crosses the intermediate stepdown altitude of 940 HAA at about 2.3 NM from the PAPI. So you need to see the runway by then. That works out to about 3.2 DME, or 0.8 Nm prior to the final stepdown fix. (CULVE 2.4 DME) So that's what we brief: If we don't see the runway by 3.2 DME, we fly the missed approach. OR, if reported ceiling and visibility were determined to be sufficient to meet cat. D mins: We'll circle to the left for rwy 21 at the 740' MDA. (we're cat C except when circling, where cat D must usually be used due to our normally >141 kt circling speed) For this reason, we like to make the straight-in or circle decision prior to beginning the approach and commit to it. I've been involved in the impromptu change of plans during mid-approach before and was not happy with the uncertainty it created. We're paid to do better than that.​

So it turns out that this "home baked" VDP calculation is quite useful for this kind of planning if you take the time to calculate it beforehand. However, I would personally never use timing for this purpose. Fortunately, we have GPS and can calculate a VDP as measured from the ARP if the GPS is not being used as primary navigation for an instrument approach, as might be the case during a visual to a runway not served by vertical guidance or a LOC without DME. Happens all the time with all the podunk airports we frequent!​

Apologies for the long-winded post. It was a boring day on call with no action and I had enough Starbuck's espresso to really get into this!​

Best,​

 

[Tref]

Thanks Charter Dog. I think we're on the same page. BTW, how did you cut and paste the AIM reference? I tried on my initial post and it wouldn't let me.

Tref

 

[charter dog]

BTW, how did you cut and paste the AIM reference? I tried on my initial post and it wouldn't let me.

Hi Tref,

I just did a normal highlight, Ctrl+C on the FAA.gov AIM and pasted it. Then I highlighted it, wrapped the .php quote tags from the reply toolbar around it and voila! Done. I've had trouble in the past with .pdf documents not letting me copy. This one should work.

Best,

 

[DC8 Flyer]

Timed VDPs are in now way shape or form required. They are however easy to use, can make a non-precision approach without DME or a published VDP much easier. When you fly something with some size to it (thats not meant as a cut, just my observation on how helpful timed VDPs are) you will appreciate the timed VDP. It is much harder to get a 275K pound airplane at 145 knots stabalized from the MDA to the runway, than it is a say a baron. Having a fixed point to know where the 3 degree path intersects the inbound course at MDA helps when making the "continue or go missed" decision.

While going missed at the VDP or your homemade timed VDP is not required, it is, IMHO, a good idea, of course no turns until you pass the published MAP.

Calculating the timed VDP is easy, takes a minute or two, so not something to do while inbound on the approach.

Some basic numbers to help understand how
3 degree glide slope = about 300 feet per NM
60 knots = 1 NM per minute

So with a VDP of 400' AGL and a time of 3:30 at 120 knots (7.5 NM from FAF to MAP) from the FAF to the MAP (assuming the MAP is at the end of the runway), and your actual GS for the day is 150 knots, my timed VDP would be about 2 minutes 15 seconds from the FAF (about 5 miles from the FAF or 2.5 miles from the FAF). That gives me 1 minute 15 seconds to go from 400' to the runway at a GS of 150 knots, 3 degree slope at 150 knots is (5 * 150) = 750 FPM. There is plenty of room in the time for me to start down, since I don't instantly go from level at the MDA to 750 FPM, and time to find the runway enviroment and line up, if need be.

The 10% rule would work out nicely here as well, 10% of your HAA, in this case 400' AGL, would be 40 seconds. Take 40 seconds off of 3:30 and you get 2:50, where I came up with 2:15.

Timed VDP is not meant to super accurate, mearly a guidance point, you could sit down and figure it out exactly but that would mean you woud have to fly a constant groundspeed all the way in, which is more difficult than necessary. If you understand how to use the numbers, you can get a fairly accurate time point and make that transistion even easier from MDA to landing or missed.

Hope this helps.