Timed VDP

[Tref]

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.

You do realize that in your example the difference between 2:50 and 2:15 is 35 seconds and at 150kts the difference is 1.5nm? You think that works out nicely? I mean, I know it's called a "nonprecision approach" but isn't that a bit much?

Also, (maybe I just have a delicate ego) but I kinda thought a 747-400 was "something with some size to it." An 875,000lb Baron would be interesting.

Tref

 

[DC8 Flyer]

You do realize that in your example the difference between 2:50 and 2:15 is 35 seconds and at 150kts the difference is 1.5nm? You think that works out nicely? I mean, I know it's called a "nonprecision approach" but isn't that a bit much?

Also, (maybe I just have a delicate ego) but I kinda thought a 747-400 was "something with some size to it." An 875,000lb Baron would be interesting.

Tref

Put your ego back in the flight kit. I said no cut intended for the aircraft size, and since I didn't quote you, I wasn't speaking to you directly, but you chose to take offense. The timing is simply a SA tool not meant to be precise or gospel, my airline uses it, works well for all of us. The time difference I noted was to show the difference between crunching the actual numbers and using a rule of thumb. They both work and give you a good idea where abouts to start looking for a landing or start thinking of missed. You don't take the 747 to the MAP and then look down at 400' AGL and try to make the runway do you?

Try it the next time you have to do a non precision and the weather is "good", you may like it. If not, no problem if I use it, is there?

 

[charter dog]

If you actually use a VDP, keep in mind the distance will be most likely greater than the visibility approach minimums for the non precision approach.

Hey Rez, that's so true. Here is the case where using a VDP/PDP has the effect of raising the minimum visibility and ceiling required for a straight-in above that which is published because it is a circling approach. At least from a practical standpoint if you hope to achieve a stabilized approach, as in the case of the SMO example. We would need 1000-2 1/2 (Cat C) for the straight-in from the PDP ( I'm starting to like this term!) and only 600-2 (Cat D) to circle. I know from personal observation that people drive it in there on a homemade baro VNAV profile to make it straight-in all the time. If GLS, LPV or baro VNAV mins ever get published for SMO, it'll even be legal!

Best,

 

[Tref]

Put your ego back in the flight kit. I said no cut intended for the aircraft size, and since I didn't quote you, I wasn't speaking to you directly, but you chose to take offense. The timing is simply a SA tool not meant to be precise or gospel, my airline uses it, works well for all of us. The time difference I noted was to show the difference between crunching the actual numbers and using a rule of thumb. They both work and give you a good idea where abouts to start looking for a landing or start thinking of missed. You don't take the 747 to the MAP and then look down at 400' AGL and try to make the runway do you?

Try it the next time you have to do a non precision and the weather is "good", you may like it. If not, no problem if I use it, is there?

The reason I thought you were replying to me is because I started the thread and you didn't quote anyone else.

Also, you can do whatever you like, but do you really think that inaccurate data increases SA? I think that it decreases it.

 

[flx757]

Inaccurate data, good idea or bad...our Company Flight Manual spells it out pretty clearly.

"For situational awareness, calculate a VDP (when one is not depicted). If the runway is not in sight untal after passing the calculated VDP, the descent glidepath is greater than 3-degrees and a descent rate greater than 600 FPM is probably needed. Caution should be used when leaving the MDA prior to the calculated VDP without glidepath guidance.
- There may be limiting obstacles.
- A shallow glidepath can lead to visual illusions during the transition to landing and can cause a long landing.

Then there is a discussion and examples on how to calculate a VDP using either distance or time.

Personally, I believe this whole discussion can be made moot by eliminating the "dive and drive" non-precision approach in favor of a constant path angle type approach. These are safer and can help reduce CFIT accidents during the approach phase. I am in disbelief that my company still has not moved in this direction even though they keep promising that this change is "coming".

 

[DC8 Flyer]

The reason I thought you were replying to me is because I started the thread and you didn't quote anyone else.

Also, you can do whatever you like, but do you really think that inaccurate data increases SA? I think that it decreases it.

I guess I wouldn't call it inaccurate data. I pulled a quick example out off the top of my head to show the numbers and how the "long hand" version and the 10% rule can be fairly close. We can argue the difference in 45 seconds, but the fact remains, if you sit down and figure it out to the inch, it is all out the window as soon as your groundspeed changes. Calculating a point at which you start thinking more about going missed than continuing helps me personally, in getting the big picture. Maybe it doesn't work for you, thats fine, but maybe my method will help someone out. I never claimed it was dead on balls accurate, but simply a way to keep track of your progress along the final approach course relative to "glide slope" intercept.

I give myself a buffer in the time, because we do a config change before leaving the MDA for the runway, flaps 35 to 50, that takes a little bit to restabalize and get going down at the proper rate so waiting until the last second will leave you high and diving for the runway so I lead it a bit.

 

[Tref]

Personally, I believe this whole discussion can be made moot by eliminating the "dive and drive" non-precision approach in favor of a constant path angle type approach. These are safer and can help reduce CFIT accidents during the approach phase. I am in disbelief that my company still has not moved in this direction even though they keep promising that this change is "coming".

That's how we did it in the 400 (and the A330.) We were not allowed to fly level at the MDA. The MDA was treated more like a DA(H) and once you "touched" MDA you had to go around. It took a bit more planning, but I think it was much safer.

 

[Tref]

I guess I wouldn't call it inaccurate data. I pulled a quick example out off the top of my head to show the numbers and how the "long hand" version and the 10% rule can be fairly close. We can argue the difference in 45 seconds, but the fact remains, if you sit down and figure it out to the inch, it is all out the window as soon as your groundspeed changes. Calculating a point at which you start thinking more about going missed than continuing helps me personally, in getting the big picture. Maybe it doesn't work for you, thats fine, but maybe my method will help someone out. I never claimed it was dead on balls accurate, but simply a way to keep track of your progress along the final approach course relative to "glide slope" intercept.

I give myself a buffer in the time, because we do a config change before leaving the MDA for the runway, flaps 35 to 50, that takes a little bit to restabalize and get going down at the proper rate so waiting until the last second will leave you high and diving for the runway so I lead it a bit.

I guess I understand you a little better now. So you don't use it like a real VDP, ie. you can't desend before this point, but more of an "expect to be on the glidepath around this point, plus or minus a bit." Fair enough, but really, we're both still looking out the window for our primary cues. Wouldn't you agree?

 

[DC8 Flyer]

I guess I understand you a little better now. So you don't use it like a real VDP, ie. you can't desend before this point, but more of an "expect to be on the glidepath around this point, plus or minus a bit." Fair enough, but really, we're both still looking out the window for our primary cues. Wouldn't you agree?

Agree 100%. To be perfectly honest however, when it is down and the wx is right at mins for a non precision, I find it very hard to "visually" fly the airplane down without some sort of vasi/papi guidance. It has always been a weakness of mine, the whole runway in the window thing just doesn't work for me, so I have to use math, ie distance from the runway to make it "look" right. Hopefully my company will get the approval to start using the FMS's we have to their full potential, nice VNAV all the way to the runway!

 

[Tref]

Agree 100%. To be perfectly honest however, when it is down and the wx is right at mins for a non precision, I find it very hard to "visually" fly the airplane down without some sort of vasi/papi guidance. It has always been a weakness of mine, the whole runway in the window thing just doesn't work for me, so I have to use math, ie distance from the runway to make it "look" right.

I know exactly what you mean.

Hopefully my company will get the approval to start using the FMS's we have to their full potential, nice VNAV all the way to the runway!

It's the only way to fly! Or, you could have a Flightpath Trajectory Indicator on the PFD like in the Airbus. Then you can say, "gimme the bird." Works great every time.

 

[FL420]

Hopefully my company will get the approval to start using the FMS's we have to their full potential, nice VNAV all the way to the runway!

Many companies fly equipment capable of performing Constant Angle Non-Precision Approaches(CANPA's) but don't want the extra costs of initial and recurrent training in CANPA's for each pilot. You must also have approval in the OpsSpecs.

When the Wx is right down to minimum visibility you might find yourself doing more missed approaches doing a CANPA instead of a dive and drive approach. That is because the MAP on a CANPA is at DA(H) up to two miles from the runway threshold while the MAP on a dive and drive is usually at the runway threshold. A non-commercial operator flying a typical GA aircraft to a long runway has a better chance of getting aboard safely using the dive and drive when not constrained by a requirement to land in the touchdown zone from a stabilized approach.