No VDP and obstacles

Maxed-out

Maxed-out

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Maxed-out
Looking for some feedback about the following statements made by a CFII today. True or false?

An instrument approach with a depicted VDP guarantees a pilot obstacle protection below the MDA all the way to the runway threshold.

An instrument approach without a depicted VDP, offers NO guarantee of obstacle protection once the pilot descends below the MDA on the way to the runway.

The CFII was talking about this today and I began to think about the non-precision night approaches I've flown (no VDP) where I broke out of the clouds, had the runway in sight at a reasonable distance, dropped below the MDA, but was not too concerned about hitting an obstacle along the final approach course. Thought it was evaluated for obstacles and as long as I didn't descend too early, I was good to go.

Thanks!
 
The part of about the VDP is mostly true. At the time the procedure is defined, if there is an obstacle that penetrates the protected area, a VDP will not be defined. In addition, the visibility minima will be raised and if the obstacle is unlit, the procedure will be NA at night.

The lack of a VDP doesn't necessarily mean there is an obstacle. There are other criteria that inhibit VDP definition (as well as the fact that the approach hasn't been revised since they thought VDPs were good ideas).

Further, your instructor is dangerously wrong in his statement. The VDP vs. obstacle criteria apply only to when the approach is defined. There have been a few crashes were people have hit trees that have subsequently grown up into the VDP-to-runway glidepath. Visual, still means visual, you gotta be looking outside during that phase.
 
The obstacle criteria for a VDP is that the visual segment be clear on a 20 to 1 slope. A three degree glideslope is 19.08 to 1. Not much clearance for many approaches. On the newer RNAV procedures, if flight test indicates that there are obstacles that are considered particularly hazardous, in addition to not having a VDP published, they will have a note "Descent angle NA" and not publish a visual descent angle. These procedures will not provide advisory vertical guidance on WAAS GPS systems (AKA LNAV+V or LP+V). Not all procedures that need this note have been updated to the new standard.
 
John mentioned VDAs, which is another topic that was being thrown around yesterday. The statement was made that anytime you see the note "Descent angle NA" you can bet that obstacles will be in the visual area. I think he also mentioned something about not flying VDAs all the way to the runway. All of this conversation was pertaining to flying instrument approaches at night and some of the traps an unsuspecting pilot might experience. I've got to read up on VDAs, but why publish a VDA if it only helps you from FAF to MDA, but doesn't necessarily protect you below MDA? Obviously, my eyeballs are outside once I've made the decision to descend below MDA on the visual segment, but at night the old eyeballs can only help me so much when looking for obstacles. And of course I'm not just looking 100% outside, but also looking inside at the VSI and altimeter etc., so I could see the possibility of not seeing something until it's too late. I guess I'm looking for some kind of guarantee that I won't hit anything below MDA when their really isn't one. Just depends on the approach. For the next night approach I'm involved in, you can bet that I'll be much more aware of possible obstacles in that visual segment. Anyway, thanks for the feedback.
 
During the daytime, you need to avoid obstacles visually. At night, unlit obstacles can't be seen. So how comfortable are you if you sink a little below the 20 to 1 slope on the approach to the runway, assuming you can't see the obstacles at night.

The VDA and VDP are relatively new things. TERPS is used to determine if a VDP may be drawn, but it takes a flight test to determine if the VDA should be charted. The basic RNAV NPA is designed with just clearing all obstacles by 250 feet. The visual segment is evaluated on a 20 to 1 slope, but this primarily affects whether or not the procedure can be flown at night and it establishes a minimum visibility of 1 NM if below the 20 to 1 criteria. IOW, for these procedures, you have to see the obstacles to avoid them. There is one procedure, the RNAV 7 at N23. If you follow the VDA below the MDA, it will take you right thru the side of a ridge. There are other examples where there is a tall tower slightly offset an below the MDA on final. It is easy to avoid visually, in the clouds, not so much.
 
I found this info out on the net which pretty much says it all about how following VDAs below MDA can get the unsuspecting pilot in the tall weeds quick.

From an Aeronautical Charting Forum way back in April of 2012.

Ironically, VDAs were added to procedures to reduce the cases of controlled flight into terrain (CFIT) by providing a means for stabilized descents. However, blind application of VDAs has resulted in misleading information that makes it look like once the aircraft is established on the published VDA it has a clear path to the runway. This is especially compelling with the increased use of RNAV avionics and glidepath guidance (albeit advisory in nature) provided for the pilot on the primary flight display.
Click to expand...
Also, in this same document -

Recommendations:
Add a warning to the approach chart that calls the pilot’s attention to the fact that there are obstructions penetrating the 34:1 surface in the visual segment and that the published VDA may not clear those obstacles below the MDA. Possibly something like, “No obstacle clearance below MDA on VDA.
Click to expand...
Is this "no obstacle clearance below MDA on VDA" warning being included on approach charts now?
 
Interesting stuff here about this subject :
At my airline we recently just switched to CANPA Constant Angle Non-Precision Approach. Basically you now used DDA ( Derived Decision Altitude) by adding 50' to the LNAV MDA , and using a FAF-to-RW distance chart to derive a Decent rate. No more briefing VDP's which is a concept I'm still getting used to. I used the technique in a Debonair today on a GPS approach and it worked flawlessly. It makes any NP approach much more like a precision approach.


from IFR magazine on this obstacle subject...

Assuming No Obstacles
Hopefully, by now you understand that TERPS don’t guarantee obstacle clearance below the MDA on non-precision approaches. Yep—ya gotta look outside. But, since we’re discussing following that VDA, it might be good to take another look at what the chart tells you about possible obstacles, especially since Jeppesen and AeroNav do it differently.

As a rule of thumb, if the Jeppesen chart has the note, “Only authorized operators may use VNAV DA(H) in lieu of MDA(H)” the visual area is clear below MDA. If AeroNav charts have the note, “Descent Angle NA” the visual area is not clear below MDA. Also, if AeroNav identifies a VDP, the visual area is clear, but lack of the VDP doesn’t indicate obstacles. Even with visual area obstacles, you could still fly the VDA to the MDA, but we don’t recommend it as it’s just natural to continue the descent below MDA.

The visual area is clear on our approach, so you can safely fly the VDA to the runway, as long as it’s in sight before you reach MDA. But unless you can program in a 3.06 degree descent to begin at KAPUT, how would you go about flying this CANPA-thing

http://www.ifr-magazine.com/issues/1_4/features/Constant-Angle-Descent_32-1.html
 
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Maxed-out said:
I found this info out on the net which pretty much says it all about how following VDAs below MDA can get the unsuspecting pilot in the tall weeds quick.

From an Aeronautical Charting Forum way back in April of 2012.

Also, in this same document -

Is this "no obstacle clearance below MDA on VDA" warning being included on approach charts now?
Click to expand...

I attend the ACF/IPG. The recommendation was not accepted at the ACF. A change was made to the instrument procedure flight test to evaluate the suitability of using a VDA on the approach and if it was not suitable to remove the VDA from the chart and add the note "Descent Angle NA". The FAA also screwed things up for a while by arbitrarily changing the database specification to code a zero glidepath angle in the WAAS LP procedures. Garmin had an error in their logic and would cause a divide by zero fault on any of these procedures, so affected procedures were removed from the database. It took a good part of a year before the software was updated on both the GNS and GTN navigators to permit these procedures to be added back into the database. Every once in a while I see a post, where is my approach, it isn't in my database. Those with down level software still don't see these approaches, there are more than sixty of them so far.
 
A post in this thread about "derived decision altitude" as it relates to continuous descent final approaches (CDFA) got me wondering about whether I (a Part 91 grunt) can also use DDAs on certain non-precision approaches. I noticed he used the procedure in a Debonair, so maybe I've been missing out on something. The minimum amount of stuff I've read about DDAs is always aimed at "operators" meaning part 121 or 135 guys, I think. It seems simple enough - just add 50 feet to the MDA and that becomes the new Derived Decision Altitude. As I execute the CDFA in IMC, I arrive at the VNAV DDA and either have the required stuff in sight and continue down the path or don't have anything and execute the missed approach, never going below the MDA. Precision flying at its best.

Anybody know if Part 91 pilots can calculate their own DDAs and apply that value to approaches (non-precision RNAV) with VNAV/VDA info available? I guess when it comes right down to it, who would know. Just as long as it's safe, right?

I'm probably missing something fundamental here, so forgive the ignorance. Of course, that 50 feet could be the difference between a pilot seeing the airport and not seeing it, so you really have to be a die-hard continuous descent advocate vs. the old dive-and-drive technique. I can see the logic in both.
 
The DDA assumes that if you don't see the runway at the higher DDA, the extra 50 feet will provide a cushion to commence the pull up without busting the MDA. When the visibility is good and the overcast is high, there is a good chance that the approach may be completed. When the visibility is poor and close to or at the approach minimums, a missed approach is likely to result whereas using the dive and drive would have resulted in a safe landing.

Most non precision approaches have a visibility requirement around 1 SM. Using the DDA often increases the effective visibility minimum to 2 SM or greater, or you simply won't see the runway when you arrive at the DDA. Unlike the long and well lit runways that the airlines fly into, many GA airports do not have approach lights, are narrow, tree hidden, and not well marked. For such a runway, it takes time to analyze what you are seeing is actually a runway in low visibility. The DDA robs you of time under these circumstances and you must make your decision further from the runway and higher above it. A turbojet doesn't have the flexibility or agility that a GA piston aircraft has in these circumstances, and is much safer flying a constant angle descent.

So my conclusion is that using the DDA and a constant angle descent is good practice under many conditions, but there are limitations when the weather is at or close to minimums that favor using the dive and drive. GA piston airplanes have greater flexibility in flying these approaches safely than turbojet aircraft do.
 
John Collins said:
The VDA and VDP are relatively new things.
Click to expand...

True as to the VDA; perhaps in the last 7 or 8 years. But, the VDP was pushed and accepted by the FAA 30, or so, years ago. Until RNAV came along they were difficult to establish. For a few years (1990s?) they were mandatory for Part 121 operators when charted. (Always a properly aligned DME fix in those days.)
 
Here is link to a rather large PDF I created comparing two RNAV procedures, one at Thermal, CA and the other at French Valley, CA.

http://www.terps.com/34

The file contains both the FAA chart and the regulatory source document (8260-3). The Thermal LNAV-only procedure has a VDP and it is 34:1 clear in the visual segment. This is indicated on the FAA chart by the small grey feather in the profile view. This 34:1-clear feather is shown on FAA charts that have straight-in non-precision minimums and 34:1 clear is indicated on the source document.

The other chart for French Valley is not 34:1 clear but is LPV, LNAV/VNAV, and LNAV only. It also has a VDP but, as indicated, is for the LNAV only approach.

The Thermal procedure was published in 2002. The present French Valley procedure was published in January of this year.

NOTE: The PDF file is large because in 2002 the regulatory source documents were bitmap scans. That changed later on as indicated by the French Valley form.
 
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