Non-Precision approach

popaviator

popaviator

Well-Known Member
When on a non-precision approach do you have to wait to the VDP to descend to the runway if you have it in sight? No right?...just looking for a textbook answer at 12:30 am :pirate:
 
My understanding is that the VDP was the point at which larger aircraft would make their decision to continue the landing or go missed. If i'm shooting an approach, and I get the runway in sight before the VDP I focus on the runway and make a normal, stabilized landing. FAR 91.175 (c) states as long as you have one of the visual references in sight, you can put the aircraft in a position from which a descent to a landing on the intended runway can be made at a normal descent rate, using normal maneuvers. Part 121 and 135 carriers have the additional landing in the touchdown zone rule.

Then again, I may be wrong, seeing as how my instrument rating is still soaking wet. :D
 
popaviator said:
When on a non-precision approach do you have to wait to the VDP to descend to the runway if you have it in sight? No right?...just looking for a textbook answer at 12:30 am :pirate:
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Do you have to wait? No, but anything inside of the VDP may preclude a "normal" descent to the runway. Anything outside will not assure obstacle clearance below the MDA.

What's this mean? Your effective visibility requirement is that of the VDP DME/ATD... I see that the actual vis mins are less most of the time.

What's nice is when a VDP is provided in conjunction with a constant angle descent profile published right on the chart. This might give you an advisory 3.xx descent from the FAF all the way down to a 5x feet TCH, even on a straight up, bare bones VOR approach. Utilizing this gives you decision height like go/no-go easy decision making at the VDP. You will reach the MDA at the VDP if you fly it right on an approach like this.



VDP.png
 
It depends on manuals and opspecs too. Our books require us to wait until a published VDP (or make up our own if one isn't published) before descending from MDA. Not every company has that stipulation though.
 
According to 5-4-5 of the AIM on page 734, The pilot should not descend below the MDA prior to reaching the VDP and acquiring the neccessary visual reference...... and as mentioned above with different carriers their procedures that govern VDP's differ according to their op's spec's, and good luck with the Interview, and remember safety 1st!!!

Thanks
 
In The Instrument Flight Training Manuel by Peter Dogan it states that the VDP "is defined as the point where a normal 3* approach path to the runway would intersect the MDA...and if you are equipped to identify it, regulations prohibit descending below the MDA before reaching this point."

I have looked through the regs and could not find anything in part 91 that confirms this; the only thing I found was in the AIM 5-4-5 as mentioned above
 
AlbertE said:
In The Instrument Flight Training Manuel by Peter Dogan it states that the VDP "is defined as the point where a normal 3* approach path to the runway would intersect the MDA...and if you are equipped to identify it, regulations prohibit descending below the MDA before reaching this point."

I have looked through the regs and could not find anything in part 91 that confirms this; the only thing I found was in the AIM 5-4-5 as mentioned above
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This is approximately right. 8620.3B has the actual formulas and they can get a little convoluted (big surprise from a TERPs regulation, right?) But basically 3 degrees (or 318'/NM) is what they use if there is no visual glide slope indicator (VASI, PAPI, FLOLS, LCVASI, etc.) If there is a VGSI, then they are supposed to design it so that it is coincident with the glide slope that the VGSI system. So it could be a little bit off of 3 degrees, if say, the PAPI's are 2.75 degrees or whatever.

It could also be designed with a steeper angle if there is no VGSI, but there was an obstacle of some kind in the visual area that penetrated either a 20:1 or a 34:1 descent gradient. In that case, he may have raised the MDA slightly to remove the obstacle, and he may need to move the VDP a little bit closer in (giving a glide slope greater than 3 degrees). But most of the time, it will still be pretty darn close to 3 degrees, if not exactly 3.
 
fish314 said:
This is approximately right. 8620.3B has the actual formulas and they can get a little convoluted (big surprise from a TERPs regulation, right?) But basically 3 degrees (or 318'/NM) is what they use if there is no visual glide slope indicator (VASI, PAPI, FLOLS, LCVASI, etc.) If there is a VGSI, then they are supposed to design it so that it is coincident with the glide slope that the VGSI system. So it could be a little bit off of 3 degrees, if say, the PAPI's are 2.75 degrees or whatever.

It could also be designed with a steeper angle if there is no VGSI, but there was an obstacle of some kind in the visual area that penetrated either a 20:1 or a 34:1 descent gradient. In that case, he may have raised the MDA slightly to remove the obstacle, and he may need to move the VDP a little bit closer in (giving a glide slope greater than 3 degrees). But most of the time, it will still be pretty darn close to 3 degrees, if not exactly 3.
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And if there's no published VDP, you can (and should, IMO) compute your own.

GuS wears a HAT. :)
 
MikeD said:
And if there's no published VDP, you can (and should, IMO) compute your own.

GuS wears a HAT. :)
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With that caveat that your computed VDP does NOT guarantee that you won't clear any terrain from the MDA to the TDZ!
 
I was 'wutting' his phraseology, not the thought that you're addressing.

I'm still not sure I'm clear on exactly what that sentence means. :pirate:
 
kiloalpha said:
Exactly...

VPD does guarantee you will be clear of terrian since you'll be visual.. hence the VISUAL DESCENT POINT.

Unless you're just a plain idiot. (Pun intended)
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He's talking about a self-computed VDP, not a published one.
 
seagull said:
With that caveat that your computed VDP does NOT guarantee that you won't clear any terrain from the MDA to the TDZ!
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True, if you're computing a 2 degree or less GP for yourself. But one can easily see if their computed 3 or 4 degree (or even two degree) descent rate can (and should) clear any obstacles from MDA to TDZ. If not, then there's a TERPs problem with the IAP and it likely shouldn't have any straight-in minima to begin with.
 
MikeD said:
True, if you're computing a 2 degree or less GP for yourself. But one can easily see if their computed 3 or 4 degree (or even two degree) descent rate can (and should) clear any obstacles from MDA to TDZ. If not, then there's a TERPs problem with the IAP and it likely shouldn't have any straight-in minima to begin with.
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Unless something has changed fairly recently, that's incorrect. Absent a published VDP there is no guarantee at all from the MDA to the TDZ. You are supposed to see and avoid. Of course, you are correct for MOST of the time, and if there is a VASI or similar, or the runway is served by a precision approach.
 
seagull said:
Unless something has changed fairly recently, that's incorrect. Absent a published VDP there is no guarantee at all from the MDA to the TDZ. You are supposed to see and avoid. Of course, you are correct for MOST of the time, and if there is a VASI or similar, or the runway is served by a precision approach.
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Thats what I said (or meant). With the VDP being the point where a visual descent is made, then there should be no problem with avoiding obstacles. What that self-computed VDP gives you, is moreso a point in space to realize that past it, the chances of (if IMC) now visually acquiring the runway and still being in a normal position to descend and land, are likely now past. The self-computed VDP is a great reference for this. For most 3-4 degree GPs, it also is a useful reference for obstacle and terrain avoidance, however computing a shallow 2 degrees or so VDP, could potentially have you dragging through obstacles that you might not see, or having to dodge the ones you do see.
 
Just realize that that there ARE approaches out there that a 3 degree path from MDA to the TDZ will NOT clear all obstacles. Not many, but they do exist. The one that got AA into BDL was one.
 
MikeD said:
Thats what I said (or meant). With the VDP being the point where a visual descent is made, then there should be no problem with avoiding obstacles. What that self-computed VDP gives you, is moreso a point in space to realize that past it, the chances of (if IMC) now visually acquiring the runway and still being in a normal position to descend and land, are likely now past. The self-computed VDP is a great reference for this. For most 3-4 degree GPs, it also is a useful reference for obstacle and terrain avoidance, however computing a shallow 2 degrees or so VDP, could potentially have you dragging through obstacles that you might not see, or having to dodge the ones you do see.
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I think its possible to even have obstacle issues on some approaches even using a 3-4 degree glideslope for your VDP computation. First of all, the approach designers always compute a VDP, but they may or may not include it. If they don't include it, one of the reasons may be due to an obstacle that penetrates a 34:1 and/or 20:1 from the landing surface. 20:1 is about a 2.86 degree glideslope, so if you were flying 3 degrees that's only .14 degrees above. At about 2 miles, that's 29 feet above the obstacle, and it's less the closer to the runway you get (14.5 feet at a mile, 7.25 feet at a half a mile, etc., etc.)

There is some technical data in the 8620.3B (page 23) that leads me to think that if the final approach course is not aligned with runway centerline (say an ADF or a VOR approach or something), you could have such an obstacle on the final approach course (but not on runway centerline, or MikeD is right- it wouldn't be authorized as a straight in). You would not be "guaranteed" clearance of if you did not see it visually, even if you were to fly a 3-4 degree descent. But of course that's why its the VISUAL portion of the approach in the first place.

I'll see if I can attach the picture..... ahah... that seems to work. The PDF comes out of 8620.3B, page 23, and explains the scenario I was referring to. Simply starting down, even on a 3-4 degree glideslope wouldn't guarantee clearance in this situation. There is still less than a 3.77 degree descent (400 ft/NM) to the runway because the obstacle is not on the runway centerline, so straight in minima are still ok. But because the path from the VDP to the runway has this obstacle, a VDP would not be published (and the approach designers would have to take some other steps, too, like increase the vis required to at least a mile).
 

Attachments

fish314 said:
I think its possible to even have obstacle issues on some approaches even using a 3-4 degree glideslope for your VDP computation.
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That is pretty much what happened to American Airlines Flight 1572. From Wiki (and accurate) "The NTSB investigation cited several causes for this accident. It faulted the FAA for designing the approach to runway 15 without taking the ridgeline into consideration. The new approach, which factors in the terrain, has increased the MDA to 1,320 ft (400 m)." I seem to recall that the NTSB missed the issue of venturi effect on altimeters due to the ridgeline, though.

The other aspect was that the approach had a published VDP prior to the accident, but, also prior to the accident, FAA removed it as they realized that the ridge line impeded on the 3 degree glideslope. Rather than fix that, they just quietly revised the approach with no published VDP and left it as a straight in.
 
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