Odd approach with LNAV/VNAV minima higher than LNAV minima

Nealoc187

New Member
1


If the above image of the RNAV (GPS) RWY 9 at KDKB doesn't work, click here to see it:

http://flightaware.com/resources/airport/KDKB/IAP/RNAV+(GPS)+RWY+09


This is the only approach that I've ever seen this on, and it doesn't seem to jibe with any other WAAS approach chart I've ever seen. I'm sitting here at work with 3 retired US Major captains, two current CFIIs, a corporate pilot, a furloughed US-Major FO, and about 6 air traffic controllers and none of us can come up with a reason why the LNAV/VNAV DA is higher than the LNAV MDA on this approach.

Can anyone shed any light on this?
 
Re: Odd approach with LNAV/VNAV minima higher than LNAV mini

none of us can come up with a reason why the LNAV/VNAV DA is higher than the LNAV MDA on this approach.

One common reason is that the required obstacle clearance (ROC) requirements on a glide slope (GS) change as one progresses down the slope. At the top, the ROC is higher for a GS than that for a non-precision approach (NPA), but at the bottom, the ROC is less. There may be an obstacle on the FAC that the MDA will clear, but the GS will not, at least with the proper ROC.
 
Re: Odd approach with LNAV/VNAV minima higher than LNAV mini

My best guess would be that it's because of the fact that an LNAV/VNAV approach has a DA and not an MDA. Since you will end up dipping below the DA if you decide to go missed at that point, there has to be some protection for that. I don't think they want you going any lower than MDA on the approach (could be due to the towers printed on the chart), so the DA has to be higher.

One common reason is that the required obstacle clearance (ROC) requirements on a glide slope (GS) change as one progresses down the slope. At the top, the ROC is higher for a GS than that for a non-precision approach (NPA), but at the bottom, the ROC is less. There may be an obstacle on the FAC that the MDA will clear, but the GS will not, at least with the proper ROC.

I was under the impression that an LNAV/VNAV wasn't using GS tolerances the way an LPV approach would, since the sensitivity doesn't change throughout the approach. And if we're talking about the bottom of the approach, shouldn't a lesser ROC make for lower minimums?
 
Re: Odd approach with LNAV/VNAV minima higher than LNAV mini

I was under the impression that an LNAV/VNAV wasn't using GS tolerances the way an LPV approach would, since the sensitivity doesn't change throughout the approach. And if we're talking about the bottom of the approach, shouldn't a lesser ROC make for lower minimums?

Good question. I just skimmed through FAAO 8260.54A and it appears to show the OCS at a shallower angle than the GS. However, there is a formula to calculate the OCS that I'll have to look at later, but it does indicate that the slope of the OCS is determined by the temperature spread likely to occur at the airport between high and low temperatures.
 
Re: Odd approach with LNAV/VNAV minima higher than LNAV mini

I've always noticed when i flipped through approach plates that a majority are this way. I was under the assumption that it was becsuse on an lnav approach your MAP is usually right over the approach end of the runway or very close. on an lnav/vnav approach your MAP is further away from the runway therefore a higher altitude for obstacle reasons.
 
Re: Odd approach with LNAV/VNAV minima higher than LNAV mini

[YT]<object width="425" height="344"><param name="movie" value="http://www.youtube.com/v/1ytCEuuW2_A&hl=en_US&fs=1&"></param><param name="allowFullScreen" value="true"></param><param name="allowscriptaccess" value="always"></param><embed src="http://www.youtube.com/v/1ytCEuuW2_A&hl=en_US&fs=1&" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="425" height="344"></embed></object>[/YT]

It played in my head, it must play in yours.
 
Re: Odd approach with LNAV/VNAV minima higher than LNAV mini

I've seen quite a few where LNAV/VNAV are higher than VNAV only.

http://204.108.4.16/d-tpp/1001/00884R14.PDF

http://204.108.4.16/d-tpp/1001/00884R32.PDF

Notice that one these approaches, the LPV DA is still very low. It appears that the TERPS obstacle clearance requirements may be greater for LNAV/VNAV (possibly due to their ability to be flown with baro-aiding rather than WAAS?) than it is for LPV.
 
Re: Odd approach with LNAV/VNAV minima higher than LNAV mini

It appears that the TERPS obstacle clearance requirements may be greater for LNAV/VNAV (possibly due to their ability to be flown with baro-aiding rather than WAAS?) than it is for LPV.

I thought that it was because on an LPV approach, like on an ILS but unlike on an LNAV/VNAV, the sensitivity of the vertical guidance increased as one got closer to the runway. But now that I'm looking for confirmation of that, I can't find any. :dunno:
 
Re: Odd approach with LNAV/VNAV minima higher than LNAV mini

AIM 5-4-5. Instrument Approach Procedure Charts

i. Vertical Descent Angle (VDA) on Nonprecision Approaches.

5. In rare cases the LNAV minima may have a lower HAT than minima with a glide path due to the location of the obstacles. This should be a clear indication to the pilot that obstacles exist below the MDA which the pilot must see in order to ensure adequate clearance. In those cases, the glide path may be treated as a VDA and used to descend to the LNAV MDA as long as all the rules for a nonprecision approach are applied at the MDA. However, the pilot must keep in mind the information in this paragraph and in paragraph 5-4-5j.


k. Area Navigation (RNAV) Instrument Approach Charts.

1. The minima lines are:

(b) LPV. "LPV" is the acronym for localizer performance with vertical guidance. LPV identifies WAAS APV approach minimums with electronic lateral and vertical guidance. The lateral guidance is equivalent to localizer and the protected area for LPV procedures is now the same as for an ILS. The obstacle clearance area is considerably smaller than the LNAV/VNAV protection, allowing lower minima in many cases. Aircraft can fly this minima line with a statement in the Aircraft Flight Manual that the installed equipment supports LPV approaches. This includes Class 3 and 4 TSO-C146 WAAS equipment.

(c) LNAV/VNAV. LNAV/VNAV identifies APV minimums developed to accommodate an RNAV IAP with vertical guidance, usually provided by approach certified Baro-VNAV, but with lateral and vertical integrity limits larger than a precision approach or LPV. LNAV stands for Lateral Navigation; VNAV stands for Vertical Navigation. This minima line can be flown by aircraft with a statement in the Aircraft Flight Manual that the installed equipment supports GPS approaches and has an approach-approved barometric VNAV, or if the aircraft has been demonstrated to support LNAV/VNAV approaches. This includes Class 2, 3 and 4 TSO-C146 WAAS equipment. Aircraft using LNAV/VNAV minimums will descend to landing via an internally generated descent path based on satellite or other approach approved VNAV systems. Since electronic vertical guidance is provided, the minima will be published as a DA. Other navigation systems may be specifically authorized to use this line of minima, see Section A, Terms/Landing Minima Data, of the U.S. Terminal Procedures books.

(e) LNAV. This minima is for lateral navigation only, and the approach minimum altitude will be published as a minimum descent altitude (MDA). LNAV provides the same level of service as the present GPS stand alone approaches. LNAV minimums support the following navigation systems: WAAS, when the navigation solution will not support vertical navigation; and, GPS navigation systems which are presently authorized to conduct GPS approaches. Existing GPS approaches continue to be converted to the RNAV (GPS) format as they are revised or reviewed.
 
Re: Odd approach with LNAV/VNAV minima higher than LNAV mini

It has to do with an obstacle in the visual portion of the approach. Look at the minimums for this approach. They are all high (way more than 200' like a typical cat I ILS) due to an obstacle down low.

If there were no obstacle, the minimums would all look normal. Because there is an obstable, LNAV is higher because you could be far out flying at the MDA. If you left the MDA at the VDP on a shallow descent path you'd be too close to the obstacle. Flying the LVAV/VNAV you would clear the obstacle because of the steeper angle. I know it has an obstacle because of the lack of a shaded area between the VDP and runway.
 
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