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Asked by: Mario Fedeli
GPS, LNAV, LPV Airspace, Instrument Rating
Washington Warren Filed (OCW) RNAV (GPS) RWY 23
LPV DA 436
LNAV MDA 420
Why is the LNAV MDA (which requires only a IFR approach cercified GPS) lower than the LPV DA (which requires a WAAS receiver).
on Apr 12, 2011
Paging John Collins… Paging John Collins…
Until he shows up here, I’ll point you to this excellent post he made over at pilotsofamerica.com about a similar issue: http://www.pilotsofamerica.com/forum/showpost.php?p=652684&postcount=3
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John D. Collins
on Apr 12, 2011
Very good question. If you look at the approach chart, you can observe several facts. There is a 168 foot obstacle on short final. The profile view does not show a gray feather extending to the threshold, which indicates that the visual segment is not clear on a 34-1 slope ratio, 34-1 is required to obtain the lowest LPV minimums. Also, the profile view does not have a VDP (Visual Descent Point) depicted on the final segment. A VDP is depicted for the LNAV minimums anytime the visual segment is clear on a 20-1 slope ratio. Since the VDP is not on the chart, one can conclude the final is not clear on the 20-1 slope. All of these facts, put together, indicate that the visual segment is obstacle challenged.
For the LNAV MDA, the minimums are determined to be 250 feet above the controlling obstacle. So the MDA in this case will be 168+250= 418 feet MSL. This is rounded up to the nearest 20 foot increment, or 420 feet MSL.
The LPV minimums are more complicated, in that obstacles are evaluated along a sloping Obstacle Clearance Surface (OCS) that is offset from the runway threshold. Any obstacle that penetrates the OCS, requires an adjustment in one of the LPV parameters, the glidepath angle, the threshold crossing height. the DA, or a combination of the three. When the obstacle is close to the runway as is true in this case, it is imperative that the obstacle be in the visual segment (below the DA) with sufficient clearance for the pilot to see and avoid it after breaking out. To accomplish this, the height of the obstacle is projected back along the approach path until it intercepts the OCS. The DA is then positioned over this point. The further you have to project the obstacle to match the OCS, the higher the DA. In this case, the result is what you see with the DA at 436 feet MSL. Note also that because the DA is further from the threshold, the visibility requirements are increased as well to 1.5 miles verses the 1 mile requirement for the LNAV. Remember the LNAV allows the pilot to motor up to the threshold, and therefore requires a lower visibility whereas the LPV has a DA and the pilot must see the runway from that point.
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