A question on this approach:
http://aeronav.faa.gov/d-tpp/1013/06006R10.PDF
Why is the LNAV/VNAV minimums so much higher than all aother minimums, including circling?
Bryon,
A good question and an interesting approach. I suspect that the primary reason for the DA being higher than both the LPV and the LNAV has to do with the way the DA is determined when there are obstacles close in to the runway.
An Obstacle Clearance Slope (OCS) is used to determine the required obstacle clearance between the glidepath and obstacles for the LNAV/VNAV and the LPV, but they are calculated differently and are shaped differently. The width of the OCS for the LNAV/VNAV is much wider than the the one for LPV. Since the LNAV/VNAV can be flown by aircraft with Baro VNAV systems in addition to a WAAS GPS, they must have built in more protection for variations in temperatures and pressure variations. The effect is that the obstacle clearance has to be greater for the LNAV/VNAV than the LPV. When an obstacle particularly one close to the threshold penetrates the LNAV/VNAV OCS, the DA of the LNAV/VNAV is pushed back and therefore higher to the point where the OCS is the same height as the close in obstacle. So for a variety of reasons, the LNAV/VNAV will often end up with a higher DA than a LPV. The higher the DA, the further from the runway at the DA, the higher the visibility requirement at the DA. By moving the DA higher when there are close in obstacles, it puts them in the visual segment where the pilot can see and avoid them.
A LNAV MDA is is based on a single level required obstacle clearance, 250 feet above the highest obstacle. The 250 foot obstacle protection provides adequate clearance in the visual segment of the approach to allow the pilot to see and avoid obstacles. Unlike a DA, the MAP for a LNAV can go right up to the threshold at the MDA, so visibility requirements can be lower since you can get closer to the threshold.
The circling minimums are based on a 300 foot required obstacle clearance that is larger for aircraft with a faster approach speed. It is a visual maneuver and you must remain clear of clouds and keep the airport environment in sight.
So, with this one approach, you note several apparent inconsistencies. The LPV has the lowest DA, but the visibility requirement is higher than the LNAV approach (at least for Category A). The LNAV/VNAV has a much higher DA than the LNAV and the visibility requirements are also much higher. Furthermore, if you just meet the visibility requirement at the DA of the LNAV/VNAV (2 miles), you won't be able to see the runway (3.2 miles away). So how can you ever complete the approach under these circumstances? Answer, "fly visual to airport", which is authorized as long as the visibility is 2 miles. The fly visual note authorizes the pilot to continue the approach even though none of the required 91.175 cues are met as long as the visibility is met. The pilot is responsible for altitude, terrain and obstacle clearance, VFR traffic avoidance and navigation to the runway and must remain clear of clouds. However, in general with a fly visual, the pilot is also responsible for determining his own missed approach procedure plan once he leaves the DA. In this case, it should not be an issue because of the requirements of the underlying approaches.
A final point, as this approach is charted, the visual segment to the runway may not be clear of obstacles on a 20 to 1 slope. This can be determined by the lack of a VDP for the LNAV being published.
