RNAV (GNSS) minima

[TeeS]

Hi Busav8r
As Nick suggests, it is all down to the different way the minima are calculated between an LNAV only and an LNAV/VNAV.

The LNAV just has the basic primary and secondary areas with different widths in the initial, intermediate, final and missed segments of the approach. Each segment has a different obstacle clearance, 1000' in the initial, 500' in the intermediate, 250' in the final and then (after a level period between the MAPt and start of climb), a 2.5% gradient up to the end of the missed approach segment. If an obstacle lies under those primary surfaces then it is a relatively simple matter of adding the obstacle clearance value for that segment to the elevation of the obstacle to give the obstacle clearance altitude for that segment (step down fixes can be introduced to allow a lower OCA after the obstacle has been cleared) The obstacle clearance in the secondary reduces from full to zero at the outer edge of the secondary area . The MDA is then calculated from the final segment OCA depending upon the system minima for that type of approach and may then have to be recalculated if an obstacle in the missed approach segment encroaches on the climb gradient.

The LNAV/VNAV approach uses the same primary and secondary areas laterally; however, the obstacle assessment surfaces are made up of a fairly complex set of 3D surfaces and gradients (a little like an ILS). Essentially, if an obstacle impinges on the surfaces then a height loss margin (dependant upon the category of aircraft [161' for Cat D]) is added to elevation of the obstacle to calculate the OCA, there is then some fairly complex arithmetic involved if the obstacle is in the missed approach segment. As previously mentioned, there is then some more complex arithmetic involved if the procedure is at an airfield where temperatures regularly fall outside standard atmosphere values.

I'm not sure whether that helps answer your question but it really depends on the luck of the draw when you work through the two different calculations and where the obstacle is.

Cheers
TeeS

My comments are based on PANS-OPS rather than TERPS by the way.