VNAV/LNAV coding issues

17.11.2004

Submission:

We have been having some discussion here about the above, and depictions on charts. The issue really revolves around the definition of the MAPt and distance to run to next waypoint.

I am currently working on a RNAV (RNP0.3) approach. The VNAV procedure will have a nominal MAPt (used only for coding) at the RWY THR (DA is around 600ft). The LNAV procedure will have a MAPt at approximately 1.7NM from the THR (which equates to an altitude of around 600ft also).

If we want to depict both procedures on the same chart, we require the two procedures to have the same distance reference. Normally RNAV non-precision procedures have the MAPt as a waypoint, and that is used as the zero distance reference. However, in this case, I believe we have to use the RWY THR as the zero reference and publish the MAPt as a distance-to-go (like a DME). From a procedure design aspect, there are adjustments to make between the two approaches regarding the obstacle clearance areas, but this is a relatively minor technical issue.

The only other option is to split the charts, but I think this is messy - essentially what we are trying to do is to mirror the ILS/DME vs LLZ/DME type of situation.

Jerry Willmott - Airways Corporation of New Zealand

Answer or Commentary:

IW
I would probably do it the other way around. Both of these approaches are non-precision approaches and Pans-Ops recommends that the threshold is used for the MAPt. My personal suggestion would be to place the MAPt at the threshold and mark the nominal 600 ft VNAV position in the profile view at its exact position. aircraft can always start the missed approach before the MAPt if it is not specified otherwise. However in both cases you will have to calculate obstacle clearance from the threshold MAPt but it is a way of depicting both procedures on one chart..