NON-WAAS question

[ememic99]

I can only conclude that there's no exact solution Why don't we use similar calculation when performing ILS/LOC/VOR approach with DME? Because it's not easy and it's not precise enough.

[ememic99]

I got the answer on another forum

My understanding of MAPt was wrong. MAPt on RNAV approaches is actually runway threshold (rather than point related to DA which I wrongly assumed), so the value shown at FPL corresponds to value at Jepp approach plate.

[linzhiming]

I got the answer on another forum

My understanding of MAPt was wrong. MAPt on RNAV approaches is actually runway threshold (rather than point related to DA which I wrongly assumed), so the value shown at FPL corresponds to value at Jepp approach plate.

That is generally correct as long as the MAPt for the RNAV approach is actually the runway threshold. There are some RNAV approaches with the MAPt (not to be confused with the MDA CDFA/DA-equivalent point) before the runway threshold.

It gets awfully interesting though as soon as you want to track your vertical profile on a non-precision approach (Jeppesen has the suggested altitudes vs distance) and/or start dealing with step-down fixes which are not named waypoints, e.g. on a LOC/DME approach or an RNAV approach with step-down fixes which are not named waypoints (Why would an approach designer want to do that? It doesn't cost anything to name waypoints in an RNAV approach where all the other waypoints are already "random"/GPS-based...). EDDR (Saarbrücken/Germany) provides good examples for that with the various Rwy 27 approaches.

In those cases it pays off to know what distance is being referenced way beforehand (during the approach briefing or by testing it out in the official G1000 simulator with the latest Nav database). It is also useful to note that for non-RNAV approaches, the Jeppesen database creates named waypoints for certain fixes (FAF, IF, IAF) based on a defined naming convention (see Jeppesen website) even if the Jeppesen or government ICAO chart does not show a named waypoint (e.g. FAP at 8 DME).

I suggest you try EDDR out in the G1000 simulator...it can quickly get confusing for a LOC/DME approach in case the GS transmitter is out of service (and of course that had to happen during my checkride)! What generally helps here is that the MAPt for LOC approaches is normally (if not always) the runway threshold and you can use that to use the distances shown at the bottom of the vertical profile view of Jeppesen charts...it still does not help that the suggested Jeppesen altitudes reference DME though so it's best to have done the calculation before flying the approach.

Wolfgang

[ememic99]

Nice example on how important is to study approach before flying it.