Originally Posted by
alf5071h
The error appears to be incorrect QNH, which results in erroneous indicated altitude - what the pilots see.
An altitude - range table, based on threshold distance, relates the altitude which should be seen at a specific distance, thus a check of the displayed altitude at x nm would identify an indicated value which was below the required chart altitude - below flight path.
The critical points here are (1) having a chart and (2) an appropriate reference position for distance - ideally the threshold. So with an adequate distance reference, the low altitude / below flight path could be identified (but not necessarily the wrong QNH).
I think the point that quite a few people on this thread have been making is that, no, you can’t do a meaningful range/altitude check on a Baro-VNAV approach because it will always appear correct: the navigation computers are following a profile based on altitude, which is determined from static pressure and QNH. Get the QNH wrong and the flightpath will be above/below what it should be but the indications will be right, i.e. if the procedure says 3,000’ at 8DME, that’s what you’ll see, even if you’re actually at 2,500’ or 3,500’ at that point.
The only way to cross-check would be to utilise another, independent source, e.g. RAD ALT or GPS. On the charts I have (Lido) there isn’t really enough information in terms of terrain elevation along the approach track to catch any but the grossest of errors. Hence the need to be vigilant over pressure settings.