Originally Posted by
AerocatS2A
The height/distance reading appears to be correct. When you do a height/distance check on an NPA you are just checking that the barometric altitude indication is correct for the distance. It cannot check that the altitude indication itself is correct because you’re not using an additional independent source for the height check. A height/distance check works on an ILS because the ILS GP is an independent source of height. For an NPA it works as a check against human error flying the approach itself but does not detect underlying problems with the data used.
Originally Posted by
Capn Bloggs
In this case, the "glidepath" is determined by the box using the set QNH, so all will appear well. The only time an altitude check will really be correct is when you check your altitude while following an externally-generated glidepath such as an ILS or GLS.
I understand that is one advantage of RNAV LPVs (Localiser Precision with Vertical guidance) using a SBAS or WAAS: a vertical profile is generated by the GPS, based on the satellites only and is therefore immune to QNH mis-setting errors, as occurs with an ILS glideslope. As per an ILS, if you mis-set the QNH, you will be closer or further away from the runway when you reach the DA, but at least you will still be on the GPS-derived GP. The worst that can happen is you'll hit the 300m markers. And, of course, if you do an altitude check somewhere on the glidepath, the error will be obvious.
Cheers a little better understood now!