Originally Posted by k.swiss
One thing I am confused by, the good captain explains that even if incorrect QNH is set, the height/distance reading still comes across as correct. Surely it would appear incorrect if the wrong QNH was set, hence this is our first line of defense?
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.