Geoffers, if you are descending on an ILS with a DH state minima at 200 feet, during the entire descent on the correct glideslope you should never, ever be closer to the surface than 200 feet MSD or indeed the published DH if above the state minima.
To be crystal clear, I have never stated it is OK to use DH when on a QNH approach. It is however highly recommended use the DH as a safety reference especially in a modern cockpit where both the BARALT and the RADALT bugs can be set independently and manipulated to provide visual indication that the minima has been achieved AND aural warning if the minima is breached.
Therefore, and this is the entire point, whatever the DA, which reflects threshold elevation essentially, setting the RADALT bug slightly below the published DH (the figure in brackets) is the safety backstop to prevent a whoopsie due to poor glide path accuracy or god forbid, a wrong QNH.
212 man, to suggest that the RADALT does not matter during a QNH approach is crazy given that the ONLY thing that matters is where the ground is as you get closer to it.
As I already stated, for a non precision approach not following a CDFA, if you dive immediately to the minima once on the FAT, it is more than likely an aural warning will sound unless the entire FAT is at the same elevation as the threshold.
To be clear, all QNH approaches I have flown use the BARALT DA/MDA to achieve minima. The RADALT is the safety backstop and in my cockpit is always referenced in accordance with the DH on the plate and the bug is set 30 feet below that reference. OM states, aural warning sounds and still IMC, immediate GA. This is sensible, safe and works.
DB
Last edited by DOUBLE BOGEY; 9th Aug 2015 at 19:40.