Originally Posted by
HIFLYR
Couple of things
The A310/300 will capture the Vnav path from above if armed for profile as long as you do not capture a altitude in the FCP first.
The calculated path is enroute profile which is a altitude to a altitude.
The path for RNAV/VNAV approach is not the same it is a Vnav path that starts at 50' above the first of the runway and goes to infinity on the angle encoded in the FMS database. This angle only provides terrain clearance from mins to the FAF as all of these approaches are see to land approaches. Good article about this is Danger below MDA in IFR magazine.
Correct, good post.
Also the "prohibition" from capturing G/S from above is because of excessive descent rates and need to arrest this on time and possibility of false lobe (false G/S capture) with analog ILS G/S signals. #2 doesn't apply for derived glide slope but #1 always applies no matter who or what is flying.
DDA allows for "dip" to arrest the descent which is why it is always above MDA for final segment in all approaches I've seen or flown. It is clear from posts here some don't really understand this, even outside of the GA world, where it is routine to encounter pilots that don't know technical and operational difference between LNAV+V and LPV. MDA is MDA. Never go below without environment in sight.
The IFR mag article you reference was also reprinted by AVWeb for those of you who don't subscribe, linked below. This kind of stuff is absolutely essential to know, even (especially???) for us hand-flying Cat B operators.
Danger Below MDA? - AVweb Features Article