Originally Posted by
Geoffersincornwall
I'm troubled by the notion that all ILS approaches universally arrive over flat terrain. I'm not sure of the relative details of places like Bristol and Leeds or especially places like Sao Paulo Congonhas but they certainly would appear to be problematic when it comes to Rad Ht at DA.
On an ILS the Da of 200 ft is about 0.65nm from the touchdown reference point.
If I recall the reference point for the GS of a CDFA is the TCH (Threshold Crossing Height), not the touchdown reference point. This puts the arrival at MDA much further back and therefore even more at risk from terrain effects/buildings etc.
G.
Geoffers, you are right about TCH being the reference for CDFA. Just bear in mind that helos are not REQUIRED (as FW are) to do CDFA. This reinforces your point about MDA being further out.