I may be wrong, but it is my understanding that for operations below the decent limit the see and avoid principle applies. For example on non precision approaches. At some point you have to decent below the OCA to land.
The designers will design a instrument approach to guide you down free of obstacles, but at some point you will take over visually. Think of some approaches where you have really high mins due to high obstacles around like mountains. The designers can only get you so low. At some point you will have to decent below the altitude they can get you. Hence, see and avoid.
Pure theoretical discussion though as there will generally be few obstacles between you and the runway but you will decent below obstacles on which the OCA is based.
Last edited by flyburg; 23rd December 2012 at 19:54.