hI,
according to my old memories..atpl theory.long time ago..
There were 2 ways to consider the situation in montaneous area, when experiencing a seious failure..for ex one engine out..
Operationnaly, we have to consider:
1) are we able to maintain , after an initial descent (due to the eng. failure) a minimum of 1000 ft above all obstacles while levelling..
This is the let down rule..
2) No we will not be able to maintain this obstacle margin, but we can select a routing enabling us to avoid all obstacles along the descent wit a min margin of 2000 ft. This is the drift down rule..
The final status is the same..maintain at least 1500 ft agl above selected field..and able to give a positiv gradient..
That's what I remember..