If there is rising ground ahead, not seen in the dark, where would be the advantage of a radalt over a baralt? Of course it is more accurate for where you are, but sometimes I'm not sure that your thinking is as lateral as it could be.
The advantage of a rad alt over a bar alt in this situation, imho, is that it's going to tell me fairly accurately how high I am over the ground that I am about to attempt the final stage of my engine off landing to.
I don't know about you, but my thinking is that a rad alt will tell me when I'm approaching 500', 200' then 100' and says, 'you know what Sid, best you start moving that thar stick & lever ... rather than a landing lamp giving me just enough info that says, 'well here's the ground Sid, whatcha going to do in the next 3 seconds to get out of this' or a bar alt that says, 'well Sid, if you were over the water you'll be getting wet just about .. now!'
Not all helicopters lose the landing lamp with a double engine failure.
I don't think any do, do they?