Terrain Avoidance at Night

From the Lycoming website:
"To reduce spark plug fouling and keep the cylinder cooling within the
recommended 50o per minute limit, the mixture should be left at the
lean setting used for cruise and then richened gradually during descent
from altitude. "

From my experience it's just the first 100 degrees you have to worry
about, so that is the first two minutes. After that you can pretty much
pull the throttle back. But the key is that 50 degrees per minute on
the CHT. Lycoming says not to exceed 50 degrees per minute of cooling.

Planning descents in mountainous terrain is not trivial. You do not
want to shock cool and you do not want to be above Va due to
turbulence. Takes some planning.

As for terrain. If you are navigating VFR and you can't see the
terrain, you shouldn't be flying in the mountains. I have done PLENTY
of mountain flying in Colorado mountains, all VFR, but not at night. Of
course there are exceptions. If you fly above the highest obstacle then
you are ok. And if you are on an IFR flight plan and obeying the
terrain clearance rules you are of course ok. Both of these are hard to
do in the west without oxygen. Some pilots HAVE designed their own
terrain clearance waypoints by flying in the daytime and creating user
waypoints with altitudes along their saved route. I never wanted to
play that game, but in theory it should work if followed carefully.
Familiarity with the route helps a lot. Be careful out there.

One other item. There is a large amount of difference in the darkeness
at night. With a full moon and snow cover, one can see quite well at
night. Over heavily populated areas the ground lights create enough
light to be able to make out most of the terrain. So like in all
things, it all depends.