It is hardcoded - the explosion due from a UFO crash is calculated when the map is generated. Changing the MCD value has no effect.
The explosion damage can vary between 180-250, with a radius of 10 tiles, if I'm reading it right.
Ok.
Would be nice if it were externalized to the ruleset.
Then, une could also give other objects of the craft the possibilty to be destroyed, for example:
UFO navigation: 25% chance for 50 HE
Alien Alloys (UFO walls): 1% chance for 100 HE
etc
to simulate hits from craft weapons.