Citation XL/XLS temperature limits
 

These aircraft have an en-route temperature limit on the airframe of approximately -64c at FL400, decreasing as cruise altitude decreases.
Anybody know exactly which components are affected by this?
Some say the brake seals..........any other offers?

These people are right. The main issue is the brake lines on both main gear and is well documented. Never found it an issue as generally you can always descend into warmer air or climb above the tropopause where of course the air 'warms'.