My two cents....
I have also heard that there may be a Hydraulic system concern in extremely cold temperatures. We don't have any TANIS heaters on our hydraulic systems... Any feedback on that.
As far as the BK is concerned, the limit is something like -45, but you reach a limit of -32 using Jet A. Below -25, we used Jet B to be safe, but had an operational limit of -40 (C and F) <img src="smile.gif" border="0"> but who wants to be out there at -40, I think at these temperatures, it becomes a real to life human factors issue: What if you go down with a precautionary, or if you have a heater malfunction? Most of us are civilian operators who really have no business flying at these extremely low temperatures. OAT affects the A/C but wind chill affects the people!