ACFT; METROLINER III; ENCOUNTERED SEVERE ICING DURING CLB OUT AND UNABLE TO CONTINUE CLB. PLUS; HAD TO DSND IN MOUNTAINOUS TERRAIN TO PREVENT STALL. EVENTUALLY THE FLC WAS ABLE TO DSND TO A LOWER ALT AND PROB RESOLVED.

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SCHEDULED PART 135 COMMUTER FLT HELENA TO BILLINGS; MT. ON TAXI OUT OF HLN RECEIVED CLRNC TO BIL -- J-136 AT 19000 FT. ON CLB OUT OF HLN ENTERED IMC CONDITIONS AT 10000 FT. WE BEGAN TO BUILD LIGHT RIME ICE. NOTE: ALL KING EQUIP ON AND FUNCTIONING. CLBING THROUGH 13000 FT WE ENCOUNTERED SEVERE ICING. SEVERE ICING! IN 2-3 MINS WE HAD LOST 40 KTS OF INDICATED AIRSPD AND PICKED UP SEVERE TAIL BUFFET. WE APPLIED MAX PWR AND WERE FORCED TO LOWER THE NOSE TO KEEP FROM STALLING. WE COULD NOT MAINTAIN ALT AND STARTED TO DSND. WE WERE 45 MI E OF HELENA. WE INFORMED ZLC OF OUR PREDICAMENT; AND REQUESTED THE LOWEST MINIMUM VECTORING ALT. SALT LAKE TOLD US 13000 FT. WE WERE DSNDING THROUGH 12800 FT AT THIS TIME. IMMEDIATELY REQUESTED A VECTOR TOWARD LOWER TERRAIN. SALT LAKE INFORMED US THAT RADAR CONTACT LOST. AT ABOUT THIS SAME TIME WE LOST COM WITH SALT LAKE AND ALL VOR NAV INFO AS WE WERE TOO LOW IN MOUNTAINOUS TERRAIN. WE REALIZED WE WERE IN CLOSE PROX TO THE CRAZY MOUNTAINS; WITH PEAKS UP TO 11000 FT. SO WE INITIATED A TURN TO THE N; WITHOUT CLRNC; TO WHERE WE REASONED THERE WAS LOWER TERRAIN. WE MANAGED TO HOLD LEVEL FLT AT 10800 FT. A PASSING ACR JET BEGAN RELAYING OUR CALLS TO SALT LAKE. WE WERE INFORMED BY THE ACR CREW THAT WE WERE FLYING TOWARD AN AREA OF 11000 FT MSA'S; AND WOULD SHORTLY BE ENTERING AN AREA OF 9000 FT MSA'S. WE CONTINUED ON THAT COURSE. A FEW MINS LATER WE WERE ABLE TO TALK DIRECTLY WITH SALT LAKE AND WERE CLRED DOWN TO 9000 FT. DSNDING THROUGH 10000 FT WE BROKE OUT IN VMC CONDITIONS AND OAT TEMP BEGAN TO RISE. WE BEGAN TO SHED THE ICE AND BUILD AIRSPD. WE NOW PICKED UP BILLINGS VOR AND BEGAN NAVING THERE. ALL ICE EVENTUALLY WAS SHED; AND WE PROCEEDED DIRECT TO BILLINGS WITHOUT FURTHER INCIDENT. THE SUCCESSFUL OUTCOME OF THIS EVENT WAS DUE TO GOOD CRM AND THE HELP OF THE ACR CREW IN RELAYING RADIO XMISSIONS. DON'T KNOW WHAT WE WOULD HAVE DONE DIFFERENT IN THIS SIT.