2 INSTRUCTORS ON A PRACTICE INST REFRESHER TRAINING FLT IN ACTUAL IMC CONDITIONS RETURNED TO LAND AFTER THE ACFT LIGHT ICING WAS ENCOUNTERED AND THE ENG WOULD ONLY RUN UNDER REDUCED PWR AFTER PULLING ON ALTERNATE ENG AIR.

Narrative

CUSTOMER DEPARTED WITH RENTAL ACFT ON IFR TRAINING FLT IN IMC. ENCOUNTERED LIGHT ICING; UPON REACHING CLRNC ALT OF 11000 FT MSL. DETECTED MOMENTARY ENG ROUGHNESS AND REQUESTED RETURN TO BZN. ATC HELD ACFT AT 11000 FT (MEA 10400 FT). AT THIS ALT ACFT CONTINUED TO PICK UP ICE. ENG SHOWED SOME REDUCED RPM BUT AUTOMATIC ALTERNATE AIR KEPT THE FUEL-INJECTED ENG OPERATING. UPON PASSING ZUBLI TURN POINT (OVER GALLATIN VALLEY) ACFT COULD NOT MAINTAIN ALT AND PLT ELECTED TO ACCEPT 100-200 FPM DSCNT (PLT AND CFI VERY FAMILIAR WITH AREA). CREW HAD INTERMITTENT GND VISIBILITY BUT IMC WITH ZERO FORWARD VISIBILITY. UPON REACHING BZN VOR; ACFT DRIFTED TO 9200-9300 FT. CREW REQUESTED AND RECEIVED VOR APCH AND LANDED WITHOUT FURTHER COMPLICATIONS. ATC CTLR AT ZLC PANICKED WITH ACFT'S BELOW-MEA ALT AND INSTRUCTED BZN TWR TO ROLL CRASH TRUCKS AND AMBULANCE -- UNBEKNOWNST TO ACFT CREW. THIS CREATED AN EMER ENVIRONMENT AT THE ARPT WHICH THE CREW WAS UNAWARE OF UNTIL SEVERAL DAYS LATER. ATC EMER CALL COULD HAVE ALSO TRIGGERED FAA INQUIRY DESPITE PLT NOT REQUESTING CFR. CREW WAS AWARE OF POS AT ALL TIMES AND AWARE OF TERRAIN SEPARATION. ATC IN THIS NON RADAR ENVIRONMENT BELIEVED THEM TO BE MI AWAY. ATC SEEMS UNAWARE OF AREA TERRAIN AND VALLEYS. ATC ACTION SEEMS TO HAVE EXTENDED BEYOND PRUDENCE TO BORDERLINE PANIC.