LTT EXPERIENCES A SERIOUS ACFT EQUIP PROBLEM AND GETS LOWER ALT ONLY BY EMER DECLARED.

Narrative

CLBING THROUGH FL300; THE COCKPIT BECAME VERY HOT. THE CREW WAS UNABLE TO CTL THE HOT AIR MIXING VALVE. THE COCKPIT BECAME EXTREMELY HOT. SMOKE STARTED POURING INTO THE COCKPIT. THE CAPT TURNED OFF THE FLOW CTL VALVE WHICH SHUT OFF THE HOT AIR AND SMOKE. THIS VALVE ALSO OPENED THE OUTFLOW VALVE WHICH EVACUATED THE SMOKE. THE CABIN ALT BEGAN TO RISE AND THE CAPT ASKED CTR FOR LOWER ALT 3 TIMES AND CTR DID NOT CLR THE ACFT TO LOWER ALT. THE WX WAS VMC; SO THE CAPT DECLARED AN EMER AND EXECUTED AN EMER DSCNT. THE CREW PROCEEDED BACK TO THE DEP ARPT AT 10000 FT UNPRESSURIZED AND EXECUTED A NORMAL APCH AND LNDG. SUPPLEMENTAL INFO FROM ACN 217512. COCKPIT TEMP GAUGE WAS CLBING AND WAS NOT CTLABLE BY EITHER AUTO OR MANUAL CTL. WE GOT A DUCT OVERHEAT LIGHT BUT NO NORMAL LOSS OF TEMP THAT SHOULD FOLLOW THIS LIGHT ILLUMINATING. ABOUT 28-30000 FT THE HEAT WAS BECOMING UNCTLABLE AND WE NOTICED SOME SMOKE IN THE COCKPIT. THE CAPT WAS FLYING AND WE AGREED HE WOULD CONTINUE TO DO SO AS WELL AS COMMUNICATE WHILE I TRIED TO FIND A CHKLIST TO MEET THE PROBLEM -- NO APPROPRIATE CHKLIST WAS FOUND SO I PLACED A PHONE CALL TO OUR MAINT FACILITY BUT THEY WERE UNABLE TO PROVIDE ANY ASSISTANCE. THE CAPT WAS TRYING TO GET LOWER; BUT CTR WAS UNABLE SO WE TURNED THE CABIN AIR SWITCH OFF TO VENT THE CABIN AND STARTED A NECESSARY DSCNT TO TRY TO CTL CABIN ALT AND SMOKE. IT WAS NECESSARY TO DECLARE AN EMER IN ORDER TO GET LOWER. WE THEN RETURNED TO ELM FOR MAINT.