AN MU2 FREIGHTER ON FINAL APCH AT 2100 FT DECLARED AN EMER DUE TO THE COCKPIT FILLING WITH SMOKE AND FUMES CAUSED BY A FAILED COCKPIT HEAT VALVE.

Narrative

WHILE BEING VECTORED FOR THE ILS 11 AT CAE THE COCKPIT FILLED WITH SMOKE DUE TO A HEATER SYS OVERTEMP. AS A RESULT OF THE SMOKE IN THE CABIN; MY ALT WAS EXCEEDED BY APPROX 500 FT. NO CONFLICT RESULTED. ENVIRONMENTAL SYS WAS TURNED OFF; CABIN PRESSURE RELEASED AND CABIN VENTED TO CLR SMOKE. ATC WAS ADVISED AND RESCUE TRUCKS WERE DISPATCHED. THE ACFT LANDED WITHOUT INCIDENT. I FEEL THAT AN AIRWORTHINESS DIRECTIVE SHOULD BE ISSUED ON THE MITSUBISHI MU2 HEATER SYS; AS THIS MALFUNCTION HAS OCCURRED 3 TIMES IN 3 DIFFERENT AIRPLANES THAT I HAVE FLOWN IN (ALL MU2'S). CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE ACFT WAS AN MU2 AND THIS WAS THE RPTR'S THIRD EXPERIENCE OF SMOKE IN THE COCKPIT CAUSED BY THE AIR CONDITIONING SYS. THE RPTR SAID THE SYS HAS A 3 POS SWITCH; MANUAL HEAT; MANUAL COLD; AND AUTO POS WITH A RHEOSTAT TO CTL THE TEMP. THE RPTR STATED WITH ANY MODE SELECTED THE COCKPIT HEAT VALVE MOVES TOO SLOWLY TO KEEP UP WITH THE ACFT. THE RPTR SAID IN THIS INCIDENT THE VALVE FAILED COMPLETELY AND ALLOWED HOT BLEED AIR TO OVERHEAT THE DUCTING CAUSING SMOKE AND FUMES TO ENTER THE COCKPIT. THE RPTR SAID WITH A 1-PERSON CREW IT'S A REAL PROB OPERATING THE ACFT AND CONTINUOUSLY MONITORING THE AIR CONDITIONING SYS. THE RPTR SAID THAT AFTER GETTING THE ACFT ON THE GND AND EXITING THE ACFT THROUGH A COCKPIT WINDOW IT WAS NECESSARY TO STAND BY FOR 15 MINS FOR THE SMOKE AND FUMES TO CLR SO MAINT COULD CHK THE ACFT. THE RPTR SAID THE ONLY WARNING SYS IS ON THE MASTER CAUTION SYS AND ILLUMINATES THE DEFOG/OVERTEMP LIGHT WHICH CAME ON LONG AFTER THE COCKPIT FILLED WITH SMOKE AND FUMES. THE RPTR SAID IT'S POSSIBLE TO MAKE A SETTING FOR TAXI OUT AT LOW ENG PWR WITH THE VALVE OPEN AND THEN GO TO HIGH ENG PWR FOR TKOF AND CLB AND OVERHEAT THE DUCTING BECAUSE OF THE HIGH BLEED AIR TEMP NOW COMING FROM THE ENG AND THE SLOW MOVEMENT OF THE COCKPIT HEAT VALVE TO MODULATE TOWARD CLOSE.