What was found
DEFOG FAN FAILED IN-FLIGHT FILLING CABIN WITH SMOKE. DEFOG AND OVERHEAD FANS ARE ON SAME CIRCUIT WHICH ALLOWS CONTINUED POWER TO FAILED UNIT WITHOUT TRIPPING CIRCUIT BREAKER. EMERGENCY DESCENT INITIATED AND LANDED SAFELY. RECOMMEND SB TO SPLIT CIRCUIT AND INSTALL CIRCUIT BREAKERS THAT WILL TRIP IF UNIT FAILS. SYSTEM IS CONFIGURED FROM FACTORY. SAME CONDITION EXPERIENCED AT LEAST TWICE PREVIOUSLY. RECOMMEND AIRWORTHINESS DIRECTIVE IF MFG DELAYS.