What was found
DURING CRUISE, WITH CABIN BLEED AIR HEATER AND DEMISTER SYSTEMS OPERATING. NOTICED A CHANGE IN THE SOUND OF THE HEATER SYSTEM AND AN INCREASE IN AIRFLOW FROM BOTH SYSTEMS. AROUND 10 SECONDS LATER NOTICED AN ODOR DESCRIBED AS HOT PLASTIC. THE HEATER AND DEMISTER WERE TURNED OFF AND VENTILATED THE CABIN. THE FLIGHT CONTINUED NORMALLY TO THE DESTINATION. MAINTENANCE FOUND THE BLEED AIR MIXING DUCT DEFECTIVE. THE INNER WALL OF THE DOUBLE WALL TUBING HAD DEBONDED AND PEELED DOWN OBSTRUCTING THE MIXING OF AMBIENT AIR WITH ENGINE BLEED AIR. THE HOT ENGINE BLEED AIR CONTACTING THE INNER WALL OF THE FAILED AIR DUCT WAS THE SOURCE OF THE ODOR. THE BLEED AIR MIXING DUCT WAS REPLACED AND THE AIRCRAFT WAS RETURNED TO SERVICE AFTER A SATISFACTORY FUNCTIONAL TEST OF THE SYSTEMS.