What was found
(CAN) PENUMATIC DE-ICE SYSTEM FAILED IN FLIGHT. DE-ICE PRESSURE WAS DOWN TO 12-15 PSI INFLIGHT AND 8-10 PSI ON GROUND. MANUAL PRESSURE 1518 +/-1 PSI. MAINTENANCE FOUND THE SYSTEM FROZEN, AND ALL THE WATER SEPARATOR FILTERS PLUGGED WITH SOOT. THE FILTERS WERE ALSO BRITTLE FROM TIME. THE INSPECTION/REPLACEMENT TIME ON THESE FILTERS IS 1800 HOURS. THE DE-ICE SYSTEM WAS REPAIRED AND NEW FILTERS INSTALLED. BASED ON THE FINDINGS ON THIS A/C, ANOTHER DHC-7 WAS INSPECTED AND THE FILTERS APPEARED THE SAME. WE ARE LOOKING AT CHANGING THE FILTER INTERVAL.