47-42-16 DOUGLAS: (Was Service Note 2 of AD-781-1.) Applies to DC-6 Aircraft. \n\nRemove combustion chamber of surface combustion Model 63A66 thermal anti-icing heaters and Model A63A68 cabin and thermal anti-icing heaters for inspection and pressure test at each 500-hour interval of heater operation. Also carefully inspect downstream end of the combustion chambers at each periodic inspection.