48-28-02 DOUGLAS: Applies to C-54 and DC-4 Aircraft. \n\n\tTo be accomplished not later than April 1, 1949. \n\n\tAs a fire protection measure in order to prevent burn through, replace the present cabin heater combustion ducts with corrosion resistant steel ducts. \n\n\t(Douglas Service Bulletin DC-4 No. 89 covers this same subject.)