59-06-02 DOUGLAS: Applies to All Model DC-7 Series Aircraft. \n\n\tCompliance required by first block overhaul after receipt of parts but not later than December 1, 1959. \n\n\tSeveral instances have occurred wherein the green indicator light for one of the main landing gear failed to go on when the landing gears were extended. In one case, after landing, it was noted that the left gear downlatch was not fully engaged and the ground lock safety pin could not be installed. Subsequent investigation and laboratory tests revealed that the orifice check valve could malfunction due to contaminants in the hydraulic fluid of sufficient quantity and size (approximately 0.003-inch diameter) and thereby prevent full extension of the gear. \n\n\tTo overcome this difficulty, remove existing orifice check valve assembly, P/N 4498423-503 (Mineral Oil Aircraft), or P/N 4498423-5503 (Skydrol Aircraft), from each main landing gear actuating cylinder and replace with new orifice check valve assembly, P/N 2230565-5-093 (Mineral Oil Aircraft), or P/N 4481262-5-093 (Skydrol Aircraft). (Douglas Service Bulletin DC-7 No. 353 dated January 30, 1959, covers this same subject.)