known ordinarily as flammables or com- bustibles. Toxic and poisonous materials (TOXIC). A line identified by the word TOXIC contains materials that are extremely hazardous to life or health. Anesthetics and harmful materials (AAHM). All materials productive of anesthetic vapors and all liquid chemicals and compounds hazardous to life and property, but not normally productive of dangerous quantities of fumes, or vapors, are in this category. Physically dangerous materials (PHDAN). A line that carries material that is not dangerous within itself, but that is asphyxiating in confined areas or is generally handled in a dangerous physical state of pressure or temperature, is identified by the marking PHDAN. Table 6-9 lists some of the fluids with which you may be required to work and the hazards associated with each one. For convenience in distinguishing one hydraulic line from another, each line is designated as to its function within the system. In general, the various hydraulic lines are designated as follows: Supply lines. Lines that carry fluid from the reservoir to the pumps are called supply (or suction) lines. Pressure lines. Lines that carry only pressure are called pressure lines. Pressure lines lead from the pumps to a pressure manifold, and from the pressure manifold to the various selector valves, or they may run directly from the pump to the selector valve. Operating lines. Lines that alternately carry pressure to an actuating unit and return fluid from the actuating unit are called operating lines, or working lines. Each operating line is identified in the aircraft according to its specific function; for example, LANDING GEAR UP, LANDING GEAR DOWN, FLAPS UP, FLAPS DOWN, etc., as the case may be. Return lines. Lines that are used to return fluid from any portion of the system to the reservoir are called return lines. Vent lines. Lines that carry excess fluid over- board or into another receptacle are called vent lines. Table 6-9.—Hazards Associated with Various Fluids Contents Hazard Air (under pressure) PHDAN Alcohol FLAM Carbon dioxide PHDAN Freon PHDAN Gaseous oxygen PHDAN Liquid nitrogen PHDAN Liquid oxygen PHDAN LPG (liquid petroleum gas) FLAM Nitrogen gas PHDAN Oils and greases FLAM JP-5 FLAM Trichlorethylene AAHM STORAGE Fabricated tubing and tube assemblies requiring storage for any length of time should be provided with protective closures at each end. Do not use pressure-sensitive tape as a substitute for protective closures. Oxygen tube assemblies require protection of the entire assembly in addition to protective closures at end fittings. The complete assembly should be stored and packaged in sealed plastic bags in accordance with Aviation Crew Systems Manual Oxygen Equipment, NA 13-1-6.4. TUBING AND TUBE ASSEMBLIES MAINTENANCE Learning Objective:    Recognize the mainte- nance practices and procedures used in the repair and fabrication of tubing and tube assemblies. Maintenance of tube assemblies at the organizational level is limited to inspection, removal, installation, repair and replacement. Inspections are performed during fabrication, installation, and on in-service equipment. During fabrication, inspect 6-19