Aircraft fluid lines are usually made of metal tubing. or flexible hose. Metal tubing (also called rigid fluid. lines) is used in stationary applications and where long, relatively straight runs are possible. They are widely. used in aircraft for fuel, oil, coolant, oxygen, instru-. ment, and hydraulic lines. Flexible hose is generally. used with moving parts or where the hose is subject. to considerable vibration. Occasionally, it may be necessary to repair or replace. damaged aircraft fluid lines.