Fuel lines, which are sometimes called fuel hoses, deliver fuel from a tank to an engine. Smaller fuel lines also transfer fuel between different parts of an engine.
Diesel fuel, petroleum and other forms of transportation fuel are potentially corrosive and can dissolve certain materials, so fuel lines need to be tested to ensure safe operation. In addition, fuel lines might have to deal with regular vibrations and extreme temperature variations, so they need to be tested properly.
Fuels lines are used on almost all vehicles that need fuel, including aircrafts and boats. Fuel is refined and filtered to be as clean as possible, but small particles can wear away at fuel lines over time. In addition, the plastic or rubber used to make fuel lines can become brittle. On an aircraft, a broken fuel line can cause a fire, which can spread quickly. Therefore, regulations dictate regular inspections to make sure fuel lines don't leak.
Engines produce a significant amount of heat when they run, so fuel needs to be kept away from running engines to prevent combustion within the storage tank. In addition, engines are heavy, and keeping fuel in a different part of the vehicle can help distribute weight more evenly. This is especially important on aircrafts, which need to be balanced appropriately.