The management of fuel is essential for the safe and economic operation of aircraft. The scope of fuel management depends on the size and type of aircraft; fuel is delivered to the engines using a variety of methods. The system comprises fuel quantity indication, distribution, refuelling, defuelling and fuel jettison. On a typical passenger aircraft, the fuel is contained within the sealed wing box structure. The fuel tanks are divided into main tanks, reserve tanks and centre wing tanks. Fuel tanks on general aviation (GA) aircraft are rubberized bags (bladder tanks) contained within the structure of the aircraft; smaller GA aircraft use metal fuel tanks attached to the wings and/or fuselage.