where rotating parts of the engine break off and exit through the case. These high energy parts can cut fuel and control lines, and can penetrate the cabin. Although fuel and control lines are usually duplicated for reliability, the crash of United Airlines Flight 232 was caused when hydraulic fluid lines for all three independent hydraulic systems were simultaneously severed by shrapnel from an uncontained engine failure. Prior to the United 232 crash, the probability of a simultaneous failure of all three hydraulic systems was considered as high as a billion-to-one. However, the statistical models used to come up with this figure did not account for the fact that the number-two engine was mounted at the tail close to all the hydraulic lines, nor the possibility that an engine failure would release many fragments in many directions. Since then, more modern aircraft engine designs have focused on keeping shrapnel from penetrating the cowling or ductwork, and have increasingly utilized high-strength composite materials to achieve the required penetration resistance while keeping the weight low. Economic considerations In 2007 the cost of jet fuel, while highly variable from one airline to another, averaged 26.5% of total operating costs, making it the single largest operating expense for most airlines. Environmental considerations Jet engines are usually run on fossil fuels and are thus a source of carbon dioxide in the atmosphere. Jet engines can also run on biofuels or hydrogen, although hydrogen is usually produced from fossil fuels. About 7.2% of the oil used in 2004 was consumed by jet engines. Some scientists believe that jet engines are also a source of global dimming due to the water vapour in the exhaust causing cloud formations. Nitrogen compounds are also formed during the combustion process from reactions with atmospheric nitrogen. At low altitudes this is not thought to be especially harmful, but for supersonic aircraft that fly in the stratosphere some destruction of ozone may occur. Sulphates are also emitted if the fuel contains sulphur. Advanced designs Ramjet A ramjet is a form of airbreathing jet engine using the engine's forward motion to compress incoming air, without a rotary compressor. Ramjets cannot produce thrust at zero airspeed and thus cannot move an aircraft from a standstill. Ramjets require considerable forward speed to operate well, and as a class work most efficiently at speeds around Mach 3. This type of jet can operate up to speeds