The temperature at which wax appears is an important factor in jet fuel specification. At the cloud point wax appears6. Specification for jet fuels take note of this, and a value of -47°C applies in some countries. In others (including Russia) jet fuels are commonly used which have a cloud point not above -50°C. When an aircraft crashed at Heathrow early in 2008 there were suggestions, subsequently dismissed, that the fuel had experienced solid deposition a few hours earlier whilst the aircraft was flying over Siberia where the temperature at the altitude of the aircraft was about -75°C.
The temperature at which a matrix structure within the fuel is formed which precludes flow is called the pour point. Often when a fuel initially reaches the pour point only something like 1% of the total mass is in the solid state, and the pour point is usually less than 10°C below the doud point. Below the cloud point the temperature dependence of the viscosity changes and with some fuel systems this makes for difficulties. With jet aircraft however it is the pour point which determines the lowest operating temperature. In an airliner there is a temperature sensor within the fuel which, if the fuel drops to a temperature below that considered safe, sends a signal to the cockpit.
Additives are available by means of which the pour point of a jet fuel can be lowered, known as ‘pour point depressants’. These work analogously to the solute in simple freezing point depression of a single liquid compound. They are added in amounts of less than 1%, and many consist of copolymers of ethylene and vinyl esters. Urea has been shown to be an effective pour point depressant under certain conditions. Ethylene glycol monomethyl ether and diethylene glycol monomethyl ether have been widely used. A jet fuel might also contain a corrosion inhibitor. This can have a side-effect on viscosity and solid deposition behaviour as will be explained more fully in the next section.