flight. This minimizes the size of the vapor space above the  liquid fuel and the amount of air and associated water vapor  present in the tank. When excessive water is drawn into the  fuel system, it passes through carburetor jets where it can  interrupt the smooth operation of the engine(s). If water is entrained or dissolved in the fuel, it cannot be  removed by draining the sump(s) and filter bowls before  flight. However, there may be enough water for icing to be  a concern. As the aircraft climbs and fuel is drawn out of the  tanks, the fuel supply cools. Entrained and dissolved water  in the fuel is forced out of solution and becomes free water.  If cool enough, ice crystals form rather than liquid water.  These can clog filters and disrupt fuel flow to the engines.  Both AVGAS and jet fuel have this type of water impurity  issue leading to icing that must be monitored and treated. The characteristics of turbine engine fuels are significantly  different from those of AVGAS. Turbine engine fuels are  hydrocarbon compounds of higher viscosity with much  lower volatility and higher boiling points than gasoline. In  the distillation process from crude oil, the kerosene cut from  which jet fuel is made condenses at a higher temperature than  the naphtha or gasoline cuts. The hydrocarbon molecules of  turbine engine fuels are composed of more carbon than are  in AVGAS.  [Figure 14-10]  Turbine engine fuels sustain a continuous flame inside the  engine. They typically have a higher sulfur content than  gasoline, and various inhibitors are commonly added to them.  Used to control corrosion, oxidation, ice, and microbial and  bacterial growth, these additives often are already in the fuel  when it arrives at the airport for use.  Fuel anti-ice additives can be added to the bulk fuel and also  directly into the aircraft fuel tank, usually during refueling.  These are basically diethylene glycol solutions that work as  antifreeze. They dissolve in free water as it comes out of the  fuel and lower its freezing point.  [Figure 14-9]  Aircraft with turbine engines use a type of fuel different from  that of reciprocating aircraft engines. Commonly known as  jet fuel, turbine engine fuel is designed for use in turbine  engines and should never be mixed with aviation gasoline  or introduced into the fuel system of a reciprocating aircraft  engine fuel system. The choice of turbine engine fuel reflects consideration of  conflicting factors. While it is desirable to use a fuel that is  low in volatility to resist vapor lock and evaporation while  in the aircraft’s fuel tanks, turbine engine aircraft operate in  cold environments. Turbine engines must start readily and  be able to restart while in flight. Fuel with high volatility  makes this easier.  AVGAS has a relatively low maximum vapor pressure  compared to automotive gasoline—only 7 psi. But the vapor  pressure of Jet A is only 0.125 psi at standard atmospheric  conditions. Jet B, a blend of Jet A and gasoline, has higher  volatility with a vapor pressure between 2 and 3 psi.  Three basic turbine engine fuel types are available worldwide,  although some countries have their own unique fuels. The  first is Jet A. It is the most common turbine engine fuel  available in the continental United States. Globally, Jet A-1  is the most popular. Both Jet A and Jet A-1 are fractionally  distilled in the kerosene range. They have low volatility and  low vapor pressure. Flashpoints range between 110 °F and  150 °F. Jet A freezes at –40 °F and Jet A-1 freezes at –52.6  °F. Most engine operations manuals permit the use of either  Jet A or Jet A-1.  The third basic type of turbine engine fuel available is Jet  B. It is a wide-cut fuel that is basically a blend of kerosene  and gasoline. Its volatility and vapor pressure reflect this and  fall between Jet A and AVGAS. Jet B is primarily available  in Alaska and Canada due to its low freezing point of  approximately –58 °F, and its higher volatility yields better  cold weather performance. Figure 14-9.  Fuel anti-icing products, such as Prist$^{®}$, act as  antifreeze for any free water in aircraft fuel. They dissolve in the  water and lower its freezing point to prevent ice crystals from  disrupting fuel flow. Turbine Engine Fuels Turbine Fuel Volatility Turbine Engine Fuel Types 14-8