Vapour locking and boiling
113. The main physical difference between kerosine and wide-cut fuels is their degree of volatility, the latter type of fuel having a higher volatility, thus increasing the problem of vapour locking and boiling. With kerosine-type fuels, the volatility is controlled by distil-lation and flash point, but with the wide-cut fuels it is controlled by distillation and the Reid Vapour Pressure (R.V.P.) test. In this test, the absolute pressure of the fuel is recorded by special apparatus with the fuel temperature at 37.8 deg. C. (100 deg. F.)
114. Kerosine has a low vapour pressure and will boil only at extremely high altitudes or high tempera-tures, whereas a wide-cut fuel wilt boil at a much lower altitude.
115. The fuel temperature during flight depends upon altitude, rate of climb, duration at altitude and kinetic heating due to forward speed. When boiling does occur, the vapour loss can be very high, especially with wide-cut fuels, and this may cause vapour locking with consequent malfunctions of the engine fuel system and fuel metering equipment.
116. To obviate or reduce the risk of boiling, it is usual to pressurize the fuel tanks. This involves maintaining an absolute pressure above the fuel in excess of its vapour pressure at any specific temperature. This may be accomplished by using an inert gas or by using the fuel vapour pressure with a controlled venting system.
117. For sustained supersonic flight, some measure of tank insulation is necessary to reduce kinetic heating effects, even when lower volatility fuels are used.
Fuel contamination control
118. Fuel can be maintained in good condition by well planned storage and by making routine aircraft tank drain checks. The use of suitable filters, fuel/water separators and selected additives will restrict the contamination level, e.g. free water and solid matter, to a practical minimum. Keeping the fuel free of undissolved water will prevent serious icing problems, reduce the microbiological growth and minimize corrosion. Reducing the solid matter will prevent undue wear in the fuel pumps, reduce corrosion and lessen the possibility of blockage occurring within the fuel system.