Most commercial, turbine-powered aircraft operations use a fuel designated Jet A. It is kerosene with inhibitors added to reduce corrosion and oxidation. The military designation of Jet A fuel is JP-5.
Jet B fuel is a blend of kerosene and gasoline fractions. This is a wide cut fuel. Jet B has a higher vapor pressure and thus is more volatile than Jet A. This aids in cold weather starting and air restarts. The military designation of Jet B is JP-4.
A third type of jet fuel is Jet A-1. It is very similar to Jet A but made for operations at very low temperatures. Jet A has a freeze point of -40°C, Jet A-1 has -47°C, and Jet B and JP-4 have -50°C.
Figure 6-1. Typical fuel system schematic
Kerosene has higher heat energy per unit of volume than gasoline. Since kerosene is slightly heavier than gasoline (6.7 pounds per gallon vs. 6.0 pounds per gallon), it does not have greater heat per unit of weight.
One disadvantage of jet fuel is its greater susceptibly to contamination. Jet fuel has a higher viscosity than gasoline, allowing it to hold more water and other contaminates in suspension than the less viscous gasoline holds.
Mixing jet fuel and gasoline can have disastrous effects in piston engines, but turbine engines are more tolerant of such mixtures. However, one major hazard in a turbine engine is that the tetraethyl lead (TEL) in gasoline will form deposits on the turbine blades. This results in a degradation of engine performance. Consult the aircraft operating handbook for acceptable ratios of gasoline and kerosene and for any operating limitations when using mixed fuels.
The primary contaminant in fuel is entrained water. This water does not readily separate out of jet fuel because of kerosene's high viscosity. When the temperature of the fuel drops below 32°F the water droplets will freeze, and combined with the kerosene, forms a substance known as gel. This can cause damage to fuel pumps and the fuel control.
Turbine-powered aircraft have fuel temperature indicators for one or more fuel tanks so that the flight engineer can determine if the fuel temperature is conducive to ice formation.
Jet aircraft are normally fueled using an underwing (single-point) pressure fueling port. This is both faster and safer than the gravity fueling method used in smaller aircraft. A primary safety advantage of pressure fueling is that it reduces the chance of fuel contamination.
Fire is a hazard when refueling any aircraft and proper precautions must always be taken. Be aware that gasoline type fuels such as Jet B produce vapors in the fuel tank that can be easily ignited at normal temperatures.
There have been a number of instances in recent years in which an aircraft received the wrong fuel load because of confusion over the units of measurement. A serious problem occurs when U.S.-based flight crews are operating in foreign countries where fuel is sold in liters instead of U.S. gallons. One U.S. gallon is equal to 3.785 liters. For example, 1,840 U.S. gallons are equal to 6,964 liters (1,840 x 3.785 = 6,964). Note: the CX-2 flight computer does this conversion automatically.
Any leakage of fuel from the tanks is potentially dangerous, but not all have to be repaired immediately. The final determination of whether an airplane can be flown with a fuel leak should be made by reference to the manufacturer's handbook. There are some general rules that apply: an airplane should not be flown if there is any leak in an enclosed area where the buildup of fumes could create a fire hazard. If an aircraft has any sort of running leak where the fuel drips or runs along the skin contour, it must be taken out of service. Stains, seeps and even heavy seeps (up to 4" in diameter) are not generally considered a hazard if they are located outside the aircraft and away from ignition sources.
It is much harder to distinguish the size and location of a new leak; however, it should be evident in a short period of time due to dirt and discoloration.
Each fuel tank has two or more electrically driven fuel boost pumps. The primary purpose of these pumps is to provide a positive flow of fuel to the engine driven pumps. These pumps also allow transfer of fuel between tanks, fuel dumping in flight, and defueling of the aircraft on the ground. The pumps are located in the tank so that at least one of them can pump fuel in any aircraft attitude when the tank quantity is low.
Regulations require that fuel systems be free from vapor lock at fuel temperatures up to +110°F. The boost pumps help prevent vapor lock condition when atmospheric pressure is low.
Aircraft with multiple fuel tanks must have some sort of fuel crossfeed system. This allows the flight engineer to maintain aircraft stability within acceptable limits. Fuel is almost always carried in the wings and there must be some way of correcting unbalanced fuel loads so that adequate lateral stability can be maintained. On some aircraft, fuel transfer also may be used to correct fore and aft balance problems.
The fuel dump system typically consists of valves, dump lines, dump chutes, and dump chute operating mechanisms. Fuel is pumped by the boost pumps through dump valves in each tank into a common dump manifold. The fuel is then jettisoned overboard through dump chutes located in each wing. Lateral stability is maintained during dump operations by the flight engineer monitoring the fuel levels in each tank and closing each tank's dump valve at the appropriate time.
It is impossible to dump all the fuel in an aircraft because each tank is equipped with a dump limit valve that shuts off fuel to the dump system before the tank can be emptied. This valve is often referred to as a standpipe valve, but in reality it usually shuts off fuel when pressure drops below a certain value rather than at a predetermined fuel level in the tank.
Regulations require that after all fuel is jettisoned using the main control, a certain amount of fuel should remain. For turbine-powered airplanes, this amount is equal to the amount required to climb from sea level to 10,000 feet and then cruise for 45 minutes at maximum range airspeed.