The design of the fuel system for an aircraft having two or more engines presents problems not normally encountered in single engine fuel systems. A large number of tanks are often required to carry the necessary fuel. These tanks may be located in widely separated parts of the aircraft, such as the fuselage and the inboard and outboard sections of the wings. The individual engine fuel systems must be interconnected so that fuel can be fed from the various tanks to any engine. In case of engine failure, the fuel normally supplied to the inoperative engine must be made available to the others.
The twin engine fuel system illustrated in figure 4-24 is the simple crossfeed type. As shown, the tank selector valves are set to supply fuel from the main tanks to the engines. These valves can also be positioned to supply fuel from the auxiliary tanks. The crossfeed valve is shown in the off position. It can also be set to supply fuel from the fuselage tank to either or both engines and to crossfeed. A few of the numerous combinations in which the three valves can be set are also illustrated.
The main feature of the four engine system shown in figure 4-25 is the fuel manifold. This fuel manifold system is actually a variation of the crossfeed. As shown, fuel is being supplied from the main tanks directly to the engines. The manifold valves can also be set so that all tanks feed into the manifold and each engine receives its fuel supply from this line. The auxiliary fuel supply can be delivered to the engines only through the manifold. The main advantage of this system is its flexibility. Should an engine fail, its fuel is immediately available to the other engines. If a tank is damaged, the corresponding engine can be supplied with fuel from the manifold.
Another advantage of this system is that all fuel tanks can be serviced at the same time through a single line manifold connection. This method of fuel servicing has greatly reduced servicing time on large aircraft because fuel can be introduced into the fueling manifold under high pressure.