Jet Transport Aircraft Fuel Systems
A large jet transport aircraft, such as the Boeing 727 has a relatively simple fuel system that supplies its three engines from three fuel tanks.
Tanks No. 1 and No. 3 are integral tanks, that is, part of the wing is sealed off and fuel is carried in the wing structure itself. Each of these tanks holds about 12,000 pounds of fuel. A fuselage tank consisting of either two or three bladder type fuel cells holds another 24,000 pounds of fuel.
Each of the wing tanks has two 115-volt AC electric boost pumps and the fuselage tank; tank No. 2 has four such pumps.
Each of the three engines may be fed directly from one of the three fuel tanks, or all of the tanks and engines may be opened, into a cross-feed manifold.
Fuelling is accomplished by connecting the fuel supply to a single-point fuelling receptacle located under the leading edge of the right wing. Fuel flows from this receptacle through the fuelling and dump manifold into all three tanks through the appropriate fuelling valves. When the tanks are completely filled, pressure shutoff valves sense the amount of fuel and shut off the fuelling valve. This prevents the tank being overfilled or damaged. If only a partial fuel load is required, the person fuelling the aircraft can monitor a set of fuel quantity gauges at the fuelling station and can shut off the flow al fuel to any tank when the desired level is reached.
The airplane tanks may be refuelled by connecting the fuel receiving truck to the manual refuelling valve, closing the engine shutoff valves, and opening the cross-feed valve from the tank to be emptied. The fuel may be cither pumped out of the tank with the boost pumps, or it may be pulled from the tank by suction from the receiving truck. If it Is pulled out by section, it leaves the tank through the boost pump bypass valve.
Fuel may be dumped in-flight by opening the fuel dump valve for the tanks to be dumped and then opening the fuel dump nozzle valve in the wing tip through which the fuel is to leave the airplane. Fuel can be dumped from either wing tip or from both tips at the same time.
There is a fuel dump limit valve in each of the three systems that will shut off the flow if the pressure drops below that. -needed to supply the he engine with adequate fuel. It will also shut off the dump valve when the level in the tank gets down to the preset dump shutoff level. This dump system is capable of dumping about 1300 pounds of fuel per minute when all of the dump valves are open and the entire boost pumps are operating,
This fuel system has provisions for heating the fuel before it enters the fuel filter if its temperature is low enough for there to be danger of ice forming on the filter.
Figure 1.6: Fuel system for the Boeing 727 transport airplane.
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