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BOEING 727 FUEL SYSTEM

 

Introduction...

(Below) B-727 Fuel Tank Locations.

(Below) B-727 Fuel System Schematic.

Fuel in the Boeing 727-200 Advanced is held within three ranks. The number 1 and 3 fuel tanks are located in the wings while tank number 2, which has three sections, lies between. The number 2 tank contains removable bladder cells. A vent surge tank is located within each wing towards the tips. Each tank can feed fuel to its own engine. Additionally, it is possible to crossfeed from any tank to any combination of engines. However, it is not possible to crossfeed tank-to-tank in flight.

Inflight Operation...

For an operational example of the tank-to-engine flow of fuel, this will describe the fuel flow from tank number 2 (center tank) to engine number 2 (top engine).

Fuel is picked up by the boost pumps and goes past a low-pressure sensor, through two check valves, and finally through the engine fuel shutoff valve. There is a boost pump bypass valve which allows fuel to be drawn from the tank by the engine pumps even if all boost pumps are off. Fuel flow is the same for tanks 1 and 3 except that they have two boost pumps each instead of four.

At least one boost pump should be working in a tank anytime fuel is being drawn from that tank. All boost pumps should be on for takeoff and landing while one boost pump per tank is used for engine starting. It's normal procedure to takeoff and land with each fuel tank supplying its own engine.

Fuel Dump System...

The fuel dump system provides a means of rapidly reducing the aircraft's weight in flight. To dump fuel, boost pumps in the selected tanks must be on. Open the fuel dump valve(s) in the selected tanks which puts fuel into the fueling and dump manifold. Finally, open the dump nozzles. Fuel will be dumped out the wing tips. (All crossfeed valves should be closed.) Fuel flow indicators will be unaffected by fuel dump operations as they indicate only the flow to the engines. There are no speeds, altitudes, or aircraft configurations where dumping is prohibited.

Dumping should automatically shutoff in a tank when fuel quantity reaches 3,500 lbs. Should this fail, the dumping procedure should be manually terminated before levels drop below 2,500 pounds in a given tank.

The correct sequence for dump operation is:

Fuel Temperature...

Should the fuel temperature gauge fail, the Total Air Temperature  (TAT) gauge may be used. This represents the air temperature next to the skin of the aircraft. Actual fuel  tank temperature is equal, or somewhat cooler, than TAT.

As jet fuel freezes between -40 and -60 C (depending on fuel mixture and additives), you will need to find warmer cruising altitudes if fuel temperatures approach the freezing point. There is no in-tank fuel heating available on the 727. Alternatively, airspeed could be increased as TAT can be 30 C or more above free air temperature because of skin friction. Small increases in MACH number can result in considerable increases in TAT.

Fueling Methods...

The 727 can be refueled by either pressure fueling through ports in the right wing or by gravity feed through overwing filler ports.

Normally, the aircraft is fueled by filling all three tanks equally until 1 and 3 are full. Excess fuel is put into tank two which has a much larger capacity. After takeoff, if there is more fuel in tank 2 than tanks 1 and 3, the excess is fed to all engines until quantities in all tanks are equal.

Pressure fueling is safer and more fuel can be loaded into the aircraft. Using gravity feed, only tanks 1 and 3 can be directly filled. Tank 2 can be serviced through ground tank-to-tank procedures only.

Only two operations require fuel boost pumps. They are Tank-to-Tank ground transfer and fuel dumping.