Large Reciprocating Aircraft Fuel Systems

Large Reciprocating Aircraft Fuel Systems

Transport-category aircraft powered by reciprocating engines are rapidly disappearing from the active fleet. One exception seems to be the venerable. Douglas DC -3. This aircraft has seen a working life of more than 50 years and is still being used for passenger and cargo applications. The fuel system installed on the DC--3 is illustrated to figure 1.5 and is typical for aircraft using large radial-type engines.

Fuel is supplied from two resin tanks and two auxiliary tanks mounted in the centre wing section of the airplane. The capacity of each main tank is 202 gallons, and the auxiliary tank holds 200 gallons each. Provisions are made for the installation of from 2 to 8 long-range tanks, each holding 100 gallons. This makes it possible to carry a fuel load of 1604 galleons, in a total of 12 tanks.

The fuel quantity is measured by the liquid meter system which consists of a float as­sembly and a liquid meter tank unit in each tank. These are connected electrically to the fuel gauge on the right instrument panel in the pilots' com­partment. There are two tank selector valves, operated by dial and handle controls in the pilots' compartment. Ordinarily the left hand engine draws fuel from the left tanks, avid the right engine draws fuel from the right tanks, but, by using the selector valves fuel may be supplied from any tank to either engine.

Two hand-operated wobble pumps are used to raise the fuel pressure when starting the engines, or before the engine-driven pumps are in opera­tion. The fuel flows from the wobble bumps through lines to the strainers located in each nacelle, through the engine-driven pumps, and from there, under pressure, into the carburettors. A cross-feed line is connected on the pressure side of each engine-driven pump, and the two cross­-feed valves in this line are operated by a single control in the pilots' compartment. The cross-feed system enables both engines to receive fuel from one engine-driven pump in case either pump fails.

On later model airplanes the wobble pumps are replaced by two electric booster pumps. Each fuel strainer is located in the centre wing near each selector valve. The fuel, therefore, flows from the selector valves, through the strainers, through the booster pumps, through the engine driven fuel pumps into the carburetor. On airplanes equipped with electric booster pumps, there is no cross-feed system. The booster^- pumps will furnish ample pressure and supply for operation of the airplane in case either engine-driven pump fails.

A vapour overflow line connects from the top chamber of the carburetor to the main tanks, and a fuel line from the back of each carburetor operates the fuel pressure gauge in the pilots' compartment. This pressure gauge normally shows from 14 to 16 pounds pressure. On some airplanes a pressure warning switch is installed in they fuel pressure gauge line. When the fuel pres­sure drops below 12 pounds, the switch il­luminates a warning light on tyre instrument panel.

A restricted fitting on the fuel pressure gaugeIine connects to the oil-dilution solenoid. This unit releases fuel into the engine oil system and the propeller feathering oil, to aid in cold weather starting. Another solenoid valve in the fuel pres­sure gauge line releases fuel into the eight upper cylinders of the engines for priming.

Vent lines from each tank vent overboard, and a vapour line connects each main tank with its cor­responding auxiliary tank.

Figure 1.5: DC-3 Fuel system schematic diagram.