Aircraft Fuel System
The Aircraft fuel system stores and supplies fuel to the engines and the APU. The fuel system consists of various subsystems.
- Fuel storage
- Engine fuel feed
- APU fuel feed
- Fuel Jettison
- Fuel quantity indicating
- Fuel pressure indicating
There needs to be some form of storing for the fuel prior to it being used by the aero engines. It is stored in fuel tanks near the aircraft’s longitudinal center of gravity in the wings and center fuselage.
Sometimes, generally for the maintenance, the fuel needs to be removed from the tanks. If personnel need to enter the tank for inspection purposes, there need to be some means of draining that fuel away. Prior to entering a tank for any reason, the tank will need to be defueled and drained completely. The only way fuel can be removed from a tank during a defuel operation. Even after a complete defuel, there are still puddles of unusable fuel left in the tanks and this has to be removed by draining. The drain ports are also used for taking fuel samples.
This method of fueling an aircraft is similar to the way most motor cars are fueled on the garage forecourt. The biggest disadvantage during overwing refueling is that of time to refuel. Access to the tanks is difficult and requires step ladders, high rise platforms, etc and the actual filing process can be slow. Great care needs to be exercised when using this method of refueling to avoid spillage. Overwing refuel points are still fitted to many large aircraft, but they are rarely used.
This is a system of refueling where fuel under pressure (max 50 psi) is supplied from a bowser, tanker, or refueling pumping vehicle. It is common at airports to have the fuel pumped underground. A vehicle connects into the ground connection after lifting a steel cover plate and pumps the fuel into the aircraft. The bowser’s (fuel tanker) fuel hose is connected to the refueling point. From this single point, there is a pipework system connected to all the tanks in the aircraft. The fuel is controlled into each tank by energizing solenoids in the refuel valves.
Advantages of pressure refueling include:
- Higher pressures and flow rates and shorter refueling times.
- Less risk of spillage.
- Ability to fill any tank with any desired quantity of fuel using the aircraft’s onboard refueling control system.
- Reduced risk of fuel contamination.
- Better access. Fueling points are accessed from the ground.
- Reduced the risk of fire.
There needs to be a system to feed fuel to the engines. Although engines will suck fuel using their own pumps, booster pumps are provided to ensure a positive flow.
As fuel is pumped into (or out of) the tanks (refueling, defueling, engine use) air has to be allowed into and out of the tank. Failure to do so could cause the tanks to rupture during refueling operations or a vacuum to occur when the engines are running and thus starve them of fuel.
When allowing venting careful consideration has to be given as a highly explosive fuel/air mix is being moved from the tank/s to the outside atmosphere. Also when allowing air into the tanks contamination may enter via the venting system.
The purpose of the venting system is to:
- Balance the air pressure within the fuel tanks with ambient air.
- Allow for thermal expansion of the fuel/fuel-air mixture in the tanks.
- Protect the tanks from excessive internal pressures.
Fuel Jettison system
For most large aircraft it isn’t structurally possible to land an aircraft at its maximum all-up weight (on long-distance flights aircraft will take-off heavier than they are allowed to land).
During the flight, fuel is burned and aircraft weight falls. If a serious malfunction causes a return to base just after take-off the aircrew needs to dump some of the fuel overboard (and hence reduce the weight of the aircraft). This is done using a fuel jettison system.
It is worth noting that due to environmental and cost concerns, aircraft do not often jettison fuel, they will usually opt for an overweight landing – with special checks to be carried out by the maintenance engineer afterward.