Basic Aircraft Fuel System

Every aircraft needs a place to store the fuel needed for the flight. You will find the tanks usually in both wings and sometimes there is a main, header or auxiliary tank in the fuselage. To get the fuel to the engine(s) under all circumstances and flight conditions the system is equipped with pumps, filters and valves.

Fuel systems are either gravity feed (most high wing aircraft) or pump feed systems where an extra pump helps fuel get to the engine. Fuel return lines are also used to deliver excess fuel back to the tanks.

The system should be designed in such a way that clean fuel, without vapor lock or bubbles, reaches the engine under all certified flight regimes for the aircraft it is installed in. Here we will discover how that is done.

System Description

As said above there are two systems basically: gravity and pump feed. In a gravity feed situation the fuel tanks sits higher than the engine so there is always some fuel pressure. Regulations require that this pressure must give a fuel flow of at least 150% of the maximum required by the engine.

As the aircraft changes attitude fuel flow might be interrupted and due to the low fuel pressure vapor lock can be expected. It would be wise to install an extra fuel pump to prevent this.

Pump feed

Modern designs use a pump feed system for reliability reasons. In such a system you will find the following components:

Fuel tanks

Made from aluminum, synthetic rubber of composite materials. Sometimes they are part of the structure, as is the case with wet wing tanks, or fitted in special ventilated spaces.

Fuel Drain

Tanks have a number of items: filler caps, expansion space, vents, baffles, sumps and drains and fuel quantity detectors. Filler caps should have an indication of the fuel type and quantity and might be lockable. Expansion space makes sure that when the fuel expands by warming up (in the sun) it doesn't overflow from the tank out of the vents. These vents let air in the tank when fuel is consumed.

Baffles are installed inside the tank for structural reasons and they prevent fuel sloshing in the tanks during taxi or turbulent flight conditions which coould cause engine surging. Sumps and drains are located lowest point of the tanks where any water and dirt can collect for pre-flight inspection. Fuel is taken from the tank slight above the lowest point so that there's always a bit of fuel unusable and any dirt will not get to the engine.

Quantity detectors can be the float or capacitance type and should indicate the amount usable. Indication is only reliable when the aircraft sits or flies in a straight and level attitude with no turbulence. Content readings should be cross checked by dipping and gauge indication, and possibly by fuel flow gauges.

Strainers and filters

Fuel is feed through conductive piping to make sure that no static charge is build up. Fuel is strained at tank exit, lowest point in the system at the gascolator and finally at the fine filter before the carburetor. The gascolator is a pre-flight inspection point where fuel can be drained and dirt removed.

gascolator

Tank selection

Usually a rotary valve and enables fuel to be routed from one or more tanks to the engines. In case of multi-engine aircraft cross feeding must be possible as a safety measure. With fuel injected engines (gasoline or diesels) the valve should be able to route the return fuel to the actual tank in use.

Fuel selector valve must provide a positive feel when selecting a tank, positioned easily without the possibility that the valve inadvertently changes position by leg movement of the pilot.

Tank Selector

Engine pumps

This pump is mechanically driven by the engine providing pressurized fuel to the injection system or carburetors. There should be a pressure gauge in the cockpit to indicate proper functioning of this pump. If it should ever fail, you must switch on the auxiliary pump.

The pump delivers fuel at a constant pressure enabled through a pressure relief valve and any excess recirculates inside the pump. Usually a bypass valve make sure that fuel keeps flowing when this pump is not operating.

Auxiliary or boost pump are installed to assist the engine pump and they also purge fuel lines from vapor, prime the cylinders of fuel injection systems and provide pressure when starting the engine.

Fuel primer

Air flow through a carburetor during engine start is just not enough for fuel to come out of the main/idling jet which makes it difficult to fire up, particularly with cold starts. Using a primer just before starting, will pump fuel into the intake manifold where it vaporizes and this procedure helps to cold start the engine.

The priming system delivers fuel bypassing the carb and the engine will suck in fuel if the primer pump is not locked. Make sure the primer is locked after use. If not, it will cause a very rich mixture at idle and rough running. Black smoke can be seen from the exhaust in such a case.