Electrically operated Fuel Pumps

Electrically operated fuel pumps are used to move fuel throughout the fuel system, the power source being ac or dc.  However, 3-phase ac is mainly used in modern aircraft because of the high flow rates that can be achieved from relatively small pumps.

The pumps are normally situated inside the fuel tanks and are used to supply fuel under low pressure (approximately 20 psi) to the engines.

These pumps are usually called BOOSTER PUMPS when supplying fuel to the engines and TRANSFER PUMPS when used to transfer fuel between tanks.  However, both these functions are often combined and they can also have a jettison facility added, where they then may also be called JETTISON PUMPS.

The pumps have a centrifugal impeller which provides a low pressure supply of fuel to the engine inlet connection.

The flow of fuel will depend on the engine demand and is not therefore dependent on pump speed.  When the flow requirement is small the fuel will simply recirculate within the pump maintaining a low pressure supply, without the need for a pressure relief valve. These are not positive displacement pumps

Integral Fuel Tanks

Integral fuel tanks are formed within the aircraft structure by sealing compartments like wing sections during manufacture.  Internal diaphragms both divide the wing into a number of tanks, and also prevent ‘surge’, external or internal pipes connect the tanks to the fuel system.

Sealant

A number of methods are used for sealing the structure, including the use of sealant as a ‘faying compound’ between mating surfaces and the use of ‘filleting compound’ at the edges.  The interior of the tank is then further protected by flexible sealant coatings, which may be applied by spreading, brushing or spraying.  All the above methods may be used to seal the tank which may also be accompanied by special paint schemes, which are designed to minimize microbiological attack.  A suitable number of large access panels, hand holds and tank connections, are included within the structure.  The access panels are normally made fuel tight by using seals and sealant at the mating surfaces.

Flexible Fuel Tanks

Flexible fuel tanks are reinforced rubberised bag type fuel tanks placed in a non fuel tight compartment.

The tank incorporates all the components as required for the rigid removable fuel tank such as a vent, drain and fuel quantity indicator etc.  The airframe compartment for the tank is used completely, with a small opening provided for the insertion of the tank.  When inserted into the compartment the tank is held in place by several ‘buttons’ or ‘snaps’ which attach the tank to the compartment.  These tanks are found on many medium sized aircraft.

Rigid Removable Fuel Tanks

A rigid removable fuel tank is a tank that is installed in a compartment designed to hold the tank.  The tank must be fuel-tight but the compartment in which it fits is not fuel tight.

The tank is usually constructed of aluminium components welded together.  A baffle or baffles may be installed inside the tank to reduce the free movement of fuel called ‘surge’.  The baffles are attached to the tank structure with apertures to allow a flow of fuel but reducing the ‘surge’ action of the fuel.

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The tank must be smaller than the tank compartment to fit into the compartment and therefore the best use of the available space is not utilized.

The rigid removable tank may be held in position by straps and/or screws.  An access panel is used to cover the fuel tank.  The tank has a fuel feed line located near to the lowest inboard point to ensure that most of the fuel load is utilized.

A fuel drain point is situated at the lowest point of the tank when the aircraft is in a landed and stationary mode.

A fuel filler cap and vent tube are fitted near the top of the tank and provision made for a fuel quantity indicator.  This type of fuel tank is found on light aircraft.