Aircraft Fuel Systems

By: Eric Spoor

•Information in this section was taken from:

Aircraft Maintenance and Repair p.467-504

Transport Category Aircraft Systems p.6-1 through 6-28

Aircraft Fuel Reciprocating engine fuels Turbine engine fuels

Jet A – which is Kerosene Jet B – a blend of kerosene and gasoline Jet A-1 – used for operation at

extremely low temperatures• Jet A and Jet B are the most common

Fuel System Contamination The higher the viscosity of the fuel, the greater

it’s ability to hold contaminants in suspension This is why jet fuels, which have a higher

viscosity than av-gas, are also more susceptible to contamination than av-gas

The main contaminants that reduce the quality of fuel are: Other petroleum products Water Rust Scale Dirt

Water Contamination Water contamination in fuel can be in two

forms: Dissolved in the fuel Entrained or suspended in the fuel

Water in fuel can cause icing in the aircraft fuel system, usually in: Boost pump screens Low pressure filters

Large amounts of water can cause engine stoppage

Microbial Growth Microbial Growth is produced by various

forms of micro-organisms that live and multiply in water which is in jet fuel

These micro-organisms form slime that can be red, brown, green, or black

The organisms feed on hydrocarbons in the fuel but require water to multiply

This buildup can: Interfere with fuel flow and quantity indications Start electrolytic corrosive action

Contamination Detection Coarse fuel contamination can be

detected visually Uncontaminated fuel should be:

Clean Bright Contain no perceptible free water

Contamination Detection (cont.) Clean means the absence of any readily

visible sediment or entrained water Bright refers to the shiny appearance of

clean, dry fuel Free water is indicated by a cloud, haze,

or water slug Water saturated in fuel is not always visible Perfectly clear water can contain as much as

three times the acceptable limit

Contamination Detection (cont.) There is no accurate method of detecting

fuel entrained water when it is frozen For this reason, it is important that fuel is

checked when the water is in a liquid state This should not be done following a flight at

altitude when the fuel would be below 32 degrees F

It is more effective to drain the fuel after the fuel has set undisturbed for a period of time, allowing the water to precipitate and settle to the drain point

Fuel Systems The purpose of an aircraft fuel system is

to store and deliver the proper amount of clean fuel at the correct pressure to the engine

Fuel systems should provide positive and reliable fuel flow through all phases of flight including: Changes in altitude Violent maneuvers Sudden acceleration and deceleration

Fuel Systems (cont.) Fuel systems should also

continuously monitor system operation such as: Fuel pressure Fuel flow Warning signals Tank quantity

Types of Fuel Systems Fuel systems can be classified in two

broad categories: Gravity-Feed Systems Pressure-Feed Systems

Gravity-Feed Systems Gravity-Feed Systems use only the

force of gravity to push fuel to the engine fuel-control mechanism

The bottom of the fuel tank must be high enough to provide adequate pressure to the fuel-control component This type of system is often used in

high-wing light aircraft

Pressure-Feed Systems Pressure-Feed Systems require the use of a fuel

pump to provide fuel-pressure to the engine’s fuel-control component

There are two main reasons these systems are necessary: The fuel tanks are too low to provide enough pressure

from gravity The fuel tanks are a great distance from the engine

Also, most large aircraft with higher powered engines require a pressure system regardless of the fuel tank location because of the large volume of fuel used by the engines

Fuel System Components Pumps Tanks Lines Valves Fuel Flow-meters Filters and

Strainers

Quantity Indicators Warning

Components Fuel Drains Heaters

Fuel Pumps Fuel pumps are used to move fuel

through the system then gravity feed is insufficient

There are three main functions of fuel pumps, they are to move fuel from: The tanks to the engines One tank to another The engine back to the tanks

Fuel-Pump Requirements Engine fuel systems require main

pumps and in some systems emergency pumps

These requirements depend on the type of engines installed on the aircraft

Reciprocating-Engine Fuel-Pump Requirements Reciprocating-engines which are not

gravity-fed require: At least one main pump for each engine These pumps must be engine-driven The pump capacity must capable of

providing enough fuel flow for all operations

Turbine-Engine Fuel-Pump Requirements Turbine-Engines require:

At least one main pump for each engine Main pump power supply must be

independent of all other main pump power supplies

Each positive-displacement main pump must be able to be bypassed

Turbine-Engine Fuel-Pump Requirements (cont.) Turbine-engines also require emergency

pumps The emergency pump must be immediately

available to supply fuel to the engine in the event of a main pump failure

Emergency pump power supplies must be independent of that of the corresponding main pump

If both the emergency and main pumps operate continuously, there must be some means of alerting the flight crew of a failure of either pump

Fuel Pump Classification One way to classify fuel pumps is

according to the pump’s function These classifications are:

Boost Pump Scavenge Pump Cross-feed Pumps

Fuel Pump Classification Another way to classify fuel pumps

is by their method of operation These pumps are:

Vane-type• Variable-volume

Centrifugal Ejector

Vane-Type Pumps Vane-type fuel pumps are the most

common They use a rotor which turns vanes

in a cylinder, the vanes act to push the fuel through the system

Vane-type pumps can have from two to six vanes and they may be variable volume also

Centrifugal Pumps Centrifugal pumps are used to move

fuel from one tank to another or from the fuel tank to the engine

They are electrically driven and some may operate at different speeds

Ejector Pumps An ejector pump is normally used to

scavenge fuel from remote areas These pumps have no moving parts

they rely on return fuel from the engine to pump the fuel

Ejector pumps work on the venturi principle

Fuel Tanks Fuel systems on different aircraft

may use several types of fuel tanks The three basic types of fuel tanks

used on aircraft are: Integral Rigid Removable Bladder

Integral Fuel Tanks Integral Fuel Tanks are commonly located in the

aircraft’s wings or fuselage These tanks are ones that are built into the structure

of the aircraft and generally can not be removed Integral Fuel Tanks are formed by the actual

structure of the aircraft The seams are sealed, usually with synthetic rubber,

to produce an area inside the aircraft structure which will contain the fuel

This type of tank is used in some light high-performance aircraft and turbine-powered transports

Rigid Removable Fuel Tanks Rigid removable fuel tanks are often

made of aluminum components that are welded together

These tanks are installed in compartments specifically made for the tank

The tanks may be held in place with padded straps

This type of tank is often found on more expensive light aircraft and reciprocating-engine-powered transports

Bladder Type Fuel Tanks Bladder type fuel tanks are basically a

reinforced rubberized bag These tanks are installed in

compartments which support the weight of the fuel

The tank is held in place with buttons or snaps on the bottom and sides of the tank

This type of tank is usually found on light aircraft and some turboprop and turbine-powered aircraft

Fuel Lines Fuel lines on aircraft are either made of

rigid metal tubing or flexible hose Most of the fuel lines are the rigid type

which are usually made of aluminum alloys

The flexible hose fuel lines are either made of synthetic rubber or Teflon

The diameter of tubing used is decided by the engine’s fuel requirements

Valves Fuel selector valves are used in

aircraft fuel systems to: Shut off fuel flow Cross-feed Transfer fuel

Selector valves may be operated manually or electrically depending on the installation

Filters and Strainers Fuel is usually strained at three points in

the system Through a finger or bootstrap strainer in

the bottom of the fuel tank Through a master strainer which is

usually located at the lowest point in the system

Through a third strainer near the fuel control unit

Quantity Indicators Mechanical

Inverted float gauge Rotating dial gauge Upright float gauge Sight-glass gauge

Resistance Capacitance

Fuel Subsystems

Some aircraft fuel subsystems allow for fuel:

Jettison Heating Cross-Feeding

Fuel Jettison The fuel jettison system comprises a

combination of fuel lines, valves, and pumps provided to dump fuel overboard during an in-flight emergency

This will reduce the weight of the aircraft so an emergency landing is possible

Fuel Heating Fuel heating is necessary for turbine

engines to thaw ice particles in the fuel that would otherwise clog the filters

Fuel is routed through a heat exchanger that uses either engine oil or compressor bleed air to bring the fuel up to an acceptable temperature

Cross Feeding Cross feed systems allow the flow of

fuel from any of the tanks to any of the engines

Some reasons that this system might be used are: Engine failure Problem with one or more fuel tanks Redistribute fuel for weight and balance

purposes