FUEL SYSTEMS

FUEL TANKS

• Aircraft fuel tanks come in a variety of types and

sizes.

• Can be located almost anywhere in the aircraft

(wings, fuselage, tail).

• Managing fuel distribution between tanks on

large aircraft can be very involved.

TYPES OF FUEL

• There are two types of fuel currently used in

aviation:

• AVGAS (Aviation Gasoline)

• Conventional piston engines with ignition systems

• SG of 0.74 at 15 °C

• Avtur (Aviation Kerosene)

• Gas-turbine engine and new diesel engines

• SG (Specific Gravity) range 0.75-0.84

Basic Properties Of Fuels

• Volatile – Tendency to change from liquid to vapour

• Vapour Pressure – Pressure at which fuel vaporises

• Flash Point – Lowest temperature at which there is

sufficient vapours above the liquid to ignite without sustaining a

flame

• Fire Point – Lowest temperature at which the fuel can sustain

combustion through vaporisation

• Auto-Ignition Temperature – Temperature at which fuel

spontaneously ignites without the presence of the source

• Freezing Point – Point at which ice crystals disappears

when it warms up

AVGAS GRADES• AVGAS is classifed accoding to grades (octane

ratind i.e. resistance to detonation)

• 80 Grade

• Red coloured

• 100 Grade

• Green coloured

• 100LL Grade

• Low Lead

• Blue coloured

• C-152 & C-172

• Conventionally used

BLADDER TANKS

• Rubber bladders are used to store fuel. Usually in the wings.

• Will deteriorate over time, but are easier to replace than metal tanks.

• Black flecks may appear in strained fuel which indicates deterioration.

• Tend to deform over time which causes water, fuel, and sediment entrapment.

BLADDER TANK DEFORMATION

Over time the bladder begins to deform and rise up between attach points.

This causes fuel, water, and sediment to collect in the valleys.

Which results in increased unusable fuel, inaccurate quantity readings,

possible contamination during aggressive attitudes.

RIGID REMOVABLE TANKS

• Welded aluminum tanks inserted into the aircraft.

• Usually fuselage tanks.

• A disadvantage of this type of tank is added weight.

• An advantage is the ability to remove and repair.

• The C-172 fleet is usually equipped with this type of

tank .

INTEGRAL TANKS (WET WING)

• Integral tanks are made by sealing off

compartments inside the wings.

• They have the advantage of utilizing existing

aircraft structure to contain fuel, which reduces

weight.

• Commonly found in large aircraft.

EXTERNAL WING TANKS (TIP TANKS)

• These fuel tanks are mounted externally.

• Tip tanks at the end of the wingtips. (C-310)

• Underwing tanks: no those aren’t bombs.

(Lockheed Jetstar)

• Tip tanks can have an aerodynamic advantage

as they act like winglets.

FUEL TANK LAYOUT

• Fuel tanks can be arranged in multiple tank designs.

• Fuel can be used simultaneously from different tanks, or one at a time.

• On large aircraft the order in which tanks are filled and burned off has an effect on weight and balance.

• Some complex fuel systems have fuel burn schedules which involve systematic burn off and transfer between tanks to ensure limits are not exceeded.

• In the case of wet wing aircraft outboard tanks are usually filled first and emptied last, to ensure wing structural integrity. The fuel in the wings counteracts the forces of weight.

Fuel burn in swept wing aircraft can

have a significant effect on C of G.

Involved fuel burn schedules

COLLECTOR TANKS

• Aircraft with long wings are subject to fuel starvation

due to sloshing (slush or splash of liquid).

• This is guarded against by incorporating collector tanks

into the system.

• All fuel goes to the collector tank prior to reaching the

engine.

• This smaller collector tank is always full of fuel which

absorbs any interruptions in feed due to sloshing.

Collector tank prevents engine fuel

starvation due to sloshing.

INTEGRAL WING TANK

COLLECTOR TANK

FUEL PUMPS

• High wing carbureted aircraft are usually gravity fed and don’t need fuel pumps. (C-172)

• Fuel injected and low wing aircraft require a fuel pump to supply positive pressure to the fuel metering system.

• Fuel pumps are also used to transfer fuel between tanks and provide crossfeed.

• Fuel pumps are usually lubricated by the fuel itself and can overheat if run dry.

• These pumps are usually engine driven.

• Fuel is fed to the engine at a rate faster than it can be used, this means return lines are necessary.

CAVITATION

• The formation of an air pocket (cavity) in the fuel.

• If the fuels pressure becomes too low it will vaporize.

• The pump creates a low pressure area as the fuel is

accelerated. Air pockets forming on the suction side of

the pump can cause cavitation.

• Fuel pumps are incapable of pumping a gas.

• This can cause pump damage, and possibly an

interruption in flow.

BOOST PUMPS (STANDBY PUMPS)

• Boost pumps are used:

• As a backup for the engine driven pump.

• Crossfeed operation.

• Priming.

• Start operation.

• Fuel transfer.

• Provide positive pressure to the engine driven pump.

• Usually on for take off and landing to guard against an engine failure due to pump failure at a critical point.

• Boost pumps are also used to provide positive feed pressure to engine driven pumps which helps prevent cavitation.

• These pumps are usually electrically powered.

MOTIVE FLOW PUMPS (JET PUMP)

• These pumps are usually used for inter-tank

transfer.

• They rely on venturi effect to create suction.

• A electrically or engine driven pump provides

flow in the line, then a venturi creates suction.

FUEL VALVES

• Used to guide the flow of fuel within the system.

• Fuel valves can be manual (C-172, B-95) or

electrically powered.

• Check valves restrict flow to one direction.

• Tank selector valves control which tank is to be

used.

• Firewall shut-off valves prevent fuel from reaching

the engine. Used to secure engine in emergency

situations.

FUEL HEATERS

• Jet fuel is prone to ice crystal formation and congealing (solidify).

• Fuel heaters are incorporated to ensure the fuel is warmed to optimum operating temperatures before it reaches the engine.

• This is usually accomplished by some form of heat transfer e.g. running the fuel lines through a heat exchanger plumbed with warm oil lines.

FUEL VENTS

• As fuel is removed from a tank it must be replaced with air or a vacuum will be created and fuel flow will stop.

• The vacuum could possibly create tank collapse.

• Provides an escape for air in the case of thermal expansion.

• Vents must be heated or flush mounted, or recessed to protect against icing conditions.

DRAINS AND STRAINERS

• Drains at the low points of a fuel system are

important to drain water which collects at the

bottom. To drain tanks for maintenance.

• Strainers collect contaminants in the fuel to

ensure they are not ingested by the engine.

MEASURING QUANTITY

• Most light aircraft utilize floats to measure fuel quantity.

• More sophisticated aircraft use capacitance type

quantity indicators.

• Jet fuel volume changes significantly with temperature.

• Mass will remain constant and can be measured by

electric probes.

• The gauges of this sort of system usually indicate fuel

quantity in pounds.

DIPSTICKS

• Dipping fuel tanks is common practice with light

aircraft.

• The gauges tend to be inaccurate and dipping the

tanks often results in more accurate readings.

• Most large aircraft have a manual method of

determining fuel load in the event of gauge failure

• Magnetic measuring sticks are one method of

accomplishing this.

CROSSFEED

• Crossfeed capabilities of a multi-engine fuel system are essential to ensure fuel on the failed engine side is available for use.

• Crossfeed also enables the pilot to correct fuel imbalance situations.

• It is important to understand how the system works for your specific aircraft.

• In some systems certain tanks may be unavailable during crossfeed.

• Specific procedures may apply. (B-95 failed engine selector must not be off)

• The decision to crossfeed fuel after an engine failure should not be taken lightly. If the engine failure was the result of contaminated fuel it could mean trouble for the operative engine.

C-172

C-210

B-95

B-95

C-402