Fuel systems of aircrafts

1. FUEL SYSTEMS

3. 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.

4. 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

5. 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

6. 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

7. 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.

9. 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.

10. 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 .

12. 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.

15. 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.

19. 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.

21. Fuel burn in swept wing aircraft can
have a significant effect on C of G.
Involved fuel burn schedules

22. 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.

23. Collector tank prevents engine fuel
starvation due to sloshing.
INTEGRAL WING TANK
COLLECTOR TANK

24. 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.

25. 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.

26. 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.

27. 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.

28. 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.

30. 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.

31. 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.

32. 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.

33. 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.

34. 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.

36. 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.

37. C-172

39. C-210

40. B-95

41. B-95

42. C-402