1. The Fuel System
Chapter 8

2. Aim
Understand the principals of operation of the fuel systems

3. Objectives Describe the fuel system and components
Describe fuel types and properties
State operational aspects associated with the fuel system

4. 1. Fuel System Fuel System Overview
The fuel system is made up of a number of individual components including:
Fuel Tanks
Fuel quantity detectors
Strainers/filters
Tank selectors
Fuel pumps
Primers

5. 1. Fuel System
Fuel Tanks
Tanks can be constructed of aluminium alloy, synthetic rubber or stainless steel
They are normally fabricated to be an integral part of the wing or fuselage
Some aircraft utilize bladder type fuel tanks
They must be designed to distribute weight evenly
Expansion space should be incorporated to account for fuel expansion on hot days
Vents are incorporated to allow air to escape as fuel expands due to heat and allow air to enter as fuel is drawn into the engine. Some aircraft use forward facing vents to maintain positive pressure above the fuel
Tanks may be pressurised to reduce vapour formation at altitude, suction relief valves will be incorporated to ensure fuel flow should the pressurisation system fail

6. 1. Fuel System
Fuel Tanks
Baffles are fitted inside tanks to increase strength and prevent fuel surging
Sumps and drains are used to allow contaminants to settle and be removed from the tank
A standpipe is used to draw the fuel into the system allowing us to avoid drawing in any contaminants in the sump, this leaves some unusable in the tank below the standpipe level

7. 1. Fuel System Fuel Quantity Detectors – Float Type
Most light general aviation aircraft utilize a simple float. Here a metallic float sits on top of the fuel, movement of the fuel is transmitted via mechanical linkage or electrical signal to the cockpit
These systems are notoriously unreliable, you must dip the tanks to ensure the actual fuel quantity is known before flight

8. 1. Fuel System Fuel Quantity Detectors – Capacitance Type
Sophisticated general aviation aircraft and large aircraft use capacitance type fuel quantity detectors, these are infinitely more reliable than float type however they do cost more
A capacitor is used to store charge by separating two conductive plates with a dielectric, the dielectric is used to polarize the electric field
The amount of charge that can be stored varies with the size of the plates, the distance between the plates and the dielectric constant
Air and fuel have different dielectric constants therefore allowing different amounts of charge, a voltmeter is used to measure this voltage and a fuel quantity is derived

9. 1. Fuel System Strainers/Filters
The fuel will usually be strained at three points:
The first is the tank outlet where the fuel will normally pass through a wire gauze to remove larger foreign matter
The second will be placed at the lowest point in the system and will also incorporate a drain
The final strainer will be located at the carburettor or fuel control unit, here the fuel will be filtered through a fine gauze before entering the engine

10. 1. Fuel System Tank Selector
Selector valves allow individual tanks to be selected to feed the engine, they will typically also include facility to isolate the system
They must be readily accessible and provide sufficient ‘feel’ so that they cannot be inadvertently miss-selected inflight
The valves must be positioned on the opposite side of the firewall to the engine so if a fire occurs it cannot spread to the tanks

11. 1. Fuel System Engine Driven Fuel Pump
The engine driven fuel pump operates automatically and provides fuel under pressure to the carburettor or fuel control unit
The most common system is the vane type:
The engine drives an impellor which uses centrifugal force to increase fuel pressure
Constant fuel pressure is maintained by using a pressure relief valve, the fuel pressure is determined by a combination of a spring tension and air pressure on top of a diaphragm
Should the pump fail a bypass valve is incorporated into the system

12. 1. Fuel System Electric Fuel Pump
Sometimes referred to as boost or auxiliary pumps
Operate on the same centrifugal principals as the engine driven bump but are driven by a small electric motor
Are designed to provide 120% of the maximum demand of the engine
Are used to:
Prime fuel injected engines
Provide fuel at the correct pressure during start
Purge the fuel lines of any vapour and reduce the possibility of vapour lock
Provide a backup to the engine driven fuel pump
Will usually incorporate a bypass valve in case of pump failure

13. 1. Fuel System
Primer
For carburetted engines insufficient fuel is lifted out of the metering jets for reliable starting, particularly on cold days where vaporisation of the fuel in the intake manifold is poor
The primer delivers extra fuel directly from the strainer bowl into the manifold of one or more cylinders
The system will normally utilize a simple hand pump operated from inside the cockpit
The primer must be locked when the engine is running to prevent excess fuel entering the intake manifold

14. 1. Fuel System Gravity Feed System
Only found on high wing aircraft where a head of fuel is maintained between the fuel tank and engine
As the level of fuel in the tank reduces the fuel pressure will reduce, the system is designed to support a flow rate of at least 150% of the maximum required during any stage of flight
The two major disadvantages of the system are:
Changes in attitude, particularly negative loading, reduces fuel flow
Susceptible to vapour lock due to the low pressure in the system

15. 1. Fuel System Pump Feed System
Can be found on low or high wing aircraft
Requires a fuel pump to pump fuel from the tank to the engine

16. 2. Fuel Types and Properties
AVGAS
Aviation gasoline (AVGAS) is manufactured and managed under more stringent quality control standards than ordinary motor gasoline (MOGAS)
In general terms:
AVGAS contains more lead than MOGAS giving it a higher octane
AVGAS is less volatile than MOGAS reducing the susceptibility of vapour lock
AVGAS is stored and maintained at higher standards to increase safety

17. 2. Fuel Types and Properties
Octane Ratings and Performance Numbers
The anti-detonation (or anti-knock) properties of fuel are designated by either an octane rating or performance number
When the system was first introduced it was believed a rating of 100 represented the limit to which the anti-detonation properties of gasoline could be taken, technological advances have seen gasoline with anti-detonation properties greater than 100 therefore fuels are now assigned a performance number
A naturally occurring group of hydro carbons with an octane rating of 100 are known as iso-octane, with the addition of lead the anti-detonation properties can increase up to 45% giving a gasoline performance number of 145
A dye additive is added to fuel to easily identify the octane
The common fuel grades can be seen here
Fuel Grade
Colour
MOGAS 91
Purple
Red as of end 2013
MOGAS 96
Yellow
AVGAS 100 LL
Blue
AVGAS 100/130
Green
AVGAS 115/145
AVTUR
Pale Yellow

18. 3. Operational Aspects Fuel Contaminants
It is the responsibility of the oil company to ensure there are no contaminants in the fuel at time of delivery, once delivered it is up to the operator
The most common contaminant is water, this can enter tanks through condensation or poor seals
In order to avoid condensation forming the tanks should be filled to full overnight
Other impurities include rust, sand, micro organisms (Mould), etc

19. 3. Operational Aspects Fuel Checks
Fuel checks should be done in accordance with CAO 20.2 and company procedures
Fuel drains are located at the lowest points of the fuel system and contaminants should settle to these points
If contaminants are detected you should continue draining until pure fuel is drained
The most common contaminant is water and will settle as a bubble in the bottom of the drain. You must ensure the whole sample is not contaminated with water
There are a number of commercially available fuel testing papers and pastes that react when water is present

20. Questions?