The Fuel System Chapter 8

Aim Understand the principals of operation of the fuel systems

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Questions?