Aircraft Fuel Systems PDF

FUEL SYSTEMS  Outline Recap Types and categories of fuel systems Fuel tanks Water draining Fuel feed components Cross feed and vent systems Fuel Jettison Fuel level sensing Refueling and defueling Summary  Recap...

FUEL SYSTEMS  Outline Recap Types and categories of fuel systems Fuel tanks Water draining Fuel feed components Cross feed and vent systems Fuel Jettison Fuel level sensing Refueling and defueling Summary  Recap Introduction to aircraft Oxygen systems Safety precautions Oxygen systems layout Chemical Oxygen generator Indications and warnings  Fuel Systems To store, deliver fuel to the engines and APU. An A/C must carry sufficient fuel to operate over long periods. Safe storing and supply to engines in a suitable condition and at a controlled rate. Consist of tanks, fuel lines, connections, fittings, which are compatible with all types of fuel. Divided into storage, refuelling, distribution, transfer, venting, indicating sub systems.  Types of Fuel Systems Fuel Systems Gravity Feed Pump Feed Fuel System Fuel System Gravity Feed Fuel Systems Gravity Feed Fuel Systems Pump Feed Fuel Systems Gravity Feed and Pump Feed systems Fuel Cell Layout Three Categories Fuel Tanks Construction Rigid Flexible Integral  Rigid Type Fuel Tank Made from metal or plastic material. Fitted internally where space permits. Cannot be shaped and fitted into odd shaped spaces. Constructed from Al alloy, stainless steel or tinned. Rigid Internal Fuel Tank  Flexible Fuel Tanks Constructed with thin, very flexible walls (bag tanks) or of thicker less flexible material. Made in shapes to fit particular spaces in A/C structure. Their flexibility enables the tanks to be folded and inserted through a small aperture. The compartment for a flexible fuel tank is made as smooth as possible. When filled with fuel, the tank expands to contact the walls of compartment. Integral Fuel Tank Intergral Fuel Tanks Tank Access Panels Integral Tank Sealing  Commercial A/C Fuel Tanks Generally, large commercial A/C have three tanks in each wing (inner, outer & a surge tank). On some A/C the fuel tanks are referred to as fuel cells. A centre tank is sometimes available as a standard option. An additional space of 2% is available for for expansion of the fuel wthout spillage.  Water Draining These valves are at the lowest points of each tank. All valves may be opened with standard tools. the outer seal of the valve is replaceable without emptying the tanks. Water Drain Valve Fuel Tank Drain Points Fuel Supply Systems Design Requirements of an A/C Fuel Feed System Fuel Collector Tanks  Fuel Feed components Fuel Pumps (Booster Pumps) Jet Pumps Sequence Valves Transfer Valves Low Pressure Shutoff Valve Cross Feed Valve  Fuel Pumps Each tank normally provided with two pumps. These are centrifugal pumps driven by 115 volts, three phase motors. O/P of each pump is about 250-300 litres/min. Maximum fuel pressure at zero flow is about 38 psi Each pump includes a non-return and a by- pass valve. Jet Pumps  Sequence Valves Give an automatic transfer from one tank to another. The valve limits the fuel pressure of the outer tank pumps from 38 psi to 17.5 psi. This is to give priority to the inner tank fuel pumps for structural reasons. When the inner tanks are empty, the engines will be automatically supplied from the outer tanks so the outer fuel pumps run continuously. Sequence Valves  Transfer Valves Used to transfer fuel in between tanks.  LP shut off valve Enables isolation of the fuel system in the event of fire and engine maintenance. Located at the top of the pylon on the outside of the front wing spar. LP Shutoff Valve Fuel Filler Caps Fuel Lines and Fittings Hand Operated Valve (Cone Type) Hand Operated Valve (Poppet Type) Motor operated Valve Solenoid Operated Valve  Hand Gasoline Pump Handle Gasoline outlet Gasoline outlet valve Housing Vane Vane shaft Gasoline inlet valve Valve seat Gasoline inlet Boost Pumps Fuel Strainers Fuel Strainers Fuel Ejector Pump Fuel Cross-feed System Vent System Drip Stick Flow Indicators  Fuel Dumping (Jettison) Allow the jettisoning of fuel in an emergency thus reducing weight. Often fitted after the installation of a centre tank, because of the extra fuel weight. The jettison pipe runs inside the wing tanks through the ribs into the outer tanks, where the jettison valves are installed. These valves are fitted to the bottom of the tank. Fuel Jettison Fuel Jettison Control  The Vent Sub Systems An air vent is fitted to the top of each tank to allow free flow of air in and out of the tank. This is known as inwards and outwards venting (to prevent over pressurisation of the tanks).  Venting Due to Heat It must be able to cope automatically with any expansion and/or contraction of the fuel. As the fuel expands, due to heat, the vent must allow air and sometimes fuel, to escape to atmosphere via vent pipes. Conversely the sub-system must allow air into the tanks during contraction of the fuel when the outside air temperature is decreasing.  Unpressurised System Venting A very simple method of venting tanks. Fuel tank vent orifices be connected to a vent pipe gallery, which leads to atmosphere directly. Found in small aircraft with low flight ceilings. The disadvantages of “open orifice” or “open vented” tanks are that they are subject to fuel venting during manoeuvres. They limit the maximum ceiling of A/C due to fuel boils at low ambient pressure at altitude.  Reid Vapour Pressure (RVP) The rate at which fuel gives off vapour. There are many inherent problems with the open vented system. It is for many reasons that most aircraft fuel systems are pressurised.  Float Valves Each wing tank is provided with an additional vent opening. This opening is connected to the corresponding venting line and controlled by a vent float valve situated at the highest point of the tank. Vent Float Valve  Fuel Level Sensing A modern aircraft will use thermistors to send signals through amplifiers to actuate warnings, sequencing, etc. Older aircraft may use float switches. Fuel Level Sensing  Fuel Level Sensing Whether they are float switches or thermistors, their functions are as follows. High level sensing. Overflow sensing. Low level sensing. Under full level sensing. Level sensing for calibration  High Level Sensing Prevent an overfilling of the fuel tanks. When the fuel washes around the respective sensor, the: Associate refuel/defuel valve closes. Blue FULL light on the fuelling panel comes on. The high level signal from inner & outer tanks could be used for computation purposes in fuel quantity computer, in AUTO MODE refuelling.  Overflow Sensing If during refuelling the high level shut off system fails, fuel enters the adjacent vent tank and washes around the overflow sensor. This is indicated by the amber FULL light on the refuel panel.  Low Level Sensing Low level sensing is divided into: - Outer tank low level - Inner/centre tank low level sensing. If the outer tank LO LVL sensor is exposed to air, the associated amber LO LVL light comes on. The inner/centre tank low level sensing have only in the AUTO MODE function.  Calibration Sensing (Fuel Trim only) Sensors are installed in centre tanks, inner tanks and trim tank. They give a signal at a predetermined filling level in the trim tank for accuracy test of the fuel quantity indication during refuelling. For the trim tank the calibration sensor switching level is corrected by the stabilizer position.  Under Full Level Sensing When the fuel quantity drops in either outer tank below a certain level, the maximum flight speed is reduced to protect the wing structure. The sensor signals are sent to the ADC (Air Data Computer).  Fuel Quantity Measurement and Indication The system has the following tasks. Measuring of the fuel quantity in the tanks. Indicating of the fuel quantity on: - The fuel quantity indicator. - The pre-selector. - The ECAM system fuel page. - ECAM/EFIS. Controlling of automatic refuelling. Fuel quantity messaging to the flight management computer.  System Components Fuel quantity computer Capacitance probes Capacitance index compensator Cadensicon sensor Attitude sensor THS position detector Associated indicator in the flight compartment  Principle of Capacitance Gauging A capacitor stores electrical charge. The amount of charge it can hold depends upon three physical properties of the capacitor itself. The surface area of the plates. The size of the gap between the plates. The insulating material (dielectric) between the plates.  Capacitance Gauging In a fuel tank “capacitor stack” two of the above are fixed. The only variable is the dielectric which, in a fuel tank, is either fuel or air or both. The amount of charge held in the capacitor, when the tank is full, will be of a preset value. As the fuel level falls, dielectric will gradually change to air and the amount of charge stored will reduce. This change in capacitance is sensed by a signal conditioner.  Fuel Quantity Indicating System Each tank has installed a group of probes arranged so that a minimum of one probe is immersed at all times, the number of probes will vary from aircraft to aircraft. The following example is from a wide- bodied twin fitted with a fuel trim system. The number of probes is: - 6 in each outer tank. - 6 in each inner tank. - 4 in the centre tank. Basic Fuel System Refuelling and Defuelling  Safety Precautions When Refuelling Use correct grade of fuel (Av-gas, Av-tur). No smoking within 15m. No metal studded or tipped footwear. Correct bonding of Aircraft and Bowser. Correct positioning of Bowser. No vehicles or Ground Equipment under the aircraft. Maintenance activity kept to a minimum. No replenishment of LOX. No transmitting of Radar Aircraft & Bowser not to be left unattended. Check and remedy fuel spillage or leakage. Appropriate Fire Appliance readily available. The electrical state of Aircraft must not change while connected to the Bowser.  Defuelling Defuelling a pressure type fuel system is almost the reverse of the refuelling procedure. A de-fuel bowser would be connected to the single fuel point coupling. Using a combination of both the bowser’s suction pump and the aircraft’s own fuel supply booster pumps, selected tanks can have their contents returned to the bowser.  Longitudinal Balance Fuel System The balance of A/C in flight changes as the fuel is used. These conditions add to the complexity of the design of an aircraft fuel system. In small A/C the fuel tank or tanks are located near the CG so the balance changes very little as the fuel is used. In large A/C, fuel tanks are installed in every available location and fuel valves allow the flight engineer to keep the A/C balanced by scheduling the use of the fuel. High performance military jets and more modern civil A/C will use a fully automatic fuel scheduling system to reduce the workload on the flight crew.  Summary Types and categories of fuel systems Fuel tanks Water draining Fuel feed components Cross feed and vent systems Fuel Jettison Fuel level sensing Refueling and defueling Any Questions?