Fuel system (aircraft)
The fuel system (engl. Fuel system ) of an aircraft is used to supply an engine powered aircraft with fuel . Depending on the type of aircraft and its structural design, whether aerobatics or not, the systems can be designed differently.
Types
Despite all the differences in fuel systems, some basic principles can be identified. Most modern aircraft are equipped with two or more fuel tanks in the wings .
- In high- wing and shoulder- wing planes , the tanks in the wings are higher than the engine. Therefore, the fuel can flow independently to the engine through gravity . These are case fuel systems.
- In low-deckers in aerobatic aircraft and jet aircraft are fuel pumps needed. When the engine is started, fuel must be pumped to the carburetor or to the injection system by means of an electric pump ( boost pump ) . If the engine is running, it can be switched off. A mechanical pump that is driven by the engine then supplies it with fuel.
construction
The fuel system, and possibly each individual tank, has a valve at the lowest point ( tank drain, oil pan drain ) which enables the pilot to drain any condensation that may have accumulated during the pre-flight check . The tanks are ventilated from the outside via a valve, depending on their fill level. In hot weather and with a full tank, the expanding fuel can also exit the system via this valve. The tanks are connected to one another via a pipe system so that volume can be equalized.
The amount of fuel in the tank is shown in the cockpit for each tank by the fuel gauge . Stock measurement for fluids is also used in aircraft for oils, hydraulic fluids, water and the like. For this purpose, floats , dip sticks and standpipes are used for measurement. In the case of electrical remote transmission, for example for the fuel supply, float meters are used as measuring transducers. The current fuel consumption is displayed by the fuel flow meter in the cockpit.
A tank selector switch in the cockpit allows the pilot to choose between the individual tanks during the flight. To compensate for weight imbalances, the pilot can set the selector switch so that only one tank is emptied. Usually, but especially during take-off and landing , the switch should be set so that the tanks in the areas are emptied at the same time.
The fuel primer is used to inject fuel directly into the cylinders when the ambient temperature is cold so that the engine can be started. The mixture regulator is required to adapt the air / fuel mixture to the altitude flown.
Thrust lever (engl. Throttle ), the Propeller (engl. Prop pitch control ) and the mixture control (engl. Mixture control ) are commonly housed together in a bracket, as they together serve the power control of the engine and thus influence the fuel consumption. As the altitude increases, the density of the air decreases . The carburetor uses less fuel if the air / fuel mixture is to remain constant. It is therefore Leanen (German leaning of the mixture ) is necessary otherwise the mixture too rich (too much fuel). This not only leads to increased fuel consumption, it can also damage the spark plugs . This leanen happens via the mixture regulator.
possible sources of error
The fuel gauges in the cockpit are subject to a number of display errors. The pilot should therefore check the remaining fuel quantity several times in flight with regard to power setting and flight time.
- In the case of low-wing aircraft, a complete emptying of a tank can lead to an engine failure.
- The fuel supply is given in units of volume . For a fuel level measurement, the fuel temperature is important when calculating the remaining flight time.
- A correct specification of the supply is only possible in the non-accelerated state.
- The fuel quantity display is relatively imprecise, at least in small aircraft. Since, for example, the float can also hang, it is essential to either visually check the tank contents or to fill in or fill up with the amount of fuel required for the flight before starting a flight.
Fuel tanks
The fuel tanks of all aircraft models are located in the wings and in the fuselage area between the wings. Only a few ultralight aircraft and military aircraft are exceptions. The wings are used as a tank for three reasons, each of which alone would be reason enough for such a design:
- If it were only to be placed in the fuselage, considerable space would be lost for the fuel. The maximum amount of fuel in a B747 is up to 220,000 liters - that's 220 m³ volume that was lost as cargo space. Fuel is liquid and can therefore be transported in any geometric shape. The interior of the wings is otherwise unused and can be used as a tank.
- If the 220,000 liters of the B747 were completely housed in the fuselage, the load on the wing root would increase extremely. The accommodation in the wings relieves the wing root on the one hand and counteracts the deflection of the wings (upwards) on the other. For the same reason, the contents of any middle tank that may be present are first used in flight before the fuel from the main tanks is used in the wings. The tanks are filled from the outside in, and consumption in the opposite direction.
- In order to maintain the trim, this weight loss must take place in the area of the center of gravity - the tanks would therefore necessarily have to be attached in the center area of the machine, attachment in the bow or in the stern would not be possible.
Airplanes always only take the amount of fuel that is actually needed (plus reserves). In addition, a full load and at the same time full tanks can lead to the permissible total weight being exceeded.
Complex fuel system using the B737 Classic as an example
The fuel system in large aircraft is much more complex than that of small aircraft. With the Boeing B737 Classic (B737–300 and B737–400), which is no longer so modern , many functions are not yet automated and require control and operation by the pilots. Newer aircraft models relieve the crew considerably through the automatic control and monitoring of the fuel system.
The B737 Classic has three tanks. One right and one left main tank each in the wings (tank No. 1 and 2) and a middle tank (center tank) in the fuselage between the wings, which also extends into the wings (Fig. 1).
With a density of 0.8 kg / liter, tanks no. 1 and No. 2 each has a capacity of 5667 liters (4530 kg) and the center tank 8743 liters (7000 kg). This corresponds to a total volume of 20,077 liters (16,060 kg).
Fuel temperature
The maximum fuel temperature is 49.6 ° C. During the flight, the fuel temperature must be 3 ° C above the freezing point of the fuel. The minimum fuel temperature is −45 ° C or the freezing point of the fuel used plus 3 ° C - the higher of the two counts here.
The fuel temperature is measured in the left tank. This is a legacy of the design of the B737–200. In this case, the fuel in the left main tank was usually colder than in the right main tank. A smaller heat exchanger for hydraulic system A was located in the left fuel tank, while the heat exchanger for hydraulic system B in the right tank was designed to be larger.
Alternating current (28 volts AC) is required for the measuring system to function.
Freezing points of some aviation turbine fuels
- Jet A-1 (JP-1A) and JP-8: −47 ° C
- Jet A (JP-1): −40 ° C
- Jet B, JP-4 and TS-1: -60 ° C
- JP-5: -46 ° C
If the fuel temperature is too low, the temperature must be increased by:
- Descent to a lower (and therefore warmer) altitude.
- Change of course to a warmer air mass or
- Increase in airspeed.
As a rule of thumb, the fuel that about 1 ° C / 10 kt IAS (Indicated Air Speed, German indicated airspeed ) heats the air friction.
Center tank
If both Center Tank Fuel Pumps (11 and 12) are switched off, the Fuel Scavenge Shutoff Valve (18) opens so that the fuel flow of the Forward Fuel Pump from Tank No. 1 (16) that can operate the Center Tank Scavenge Jet Pump (40, return pump). The remaining fuel is pumped from the center tank into the left tank. The Fuel Scavenge Shutoff Valve (18) closes again automatically after 20 minutes.
The tanks are ventilated in a right and left surge tank, which in turn balances the pressure with the outside air through an opening at the wing tip. Too high or too low air pressure in the fuel tank could damage the structure of the wings.
Center Tank Scavenge Pump
The Center Tank Scavenge Pump (German return pump ) pumps fuel from the center tank into the main tank no. 1. The pump rate is at least 100 kg / hour, but mostly almost 200 kg / hour. The return pump starts up when both fuel pumps for the middle tank are switched off. The return pump then runs for 20 minutes.
If you start with less than 1000 kg of fuel in the middle tank, the fuel in the left main tank can increase significantly compared to the right main tank and lead to an imbalance. Since both fuel pumps in the main tank are arranged in such a way that the right one pumps in the front area and the left one in the rear area, the delivery rate of the right pump drops to zero when the main tank is relatively empty when climbing. This right pump normally supplies fuel for the right engine, which now only gets its fuel from the right main tank. At the same time, the left fuel pump supplies the left engine from the center tank, so that no fuel is consumed from the left main tank. The right wing is getting lighter compared to the left wing. If the main tank is empty and the fuel pumps in the main tank are switched off, the return pump starts up and pumps the remaining fuel from the middle tank into the left main tank. The already existing imbalance increases and the left side becomes relatively heavier.
APU
The APU (auxiliary power unit) draws its fuel from the left main tank. To start the APU, the fuel pumps of the left main tank must first be switched on. The left fuel pumps must also be switched on for further operation of the APU.
The fuel consumption of the APU is around 160 kg / hour (with the packs and the electrics switched on). This value is also used to calculate the fuel consumption when the aircraft and passengers are on the ground for a long time.
Refueling
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Refueling takes place at a maximum of 800 kg / min. Refueling the wing tanks takes 12 minutes. It takes 20 minutes to fill up all three tanks.
Refueling takes place at the fueling station (7) on the right wing. This is also where the tanks are emptied or the fuel is pumped over from one tank to the other at the bottom. When the tank is full, a valve automatically closes the further flow to the tank. If no tank truck with pumps is available, the refueling can also be done by gravity fueling by filling at the top of the wings, where there is a tank inlet for each tank no. 1 and tank no. 2 lies. In this case, the center tank must be filled by pumping over from tank no. 1 or 2 using the aircraft's own pumps.
The manual defueling valve (6) must be opened to drain the fuel. It connects the line for the fuel supply to the engine with the fueling station.
Fuel control panel in the cockpit
The fuel temperature in the left tank is displayed on the fuel control panel (32) in the cockpit. The pilot manually controls the crossfeed valve (3) with the crossfeed selector (27). In the open position, tank no. 1 and No 2 connected to each other. For example, if one engine fails, the fuel can be used by both tanks and the aircraft can also be trimmed better about the longitudinal axis. The maximum difference in weight between the right and left main tanks for the B737 Classic is 453 kg.
The six Fuel Pump Switches (28 to 31, 35, 36) Fuel Control Panels (two per tank) are used to switch the electrically operated fuel pumps (Fuel Pump - 8, 9, 11, 12, 15, 16) on and off.
The Fuel Quantity Indicator (FQI) shows the usable amount of fuel in the respective tank (Fig. 4). The accuracy is 2.5%. The amount of fuel is only displayed when standby AC power (alternating current) is available.
The manual measurement of the amount of fuel is done with the built-in float stick or drip stick (one of the two is built-in). There are five dipsticks in each main tank.
Dripstick - for reading, the rod is slowly pulled out of the tank until the fuel begins to trickle steadily from the opening in the dripstick at its base.
Floatstick - the flexible floatstick Sakala is pulled out of the tank until it “sticks” or “sticks”. The reading is made at the point on the scale that is at the level of the underside of the wing. Depending on the inclination of the aircraft, the reading off must still be corrected using a conversion table / correction table.
The manual reading allows a control of the display in the cockpit (FQI).
Refueling order: If the center tank contains more than 453 kg of fuel, the right and left main tanks must be full.
Tank ventilation
The ventilation openings of the two main tanks (English. Fuel tank vents ) are on the B737 Classic near the wing tip, at the trailing edge. Their shape creates a slight positive pressure in the fuel tanks. This positive pressure prevents negative pressure from developing in the fuel tanks. This reduces the evaporation of the fuel in the tanks. In addition, the positive pressure in the tanks supports the work of the fuel pumps. If the fuel tanks are overfilled, the excess fuel will flow out of these ventilation openings.
Fuel filter bypass
The fuel filter bypass is activated when there is a pressure difference of more than 11.5 PSI on the fuel filter . The bypassing of the fuel filter is never triggered by icing as the fuel is heated before it enters the filter.
Failure of the fuel pumps
If the fuel pumps for a main tank fail ( main fuel tank boost pump inoperative ), it must be ensured that there is no imbalance between the right and left main tanks. Therefore, the crossfeed valve must be opened when the fuel quantity falls below 2600 kg. However, if both fuel pumps in a main tank fail at the same time, the crossfeed valve must be kept closed. Performance degradation may occur at altitudes over 30,000 feet. However, descent to a lower altitude is only necessary if the deterioration in flight performance or a flame-out occurred immediately after the failure of the fuel pumps.
Fuel pumps
When operating on the ground, the fuel pumps of the middle tank must be switched off if the tank content is less than 453 kg. The only exception to this is for draining or pumping the fuel.
If both lights for the "Low Pressure" display on the fuel panel light up, ie the fuel in the middle tank is running low, the fuel pumps for the middle tank must be switched off. For the same reason, the fuel pumps for the medium tank may only be switched on if at least one pilot is in the cockpit who can monitor the "Low Pressure" display.
Since the fuel pumps in the middle tank are cooled with fuel, they must not run dry. Blown fuses for fuel pumps must not be reactivated during the flight.
If the "Low Pressure" light of the fuel pumps lights up in the cockpit, they must be switched off. On the one hand because of the overheating of the dry pump and the associated risk of explosion, on the other hand because of the threat of tearing off the liquid column in the pump and which is then no longer provided when the pump is started up again (when the tank is now full) because of the air it contains. If the pump continues to run dry for about ten minutes, it will no longer provide any pumping power and must be filled with fuel manually ( prime ).
If the "Low Pressure" light lights up immediately when the fuel pump is switched on and the tank is sufficiently full and does not go out after more than 20 seconds, it is probably a fuel pump that has run dry and is no longer delivering capacity and needs to be primed.
Fuel crossfeed
The “cross feed” of the fuel from the other side is opened in flight when the fuel quantity falls below 2600 kg.
Tank explosions on the B737
On May 11, 1990, the middle tank of a Philippine Airlines Boeing 737-300 exploded before take-off from Manila . The machine was devastated. The cause was the continued running of the fuel pumps in the empty central tank. Since the fuel pumps are cooled with the pumped fuel, they ran hot without lubrication. In another, similar explosion on the ground, the central tank was only almost empty. There were also high outside temperatures. Even in an empty fuel tank, there are still small amounts of fuel that evaporate when heated and, together with oxygen, form an explosive mixture.
Because of 15 similar explosions of empty fuel tanks since 1959, the FAA demanded in June 2001 improvements to the design and maintenance of the fuel tanks (SFAR88) in order to reduce the likelihood of such explosions in the future. The improvements concerned the design of the fuel pumps, the display of the fuel quantity, the electrical installations in the tank and the pneumatic systems and hot air lines of the air conditioning system for the pressurized cabin that run near the tanks . Since May 2004 the B737 have been supplied with fuel pumps that switch off automatically when the sensors indicate a low outlet pressure on the pumps.
At Boeing, research is still being carried out into fumigating the middle tank with inert gas - an inert gas that does not contain oxygen and therefore makes it impossible to ignite the evaporated fuel gases . The inert gas used is nitrogen, which makes up 78% of the air and is to be extracted from the air in flight. The aim is to reduce the flammability of the fuel in the middle tank to or below the level in the main tanks. A “nitrogen generator” (Nitrogen Generating System - NGS) is used to generate nitrogen. The actual separation of atmospheric oxygen and nitrogen takes place in the Air Separation Module (ASM). After the separation, the nitrogen enriched air (NEA) is pumped into the middle tank and the oxygen enriched air (OEA) is displaced from the tank via the ventilation nozzles. No pure nitrogen atmosphere is created in the tank. The oxygen content in the air is only reduced from 21% to 12%, which greatly reduces the flammability. With the B737 this can be achieved with a "nitrogen generator", while six nitrogen generators are required for a B747.
As an alternative, nitrogen generation and supply on the ground is being explored. Because of the closed tanks, the nitrogen gas supplied remains in the tank during the flight with this technology, thus reducing the risk of explosion.
Safety regulations when refueling
Because of the risk of fire and explosion, special safety precautions must be taken when refueling. Aircraft may not refueled with running engines (Engl. Fueling ) or defuelled (Engl. Defueling ) are. Aircraft may not be refueled or defueled in a hangar or other enclosed space. If an aircraft is refueled or defueled, it must be connected to the connected fuel supply devices and be earthed.
During the refueling and defuelling of aircraft with kerosene or aviation fuel, no vehicles or other ignition sources (apart from the tank vehicles themselves) may be located within the tank ventilation area; A safety distance of four meters must be maintained around tank openings from which a gas-air mixture can escape. Refueling and defueling is not permitted during thunderstorms. There are further safety regulations for refueling from underfloor refueling systems .
Example of safety regulations for smaller aircraft:
- Before moving the aircraft for the first time, a daily condensation check in the fuel must be carried out.
- No passengers are allowed on board when refueling.
- The aircraft must be grounded for the refueling process.
- If the wings are wet, make sure that no water flows into the tanks.
- If there is the slightest doubt about the quality of the fuel, the fuel must be checked again after refueling.
Refueling with passengers on board
Special safety measures must be observed for refueling with passengers on board. In the ICAO document “Airport Service Manual”, Part 1, “Rescue and Firefighting”, it is stipulated how escape possibilities are ensured in the event of a fire when refueling with passengers on board. All on-board stairs must be ready for use and free of obstacles. Overall, however, these regulations are kept very general: In refueling operations with passengers on board, "the aircraft operator must provide suitable fire protection or request fire protection from the airport fire department ". "Safety measures in the cabin, securing the escape routes, keeping the evacuation areas free" are prescribed.
The corresponding EU regulation 965/2012 states:
No aircraft may be refueled / defueled with Avgas (aviation fuel) or a fuel with a wide boiling point range (Wide Cut Fuel) or a mixture of these fuel types when passengers board, are on board or disembark.
For all other types of fuel, the necessary precautionary measures must be taken, and the aircraft must be properly manned by trained personnel who are available to initiate and direct an evacuation of the aircraft expediently and quickly using the means available.
The regulations of the respective airport operator, which can differ in their severity, are also binding. These regulations show which regulations have been made with regard to the fire brigade. In addition, the respective airline can issue its own rules. Only a few airlines have internal service regulations that require the fire brigade to be ready to extinguish at the refueling point.
If kerosene escapes in an uncontrolled manner, for example due to overfilling or a burst fuel hose, the refueling process must be stopped immediately, the passengers must be evacuated from the aircraft and the airport fire brigade must be alerted.
Example (Stuttgart Airport): Refueling with passengers on board is only permitted when using kerosene with a flash point above 38 ° C. A fire truck from the airport fire brigade with a crew of two must be present. Exceptions can be approved in a standard exception procedure according to JAR OPS-1. For this purpose, it is stipulated, among other things, that when refueling with passengers on board a trained supervisor (usually a flight attendant) must be on board at a specified place (usually in the door area) in order to carry out emergency fire protection and fire fighting procedures, if necessary, to initiate the evacuation and steer. The supervisor must ensure compliance with the unconditional smoking ban. Handling vehicles (catering, baggage carts, etc.) must not obstruct the access route for the fire brigade or the evacuation routes for passengers. There must be a radio connection between the supervisor and the cockpit. Defueling with passengers on board is not permitted as there is a higher risk of accidents for technical reasons (formation of a fuel-air mixture).
literature
- Götsch, Ernst - Luftfahrzeugtechnik , Motorbuchverlag, Stuttgart 2003, ISBN 3-613-02006-8
- Jeppesen Sanderson - Privat Pilot Manual 2001, ISBN 0-88487-238-6
- Wolfgang Kühr - The private pilot , technology I, volume 1, Friedrich Schiffmann Verlag, Bergisch Gladbach 1981, ISBN 3-921-270-05-7
- Bachmann, Faber, Senftleben - Danger Handbook for Pilots , Air Report Verlag, Stuttgart 1981, ISBN 3-87943-656-8
Web links
- SFAR 88 / Related Operating Rules Special Maintenance Requirements & Compliance Planning Briefing (English; PDF file; 178 kB)
- Aviation Fueling Handbook (PDF file; 290 kB)