Powerful Facts About Aircraft Fuels and Fuel Systems

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All You Need to Know About Aircraft Fuels and Fuel Systems

Just like other in other vehicles, aircraft engines need fuel to provide the energy that keeps them going. However, the requirements of aviation fuels are very specific. Tonnes of things could go wrong while the aircraft is in the air, and with no immediate relief possible, extra care should then be taken as regards the type of fuel used, the fuel systems and the handling of the fuel.

Aircraft fuels and fuel systems

In this article, I will talk about the following things:

The required properties of aviation fuels.
Types of aviation fuels.
How aviation fuels can be handled safely.
The operation of a typical aircraft fuel system.

Properties of Aviation Fuels

Fuels used in aviation should have the following properties:

High combustibility: the fuels should be easily combustible as it is the combustion of fuels that provides energy for the engines to work. When combusted, the fuels should also release a large amount of energy. The more energy released, the more energy for the engines to work with and the more power can be provided to the aircraft.
Low flammability: the fuels should not catch fire easily as this can be catastrophic for the aircraft.
Low toxicity: the fuel is handled by maintenance personnel during fuelling processes. The fuel should not be too toxic so that when being handled, it does not cause significant damage to the body of the handler.
Reasonable volatility: the fuel should not vaporise too easily as this will result in fuel loss. However, the volatility should be high enough so that the fuel combusts easily.
Reasonable viscosity: the fuel should have a low enough viscosity so that it can easily flow within the fuel systems. High viscosity also results in pressure drops, causing the fuel pumps to use more energy to sustain a constant flow rate of the fuel. This results in energy losses.
Thermal stability: the fuel should not dissociate easily in high temperatures. Dissociation of the fuel disrupts the flow of the fuel within the tanks and makes it less useful as a heat exchange for the oil systems within the engine and the hydraulic systems of the aircraft.
Low electrical conductivity: static electricity tends to build up as the fuel moves on the surfaces of the fuel systems. The build-up of too much static electricity can result in explosions within the system, therefore, aviation fuels should not be able to easily conduct electricity.
Should not cultivate the growth of microorganisms. Microorganisms in fuel can cause corrosion of the surfaces of the fuel systems and clog the strainers, preventing the easy flow of the fuel.
Should have lubricant properties to reduce the wear of internal fuel systems.
Should not corrode the fuel system components

Types of Fuel Used in Aviation

As you know, there are several different types of aircraft and aircraft engines. This calls for the use of several different types of aviation fuels as pertaining to the specific aircraft/engine type.

All You Need to Know About Aircraft Fuels and Fuel Systems: Colour coding of different aviation fuels on their equipment. Image source: FAA

Some types of aviation fuels are:

Jet fuel (Jet A-1): it is also called JP-1A and is used in turbine engines of commercial aircraft. It is light and mostly made of kerosene, with a few other additives. Its flash point is above 38 degrees Celsius while its freezing point is -47 degrees Celsius. The additives in the fuel make sure that the fuel does not freeze, does not get charged (electrically), does not accommodate the growth of organisms and that the fuel does not kindle uncontrollably. Jet A fuel has very similar properties to the Jet A-1 fuel and is only used in the USA.

Kerosene-gasoline mixture (Jet B): it is also called JP-4 and is used in fighter aircraft. Its composition is about 65 % kerosene and about 35 % gasoline. Its flash point is as low as 20 degrees Celsius meaning it can fire up easily and its freezing point is -72 degrees Celsius. This makes it especially ideal for flights in extremely cold regions.

Aviation gasoline (avgas): this is a high-octane-number fuel (100 octanes) with lead as one of its constituents. It is used in older piston engines of sports aircraft or small aircraft that require the aforementioned properties. This fuel can only be used in gasoline engine-powered aircraft. The variant of the avgas fuel that is still in use today is low-lead (100 LL).

Sustainable aviation fuel (SAF): in recent years, the increased air pollution caused by the combustion of traditional fuels has called for the development of environmentally friendly alternatives. Scientists and engineers have started adopting the use of biofuels, so-called because they are developed from renewable plant or animal sources. They are not yet in widespread use as extensive tests must be carried out with engines to ensure they are safe and adequate for use. They are also combined with traditional fuels to form what are called SAF blends.

Some examples of SAFs are:

Bioethanol: alcohol produced from the fermentation of crops like corn.
Biodiesel: it is made from vegetable oils and animal fats.

Safety Precautions for Handling Aviation Fuels

When handling aviation fuels, certain precautions must be taken by the handlers to ensure no harm is done to them or to the aircraft. These are:

Aircraft must be grounded before fuelling or refuelling processes are commenced. This means such operations cannot be performed while the aircraft is moving.
Firefighting equipment should be nearby, so possible fire breakouts can be handled quickly.
Handlers should wear gloves and other protective wear if the fuel has a likelihood of coming in contact with the skin.
The skin of the handlers must be washed immediately if aviation fuel touches it.
Once clothes are soaked in oil, they should be changed.
When the maintenance personnel is to go into the fuel tank, s/he must be well-trained beforehand and face masks must be worn to prevent damage that can be caused by inhaling the toxic vapours of the fuel.

Aircraft Fuel Systems

Aircraft have assemblies of components that work together to ensure the proper storage, circulation and distribution of fuel around the aircraft, and especially to the engines.

The main functions of the fuel system are:

Storing fuel in the tanks.
Supplying the right amount of fuel to the tanks during refuelling.
Supplying fuel to the engines.
Fuel circulation for the cooling of generators which drive the engines is called integrated drive generators (IDGs).
Keeping fuel in the outer tanks of the wings to improve their aeroelasticity.

Components of an Aircraft Fuel System and their Functions

Below are some components of a typical aircraft fuel system.

Refuel/defuel valves: when the fuel needs to be replenished, the refuel valve allows the fuel to flow from the refuelling gallery to the tanks. They close once the desired fuel load for the aircraft has been attained. During the refuelling process (maintenance), they close once the fuel load has been reduced to the desired level.

Cross-feed valves: it is run by a double motor and allows the engines to be supplied with oil from tanks on either sides or both sides. This is particularly useful when there is a fault in one engine’s tank. It will ensure that the fuel supply is not cut off from either engine.
Non-return valves (NRV): they perform the same function as in hydraulic systems, i.e., restricting the flow of the fuel in just one direction. This makes sure the system operates the correct way, i.e., there is no backflow.
Sequence valves: they allow for the sequential operation of the fuel systems. If fuel needs to be distributed to different parts of the system in steps, rather than the fuel being distributed at once, the sequence valves allow this to happen,.
Shutoff/ Low Pressure (LP) valves: they “shut off” the fuel from flowing when required. This can be by preventing fuel from flowing between tanks or to an engine.
Fuel dump valves: in an emergency, they dump excess fuel from the aircraft tanks so as to reduce the amount of fuel to the required levels. They are only used in emergency settings during flights. For example, if a plane suddenly needs to reroute to a different closer destination during a flight, there will be excess fuel, so that fuel needs to be dumped on the ground while the aircraft is still in the air.

NB: aircraft are fuelled with the exact amount of fuel they need for their trips. The amount of fuel is always calculated by the flight planner before a flight. It’s not like with cars where you can just fill your tank and keep using it for as many journeys as the fuel can carry. This is primarily due to the required weight savings in aviation. Landing lighter will impose less stress on the airframe and allow for a safer, less impactful landing. Also, ironically, increasing the amount of fuel onboard causes the plane to consume more fuel. This is because the weight has an adverse effect on fuel efficiency. I talk more about fuel dumping in the system operation section.

Selector valves: they are used to control the direction of flow of the fuel. For example, the right and left selector valves direct the fuel received from the main tanks to right and left engines respectively.
Transfer valves: if the fuel level falls below a certain mass (usually 750kg) in either of the inner tanks, the transfer valves open. This allows the fuel to flow from the outer tanks to the inner tanks.
Suction valves: they allow fuel to be supplied to the engines from the wing tanks by means of gravity in the event that the tank pumps fail. Gravity would allow for a free fall of the fuel into the engines. Of course, this is only possible with a configuration that has the engines below the wings.
Pumps: they are driven by the engines and provide the pressure which drives the fuel through the system. Pumps are usually spread over the centre tank and side tanks.
Strainer: it works like a filter in the hydraulic system. It filters out impurities that may be found in the fuel. Without filtering, these impurities may cause damage to the tanks or even the engines if they flow into them.
Fuel pressure gauge: it measures the pressure within the fuel system, allowing it to be monitored by the flight crew.
Tanks: they are the outer tanks, inner tanks and centre tanks through which the fuel moves to the engines are fitted with pumps. The centre tank is the first to be emptied followed by the inner tanks and then the outer tanks which transfer fuel to the inner tanks when the fuel load in them falls below a specified mass. Some of the fuel supplied to the generators is used as a heat sink to cool the oil systems and then goes back to the fuel pump or outer tank. The centre tank is located in the fuselage while the outer and inner tanks are located in the wings.
Level sensors: they detect the level of fuel in the tanks so that it can be monitored. This is useful when refuelling, refuelling and during flight, so that appropriate action can be taken.

Aircraft Fuel System Operation

For this section of this article, I will be using the Airbus A320, a narrow-body passenger aircraft, as a reference. The operation of other aircraft fuel systems may slightly differ.

The A320 has 6 main fuel pumps. The fuel is delivered to the pump from the centre tanks and inner tanks. An additional pump is fitted to the Auxiliary Power Unit (APU) as a backup if there is no pressure provided by the left tank pumps, which normally supply the APU.

The availability of multiple fuel tanks which supply more fuel than needed ensures that the engines will always have a fuel supply in flight. The fuel dump valves also contribute to safe operation in the event that there is excess fuel than needed on board, due to any issues developed on a flight that requires the aircraft to fly a shorter distance. The use of fuel in cooling systems ensures there is no overheating within the hydraulic and oil systems (within the IDG engine), thereby reducing the chances of fire outbreaks.

Refuelling:

It is usually done from the right wing of the aircraft. The refuelling valve opens on the outer tank (on the wing) and then fills the inner tanks and lastly, the centre tanks, if they need filling. Refuelling can only occur on battery power and at nominal pressure, it usually takes up to 20 minutes for all the tanks. The wing tanks can supply fuel to the engines under the effect of gravity.

Preparing the cockpit:

The fuel pumps are switched on when preparing the cockpit. After the pumps finish their operation, the pilots must then verify the fuel distribution in the tanks using the cockpit display which shows the fuel levels.

After one engine is started, a 2-minute timer is started and then once again after the second engine is started. This is the time the centre tank pumps will be in operation before they automatically shut down once the slats are extended. In that case, the inner wing tanks will supply the engines for take-off. If the slats are not extended, the centre tank pumps will keep running as long as there is fuel in the centre tanks.

During the normal climb, cruise and descent:

When the slats retracted after the plane has gotten in the air, the centre tank pumps supply the engines once more till the plane reached a straight-and-level flight or cruise.

During the cruise, the centre pumps then automatically shut off. During this time, some fuel is used for cooling the oil systems in the IDGs because centre tank pumps are required for this action, in the event that wing tanks are full, while the inner tank fills the IDG engines. When the fuel quantity drops below 750kg in the inner wing tanks, the centre pump automatically restarts.

During descent, the centre tank is empty so the pumps shut off automatically. The inner tanks are feeding the engines at that point. When the inner tank reaches a low level, the transfer valves open, transferring fuel from the outer to the inner tanks. The transfer continues until the outer tanks are empty.

Fuel Dumping

As discussed earlier on in the article, an aircraft might need to dump some fuel in emergency situations. The exact amount needed for the flight is calculated beforehand, taking into account the duration of the flight, plane conditions, number of passengers, weather conditions and so on. The plane is then fuelled with an amount within that range so that enough fuel can be burned before landing, and the plane can land lighter. The fuel streams behind the plane in a contrail-like manner.

Some pilots might prefer to “dirty up the plane”, meaning they will fly around in circles to create (vortex) drag and burn off the excess fuel. This obviously takes more time, as fuel dumping can let go of thousands of pounds of fuel in an instant. However, it is useful if the plane is not able to dump fuel, or if the plane is in a region where it’s unsafe to dump fuel. Pilots have regrettably dumped fuel on people on the ground before and when this happens, it can result in serious health complications.

The Federal Aviation Administration (FAA) regulates the following as regards fuel dumping:

The plane has to be at least 2,000 ft above the highest body along the flight path.
The plane has to be at least 5 miles away from any other plane.
The plane has to be away from populated areas and water bodies. Fuel dumping into the sea is not allowed. Air Traffic Control (ATC) advise the pilots on zones that are safe for fuel dumping.

The environmental effects are also noteworthy. The dumped fuel is supposed to evaporate before it reaches the ground, however even if it does not fall to the ground in liquid form, it can still be suspended in the air and form a component of smog.

Nonetheless, fuel dumping occurs on very rare occasions, also because aircraft operators would rather save fuel than lose thousands of dollars worth of fuel due to dumping. Pilots may opt to dirty up the plane, or simply land overweight with caution.

Conclusion

It’s clear that aircraft fuel systems are rather complex, much like most aircraft systems. They have intricate workings that ensure the safe operation of flight, and also enhance the operation of some other systems.

Coming years might see the widespread use of SAF in commercial aircraft as engine manufacturers are already performing testing with such fuels before launching them.

References

Products Company, C. (2007). Aviation Fuels Technical Review. [ebook] California: Chevron Products Company, pp.3-9. Available at: https://www.chevron.com/-/media/chevron/operations/documents/aviation-tech-review.pdf
Mabanaft.com. (2019). Aviation Fuels | Glossary | Mabanaft. [online] Available at: https://www.mabanaft.com/en/news-info/glossary/details/term/aviation-fuels.html
Fuel flight crew training manual. (n.d.). [ebook] pp.1-8. Available at: http://www.smartcockpit.com/docs/A320-Fuel.pdf
Hardiman, J., 2021. What Causes Aircraft To Need To Dump Fuel?. [online] Simple Flying. Available at: <https://simpleflying.com/why-aircraft-dump-fuel/> [Accessed 16 July 2022].
Cox, J., 2020. [online] Usatoday.com. Available at: <https://www.usatoday.com/story/travel/columnist/cox/2020/01/17/ask-captain-why-do-planes-dump-fuel-before-emergency-landings/4488854002/> [Accessed 16 July 2022].
Business Insider. 2020. Why do planes dump fuel?. [online] Available at: <https://www.businessinsider.com/planes-dump-jet-fuel-aircraft-landing-emergency-2019-12?r=US&IR=T> [Accessed 16 July 2022].