Being such complex machines, aircraft have many components and systems that must work in synchrony to guarantee a smooth and safe flight.
One of those systems is the fuel system. The fuel system of an aircraft has a very specific purpose, delivering a steady flow of fuel from the fuel tanks to the engines while meeting the flow rate and pressure established by the manufacturer. No matter the type of fuel or the type of engine, that is the ultimate goal.
Of course, there are many more details behind the operation of a fuel system, and they will widely vary from one aircraft to another. Yet, we will do our best to describe the most critical aspects of fuel systems found in different types of aircraft, so keep reading to find them out.
The first component in an aircraft fuel system is the fuel tank. Aircraft fuel tanks are critical components of fuel systems since they store a flammable fluid while being exposed to vibration, aerodynamic forces, heat, cold, inertial loads, and even lightning strikes during flights.
Therefore, the fuel tanks installed on aircraft must be capable of withstanding even the toughest conditions that can be found during a flight without suffering any deformation, surface alteration due to corrosion, or any similar issue.
Moreover, it is vital that fuel tanks keep sealed at all times to avoid any external contamination to reach the fuel system, including dirt, dust, and water. There are some tasks that an aircraft fueler is usually responsible for.
Fuel tanks must not be installed on the engine side of the firewall and there must be half an inch clearance, at least, between the tank and the firewall if installed in the fuselage.
Tanks must be isolated from, and cleared of personnel compartments by a fume and fire-proof enclosure. It is important that tanks remain ventilated at all times so that no accumulation of dangerous fumes or vapors occurs while in the air or on the ground.
Fuel tanks are usually classified in three types. They are integral, rigid, and bladder tanks. Let’s see each type in detail.
Integral fuel tank
Often called wet wings, this is formed through sealed wing construction within regions referred to as fuel cells. An integral tank generally consists of aluminum frames which do not corrode when immersed in fuel.
The storage space in the wing must be properly sealed to prevent rivets, bolts, nuts, fuel lines and hose to leak. This type of fuel tank must be capable of bending under aerodynamic stress, and to withstand expansion or contraction under ambient temperatures changes. The seal must support this through the operating temperature range as well as flight envelope.
Limiting the amount of fuel that can flow during maneuvers is critical since the fuel is free to move about the wing structure. Baffles and guides are built into the wing to achieve the limit to do so.
Integral tanks are usually lighter than the equivalent rigid metallic tanks, but they tend to be more difficult to maintain and repair as there is no way to remove or replace the tank alone.
Rigid fuel tank
Typically, this type of tank is built separately to the aircraft and then installed in either the wings or the fuselage.
They are self-storage units which are usually made of aluminum alloys or stainless steels in 3-series configurations and installed in specially adapted fuel bays.
This fuel tank has no part in the structure of the aircraft, so the structure must support it adequately throughout the entire flight envelope. Rigid tanks simplify repair and maintenance as tanks can be easily removed from aircraft to be repaired and reinstalled, or replaced by a new one.
Rigid tanks are pressure tested to ensure that they will not leak or collapse during flight.
Bladder fuel tank
A bladder tank is similar to a rigid tank except for being produced in reinforced flexible materials such as synthetic rubber. So, unlike rigid tanks, it does not require large cuts into aircraft structures for installation.
Instead, installation could be done by deflating the tank and folding or rolling it to its minimum expression, placing the bladder through a check hole, and unfolding it in the desired place. During installation, the personnel must guarantee there are no wrinkles to avoid contamination from entering the tank.
What are the types of aircraft fuel systems?
The types of tanks described above are suitable for specific types of aircraft and their fuel systems. So, let’s see the types of systems to better understand how the tanks adapt to them.
Gravity feed fuel system
The gravity feed fuel system, or only gravity feed system, uses gravity to pump fuel from the tank to the engine. Therefore, installation is required to allow that gravity to transport the fuel. Consequently, this fuel system is often used in light aircraft with a high-wing desing such as the Cessna 172.
While integral tanks could be used with this system, the simplicity of the system and the aircraft that use it make the use of rigid or bladder tanks installed in the wings a more suitable solution. This design places the fuel tanks installed on high-wing aircraft above the carburetor.
Regulatory requirements specify that the system must allow flow rates to exceed 150 percent of the takeoff fuel consumption for the aircraft engine. Occasionally booster pumps are also used as a means for augmenting gravity fed systems – typically for engines with fuel injection instead of carburetor-based systems.
Fuel selector valve
This is an important component for these aircraft fuel systems. According to the Aircraft Owners and Pilots Association (AOPA), the fuel selector valve “allows the pilot to choose which tank is feeding fuel to the engine. Some systems require you to alternate between Left and Right tanks, while others offer a Both position. Some aircraft might favor one side over the other when Both is selected; select the appropriate side to rectify any fuel imbalance. The selector also has an Off position.”
Fuel pump system
Low-wing airplanes with tanks on installed in their wings need pumps to transport the fuel from the tanks to the carburetor or injectors. A system dependent for pumps feeding the engines needs a minimum amount of redundant power achieved by using two fuel pumps, one driven by the main engine and one electrically driven. The electric auxiliary fuel pump operates from an electrical switch inside the cockpit.
This auxiliary fuel pump or boost pump is generally used to start the engines, but it also provides added reliability to fuel systems. According to regulations, both pumps must be independently capable of supplying the engine at a rate of 125% of the maximum requirement.
Fuel pump systems are the ones used in more complex aircraft with low-wing designs, and they are more suitable when using integral tanks. Yet, the other two types of tanks can also be used with a fuel pump system.
Let’s now see some of the most essential components of a these systems.
Other essential components of aircraft fuel systems
Fuel strainers and sumps
After leaving the fuel tank, and before reaching the carburetor or injector, the fuel passes through a fuel strainer to get rid of any moisture in the system. These pollutants are heavier as fuel for aircraft so they settle in sumps in the bottom of a strainer. Depending on fuel type and location, the fuel pump may include a drain pipe or outlet.
Fuel samples can be taken from the sumps. The fuel sample should be cleaned and examined for contaminants. Water inside sumps is dangerous as it can freeze in winters and block a power line.
Every tank has a drain installed below to allow all sediments and water to be safely drained from the tanks together with any fuel remaining. Also, because fuel is not pumped from the bottom of the tank but from higher levels, it is the bottom of the tank where the fuel is more susceptible to contamination. This explains why certain amounts of fuel in the tank are always considered unusable; therefore, aircraft manufacturers provide the usable fuel capacity compared to the total fuel capacity.
All tanks are vented to the atmosphere, no matter the type. This is done so that ambient atmospheric pressure is maintained in the tank at all times to help prevent airlocks in the system which could occur if a vacuum was present in the tank.
As the temperature in the tank changes, the density and hence volume of the fuel will also change. Therefore, a vented tank is a good idea because it allows fuel to escape if expansion has occurred at elevated temperatures.
Finally, by venting the tank to atmosphere, any fuel vapors that have formed in the tank will not dangerously accumulate. This prevents a vapor lock which could result in the engine no longer receiving fuel and ultimately stopping.
Fuel quantity gauges
Fuel quantity gauges are used to show the fuel quantity that a sense device measures within each fuel tank. This gauges ensure that the planned fuel quantity required for the flight is accurate.
Fuel pressure gauge
If fuel pumps are installed within the fuel system, a fuel pressure gauge may also be provided. This gauge measures the pressure within fuel lines. The normal operating pressure can be found in the AFM/POH or on the gauge by color coding.
Both gravity feeding and pumped fuel systems can include fuel primers. The fuel primer can be used for injecting gasoline directly into the cylinder before launching the vehicle. In the cold weather when engines find difficulties while starting, fuel primers help as long as the fuel is not frozen.
This primer should remain safely secured if not used. The levers may move in flight without being manipulated, so an excessive fuel-air mixture may be generated. If a plane has a problem with priming read the instructions provided by the manufacturer.
Some final words
As you know now, the engine fuel system of an aircraft is simpler than many people imagine, especially when it comes to light aircraft with a single engine. Obviously, the bigger the airplane the more necessary fuel pumps become to make sure the flow is steady as required for proper operation.
Also, the type and number of components can be different from one aircraft design to another as we described above, but they all essentially have the minimum we described above.
Of course, the descriptions offered within this guide are simplified for general understanding, and you can rest assured these systems will evolve as well as the rest of the technology applied on future aircraft.
Moreover, the way aircraft are powered is changing with the advent of hybrid aircraft and the potential of having fully electrically powered aircraft in a not so far away future. Therefore, we can expect these systems to be very different sooner than anyone could imagine, and they could even disappear for good.
When time comes, we will share a guide with the new systems for you to have a clear understanding of their composition and how they work.
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Jet pilot @NASA
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