Aircraft Fuel Tank – 10+ Important Features and Design Characteristics

Aircraft Fuel Tank

Image Source: Dylan Ashe from San Jose, USA, Classic Colors Southwest Airlines N648SW Boeing 737-3H4 SJCCC BY-SA 2.0

The subject of discussion: Aircraft Fuel Tank and its design characteristics

Aircraft Fuel Tank System

In our previous article, we learned about the Aircraft Fuel System and what role an Aircraft Fuel Pump plays in it. In this article, we shall take another step ahead in this journey and learn further about another component of the aircraft fuel system, which is the Aircraft Fuel Tank.

What is an Aircraft Fuel Tank?

An aircraft fuel system, as incandescently expressed, permits the crew to pump, manage, and transport aviation fuel to the aircraft’s propulsion-system and APU, and aircraft fuel is kept in Aircraft Fuel Tank, which is a crucial component of aviation fuel systems. The tanks are integrated type sealed constructions that are vented to the atmosphere; under all conditions, there is at least one open vent valve (for each tank). The vent system is meant to keep tank pressures within safe ranges. Spars, ribs, and stringers make up the majority of the tank’s structure.

The fuel tanks are normally found within the airplane’s wing box. A minimum of one tank is provided for each engine. A twin-engine plane, for example, has one main tank on each side of the fuselage and if the plane’s size and range necessitate more fuel, the middle wing box is built to accommodate it. On a 4-engined plane, there are two major tanks on each side of the fuselage, with the central tank providing supplementary capacity. Reserve and surge tanks, as well as body tanks, may be included in the fuel system.

The design of each fuel tank must allow it to sustain the vibration, inertia, and various kinds of impact load that it may experience while operating, without any error whatsoever. The total exhaustible volume for any tank must be enough to support 30 minutes of continuous operation to the very least, at maximum power. In addition, the useless fuel supply must be factored into each fuel quantity indicator.

Aircraft Fuel Tank Dipstick

A calibrated transparent plastic tubular dipstick for gauging aircraft fuel levels quickly and accurately. It is simply immersed in the aircraft fuel tank by placing the thumb over the end of the dipstick and lifting it out. The calibrated scale on the tube’s side is used to determine the fuel level. A blank calibration chart is included with the ‘universal’ fuel dipstick.

How does an Airplane Fuel Tank work?

Aircraft Fuel Tank Load Test

There are a variety of tank testing standards available to ensure that the aircraft fuel tank is capable of enduring the stresses and forces encountered during the entire flight operation. One of the key goals is to make sure that tanks are sturdy enough to stay functioning and not distort under varying loads. The ability to withstand vibrations without leaking is also a consideration. Tanks are put through their paces in the most extreme conditions possible. The structure supporting the fuel tank shall be designed for critical loads which could arise when flying or landing with fuel pressure loads.

The fuel system has been meticulously developed to maximize system protection during both wheels-up landings and accident circumstances. To reduce the risk of gasoline leakage and ignition in the case of a wheel-up landing, fuel-system component part are positioned in locations shielded by the aircraft structure and outer to the “wipe-off” zone. The fuselage skin and hefty structural elements absorb the energy of the landing shock and guard against scraping on the ground.

Breakaway landing gear, breakaway strut attachments, and breakaway flap attachments are all meant to keep fuel tanks from rupturing. Inside the fuselage contour, all tanks are designed to resist particular emergency landing loads.

Aircraft Fuel Tank Design | Aircraft Fuel Tank Construction

When it comes to fuel management, the design goal is to reduce the risk of fire and explosion. A fire or explosion requires three elements: combustible material, oxygen, and an igniting source. The risk of fire is reduced to zero if any of these items are removed and ignition sources are under the control of the designer.

As a result, a lot of cares have been put on to remove potential ignition sources and in situations where ignition sources cannot be avoided, efforts have been undertaken to reduce unintended flammable fluid leaks and provide ventilation to prevent vapor buildup. Furthermore, structural designs have been made crash worthy to limit the risk of fire in the event of a collision. Ignition source control, flammable fluid control, and crash worthiness are main 3 method to make a fuel system safer. The next sections go over how various methods are used in fuel system installations.

Ventilation and drainage are supplied in areas near fuel tank, in the wing and part of it. This keeps dangerous fumes and liquid fuel from accumulating. To avoid dumping combustible substances into potentially hazardous regions, ventilation and drain outlets are strategically placed. Flight tests are conducted to ensure that vent regions are adequate and that no pressure buildup has occurred. Fume-proof and fuel-proof barriers are always used to separate fuel tanks from occupied compartments. An aircraft fuel tank can be damaged by an uncontained engine failure, resulting in a fuel leak.

Aircraft Fuel Tank Leak Detection

Tank Installations

Aircraft Fuel Tank installations are subject to a variety of requirements. It is not recommended to install the tank on the engine side of a firewall, while a minimum distance of 12 inches between the fuel tank and the firewall is a positive. A fume-proof and a vented fuel-proof enclosure should separate the inner compartments of the aircraft from the aircraft fuel tanks. The tank should not be affected by pressurization loads. Dry bays seal the fuel in the zone on top of the engine, where spillage onto hot surfaces can break out a fire.

Fuel Shut-off

Fuel shutoff capability is provided at each engine and auxiliary power unit, along with the wing spar. A valve prevents the fuel from freeing in huge amounts through a damaged line during full engine separation when it is closed. There are two actuation modes: a fire handle and a power stop. The cables to the valve are duplicated and isolated. The valve is set to close upon turning off the engine, while it remains attached to the aircraft fuel tank in case of a broken.

Fuel-carrying Components

Fuel-carrying components and lines are sometimes found in or near fire zones, posing a risk of fuel leakage. These components and wires are rendered fireproof inside the fire zones. The chance of leakage from fuelline and components is reduced by enclosing the source with a 2nd sealed barrier.

The shroud is drained overboard, and the drain outlet is placed somewhere safe and visible, allowing leaks to be spotted and rectified before they become dangerous. Fuel lines that pass through pressured areas are encased in a drainable and vented shroud. The vent line is connected to a drain mast that is situated safely.

Aircraft Fuel Tank Sump

The appropriate construction and installation of the fuel tank is the first step in keeping impurities out of the fuel fed to the engine(s). The greater value of the sump between an effective 0.25% of the tank capacity and 1/16 gallon, must be drained in normal ground and flight attitudes. This includes drainage of any hazardous quantity of water from tank areas to its sump as well and accessibility to a sediment bowl or chamber should be provided in reciprocating engine fuel systems, with a capacity of 1 ounce per 20 gallons of fuel onboard.

Aircraft Fuel Tank Filler Caps

Each filling connection to a fuel tank must be marked. Filler apertures on aircraft powered solely by fuel must be no greater than 2.36 inches in diameter. The filler ports on turbine fuel aircraft must be no less than 2.95 inches in diameter. Spilled fuel is not allowed to enter the fuel tank compartment or any other part of the plane except the tank itself.

For the primary filler aperture, each filler cap must offer a fuel-tight seal. Small apertures in the fuel tank cap may, however, be included for venting or permitting the passage of a fuel gauge through the lid. The airplane must be electrically connected to the ground fueling equipment at all fueling stations (except pressure fueling connection points).

Aircraft Fuel Tank Outlet

The fuel tank outlet or the booster pump necessitates a fuel strainer, with 8-16 mesh/inch on reciprocating engine aircraft and it should be within definite reach of inspection personnel for cleaning and inspection. There should be a clear space five times the outlet line’s diameter and a strainer diameter equivalent to the fuel tank outlet’s diameter. Fuel strainers on turbine-engine aircraft must prohibit the passage of any object that could obstruct fuel flow or cause damage to fuel system components.

Aircraft Fuel Tank Location – What influences it?

Planes carry vast amounts of fuel to reach necessary destinations, especially those that are far from the place of departure. Notably, the weight of fuel can occasionally around 1/3rd of the total weight of the aircraft! But have you ever considered where it is kept? Yes, you’ve got it exactly right. Fuel is stored inside the wings of most planes, both small and large. Are you curious as to why? The following are some of the most important reasons:

  1. To balance the weight: Inside the airplane, it’s necessary to examine not just the seat configuration and cargo position, but also the positioning of heavy fuel. The fuel, in particular, keeps the aircraft’s center of gravity close to where it ought to be.
  2. To counter the stress: Within a small duration from take-off, the aircraft mass creates tension on the wings, and the fuel works as counter stress. This prevents drastic changes in the wing dihedral angle. In larger aircraft, leaving the wing tanks empty could result in snapping off of the wings.
  3. To reduce wing flutter: The fuel’s weight provides stiffness to the wing, thereby lessening the vibration of the wings from the airflow. Large flutter is so dangerous that it can cause the wing to completely collapse. As a result, putting fuel in the wings is a brilliant idea that keeps planes flying!

Aircraft Fuel Tanks in Wings

Advantages

Fuel tanks are frequently built into the wings of passenger planes, and when there are also tanks inside the aircraft’s body, the wing tanks are used first. Injecting heavy fuel directly into the source of lift reduces stress on the wing during takeoff and the overall flight. Placement of aircraft fuel tank in the main wings deviates heavy mass accumulation from the plane’s center of gravity, which improves flight efficacy and facilitates lesser elevator use.

Wings are frequently useless for cargo storage or passenger seating due to their uneven shape and lack of windows. However, its hollow construction allows for in-wing fuel storage and effective space utilization; structural spars in “wet wing” tanks decrease sloshing. Fuel tank usually placed in the wings, which keeps them away from traveller and crew in the case of a leak or accident.

Disadvantages

But such a location of the Aircraft Fuel Tank attributes to few disadvantages as well. Fuel sloshed laterally in the tanks due to turbulence or uncoordinated flight might result in lateral weight shift and possibly lateral instability. When there is a lack of fuel and the flight is uncoordinated, the engine may suffer from fuel starvation merely because the fuel has poured out of the sumps in the tanks. These issues can be solved by properly baffled fuel tanks and the use of feeder hoppers fed by the main tanks from which the engine drinks.

Also, fuel cannot be uniformly drained from both tanks at the same time on aircraft that use a siphon feed fuel system, such as low-wing aircraft. This is especially problematic in single-engine aircraft when separate fuel systems are dedicated to two engines. In these instances, the engine will draw fuel from either the left or right-wing tanks, which is controlled by a fuel selector valve in the cockpit.

Engine fuel feed must be manually selected on airplanes without autonomous fuel management systems. To avoid a lateral imbalance and a reduction in fuel supply, alternate feed from both tanks regularly. Furthermore, if this fuel tank swapping schedule is disregarded for an extended period, the engine may become fuel-starved, resulting in a forced landing.

Aircraft Fuel Tank Ventilation

To avoid the accumulation of flammable fluids or fumes, each tank compartment required to be ventilated and emptied, the tank-adjacent compartments required to be ventilated and emptied as well.

Aircraft fuel tanks must be built, placed, and placed in such a way that they retain fuel while subjected to inertia loads caused by ultimate static load factors, as well as under conditions similar to those encountered when the plane lands on a paved runway at a normal landing speed with retracted landing gears.  Fuel must also be available in case one of the gears fails or an engine mount separates from the engine.

Aircraft Fuel Tank Vent System

The concept of fuel tank venting is simple to grasp. The vent exists so that the tank can breathe; it provides a way for air and fuel to escape when the tank is overfilled. Because air pressure is affected by atmospheric changes, venting is particularly crucial while the plane climbs and descends. When fuel warms up, it increases in volume and decreases as it cool-down. Even if you aren’t flying the plane, the fuel level in your tanks changes during the day.

Because your tank needs to breathe, it needs a vent that can alleviate both vacuum and pressure. Because fuel is pulled from a tank to feed the engine, it must be replenished by something—air. The aircraft fuel tank cannot be fueled unless it can let the air out, and it cannot take fuel out without letting air in.

If the vents get blocked during flight, say when the tank holds 50% fuel and 50% air, the fuel will continue to be sucked out, but the remaining air will have to expand to occupy a larger volume. This results in a pressure drop—or, if you prefer, a partial vacuum—relative to the outside pressure. In any case, the fuel will soon run out or the tank will fall in on itself, imploding.

Why is it necessary to vent all aircraft fuel tanks?

To summarize, the aircraft fuel tank must be vented to:

  1. Maintain a +ve head-pressure for a sub merged booster pump.
  2. Keep the pressure difference between the tank and the atmosphere to a minimum.
  3. Get rid of the vapors from the fuel.

A vent line is present in every carburetor with vapor elimination connections and every fuel injection engine with vapor return facilities to return vapors to one of the fuel tank tops. Multiple tanks account for utilization in a specified order which causes the vapor vent line to return to the first-used fuel tank unless the relative capacity of the tanks makes it advantageous to return to another tank.

Excessive fuel waste during acrobatic maneuvers, particularly short periods of inverted flying, must be avoided for acrobatic category airplanes. When a regular flight is resumed after an acrobatic move for which certification is needed, fuel siphoning from the vent must be impossible.

Aircraft Fuel Tank Capacity

An aircraft’s fuel tank is separated into three sections: Wing Tanks, Centre Wing Tanks, and Trim Tanks.

Jet Liner’s main Fuel Tanks; Image Source: TosakaJet-liner’s main fuel tanksCC BY-SA 3.0

Wing Tanks

The Wing Tanks, as the name implies, are the tanks positioned in the wings of the aircraft. They contain roughly 70% of the aircraft’s total fuel. They’re further broken down into-

  1. Outer Tanks– Outer Tanks are positioned at the tip of the wings, at the extremity of the wings.
  2. Center Tanks– Tanks at the center of the wings are known as Centre Tanks.
  3. Inner Tanks– These tanks are positioned near the wing’s root. The main feed tanks are made up of the Centre and Inner tanks.
  4. Overflow Tanks– The Overflow Tanks are positioned towards the aircraft’s tip. If the fuel in the main tanks overflows, it will be gathered in these tanks.

Centre Wing Tanks

The Centre Wing tanks are those positioned in the belly of the plane’s fuselage, between the roots of the two wings.

Image Source: Maxxl2Adam a700 fr, marked as public domain, more details on Wikimedia Commons

Trim Tanks

Trim tanks are positioned at the tail of the aircraft in the tail wings or horizontal stabilizers. They have the smallest quantity of fuel in them.

How big is an airplane fuel tank?

A small plane can have a fuel capacity of 4000–5000 liters, a mid-sized plane can have 26000–30000 liters, a wide-body jet can have 130000–190000 liters, and a very large jumbo jet can have 200000 liters to 323000 liters.

Consider the fuel capacity of a large plane like the Airbus A380. Because of its size, the Airbus A380 has a large fuel capacity. The fuel is split in-between the horizontal stabilizer tank and the wing tank and each of wing’s tank constructed of outer tank, surge tank, mid tank, and inner tank etc. Trim tank and an aft-vent tank are located on the horizontal stabilizer.

The vent tank is a storage tank for the fuel that spills over from the main tanks. Each wing has a total fuel capacity of 120 tonnes. The trim tanks hold 18800 kg of fuel, which is equivalent to an Airbus A320’s fuel capacity. The total fuel capacity is 2*120 (wing tanks) = 240 tonnes + 18.8 tonnes (trim tanks), for a total of 258.8 tonnes (323500 litres) of fuel.

About Esha Chakraborty

I have a background in Aerospace Engineering, currently working towards the application of Robotics in the Defense and the Space Science Industry. I am a continuous learner and my passion for creative arts keeps me inclined towards designing novel engineering concepts.
With robots substituting almost all human actions in the future, I like to bring to my readers the foundational aspects of the subject in an easy yet informative manner. I also like to keep updated with the advancements in the aerospace industry simultaneously.

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