Aircraft Fuel Tank System and its 5+ Important Subsystems

Aircraft Fuel Tank System

Image Source: U.S.Air Force, AIM-4 and AIM-7 on F-4E, marked as public domain, more details on Wikimedia Commons

The subject of discussion: Various Important Subsystems of Aircraft Fuel Tank System

In the previous article, we have learnt about various features and design characteristics of an Aircraft Fuel Tank. If you haven’t read it already, check it out here; as it will serve as the background knowledge for what is upcoming in this article. In this piece, we shall go deeper and learn about the different subsystems of the Aircraft Fuel Tank System.

aircraft fuel tank system
Aircraft Fuel Tank System; Image Source: “David Biller Pumps Gas into Jet, circa 1968” by Archives Branch, USMC History Division is licensed under CC BY 2.0

Types of Aircraft Fuel Tank System

Aircraft Fuel Tank System can be differentiated into two variations- Internal or External Aircraft Fuel Tank, and then further categorized by construction method or intended usage.

Internal Aircraft Fuel Tank

Aircraft Integral Fuel Tank | Aircraft Fuel Tank Baffles

This part of the aircraft fuel tank system lies on the wings or fuselage, usually sealed with a fuel resistant 2-part sealant to form the fuel tank system on some aircraft, particularly transport class and high-performance one. The sealed skin and structural elements provide the most room for the least amount of weight. Because it formed a tank as a unit within the aircraft construction, this type of tank is commonly known as an integrated fuel tank.

Integral fuel tanks are most frequent in the otherwise vacant space inside the wings. Wet wings refer to aircraft with incorporated fuel tanks in the wings. The long horizontal shape of an integral wing tank necessitates baffles to keep the fuel from sloshing as an aircraft maneuvers. Baffles are built into the wing ribs and box beam structural components, and others may be added particularly for that purpose.

Check valves with baffles are often used to allow fuel flow into the tank’s lower, inboard sections while preventing it from spilling outboard. This guarantee the placement of the fuel booster pump at the bottom of the tanks, irrespective of aircraft attitude. Access panels for examination and maintenance of integral fuel tanks and other fuel system components are required. Technicians physically enter by a dozen oval access panel to the tank for repair works in bigger planes.

All fuel must be evacuated from an integral fuel tank before entering and performing maintenance on it, and certain safety precautions must be followed. Fuel vapors must be evacuated from the tank, and the technician must employ respiratory apparatus. A full-time spotter must be stationed at the tank’s edge to assist if necessary.

Fuel systems for aircraft with integral fuel tanks are typically sophisticated, with in-tank boost pumps. Each tank normally has at least two pumps that provide fuel under positive pressure to the engine(s). this pump may be used to de-fuelling time also.

Rigid Removable Fuel Tank

For the building of fuel tank, many aircraft, particularly for aged ones, choose an apparent choice and this tank is attached to the air-frame construction and is built of various materials. The tanks are frequently riveted or welded together, and they may have baffles in addition to the other fuel tank elements mentioned previously.

To avoid movement in-flight, removable metal tanks must be supported by the aircraft and held in place with some form of cushioned strap arrangement. They’re joined together on wings using electric resistance welding, and then a compound is poured into the tank and left to cure. There are several fuselage tanks as well. The structural integrity of the air-frame is unaffected by the placement of the tanks under any circumstance; hence the tank is not considered as integral part.

If there is a leak or a fault with the tank, being able to remove it and repair or replace it is a huge benefit. Fuel tank repairs must be carried out following the manufacturer’s instructions. When welding repairs are conducted, it is very important to observe all safety requirements. To avoid an explosion, fuel vapors must be evacuated from the tank.

Aircraft Fuel Bladder Tank

A bladder tank, which is built of strong flexible material, can be utilized as an alternative of a rigid tank. This has many of the same characteristics and components as a rigid tank, but it may be installed through a smaller hole in the aircraft’s skin. The tank, also known as a fuel cell, can be rolled up and inserted via a small hole, such as an inspection hole, into a specially constructed structural bay or hollow. It can be unfolded to its full extent once inside.

Aircraft Fuel Tank System: Bladder; Image Source: Picryl

Clips or other attaching methods must be used to secure bladder tanks to the structure. In the bay, they should be smooth and wrinkle-free. It’s also vital that there are no wrinkles on the bottom surface since this will prevent gasoline impurities from sinking into the tank sump.

Bladder fuel tanks can be seen on planes of all sizes. They’re tough and long-lasting, with seams only around installed features like tank vents, sump drains, and filler spouts. When a bladder tank leaks, the technician can repair it according to the manufacturer’s recommendations.

Aircraft Fuel Tank System; Internal Bladder; Image Source: BrokenSphereC-17 internal fuel bladderCC BY-SA 3.0

The cell can also be taken out and transported to a fuel tank repair facility that is knowledgeable and equipped to handle such jobs. Bladder fuel tanks must be kept wet due to their soft, flexible nature. If a bladder tank is to be stored without gasoline for a lengthy length of time, it is usual to practice covering the inside of the tank with clean motor oil.

Tip Tank

Fixed tip tanks are positioned at the end of each wing in several aircraft designs. The weight of the tank and fuel brings down strain on the spar structure by counter acting wing bending stress during movements.

External Aircraft Fuel Tank

Conformal Fuel Tank

Conformal fuel tanks (CFTs) or “fast packs” are supplementary fuel tanks that are fitted closely to an aircraft’s profile and enhance the aircraft’s range or endurance with a lower aerodynamic penalty than exterior drop tanks.

Drop Tank | Aircraft Auxiliary Fuel Tank

Auxiliary externally installed fuel tanks are referred to as drop-tank, external-tank, wing-tank, pylon-tank, or belly tanks, are usually disposable and easily discarded. External tanks are ubiquitous on modern military aircraft, and they are also occasionally found in civilian aircraft, albeit the latter is less likely to be ditched unless in an emergency.

Drop tanks were meant to be ejected when they were empty or in the event of a fight or emergency to minimize drag and weight while boosting maneuverability and range. Modern external tanks are not designed to withstand the stresses of supersonic flight and are kept in combat to be thrown away in an emergency.

The most significant disadvantage of drop tanks is that they increase the drag of the aircraft. External fuel tanks will also reduce air maneuver roll rates by increasing the moment of inertia and Some of the fuel in the drop tank is utilized to offset the tank’s added drag and weight. This tanks reduce the number of external hard-points for weaponry, reduce weapon-carrying capacity, and increase the radar signature of the aircraft. The fuel in the drop tanks is often used first, with the fuel selector switching to the airplane’s internal tanks only after the drop tanks have been depleted.

How do fighter aircraft’s external fuel tanks work? | External Fuel Tank Fighter Aircraft

Conformal fuel tanks (CFTs) are used instead of or in addition to traditional external fuel tanks on some modern combat aircraft. CFTs have less drag and don’t require external hardpoints, however, some variants can only be removed on the ground.

External fuel tanks, which are found in jet fighters and most current commercial planes, are attached directly beneath the fuselage through their hardpoints near the wings. On the jet fighter, the gasoline pump for the external tank is already mounted. The external tank option is built into the majority of fighters. However, if the pilot wants to extend the range by installing an extra fuel tank, it will have to sacrifice the number of armaments it can carry.

For a typical F-14 fighter aircraft, engine bleed air extracted downstream of the primary heat exchanger, which is pressure-regulated to roughly 25 psi, is used to transfer fuel from the external tanks. Each external tank’s maximum fuel transfer rate is around 750 pounds per minute. With the landing gear retracted, the fuel transfer is automatically scheduled. External tanks will be transferred first, followed by wing tanks.

Each external tank receives pressured bleed air (25 psi) via the fuel and air disconnect, the vent valve, and the tank itself. Fuel flows out of each box beam tank through the fuselage level control system’s fuelling and transfer cutoff valve.

Aircraft Fuel Surge Tank

Surge tanks are used on a few aircraft to prevent fuel from spilling onto the ground as it expands. These tanks must be drained regularly to avoid fuel from spilling, which happens rather frequently. The vent surge tanks protect the fuel from thermal expansion. Without spilling, fuel can increase by at least 2% (equal to 20°C). There are no tank indicators in the cockpit for these tanks.

Aircraft Fuel Tank Material

To avoid leaks, the aircraft fuel tank system is often made of Al- 3003 or 5052  type alloy or SS material and is riveted and seam welded. Many early tanks were composed of terneplate, a lead/sin alloy plated on thin sheet steel. The seams on the terneplate tanks are folded and soldered.

Aluminium aviation fuel tanks have been widely utilized for two decades due to their advantages of a lightweight, ease of construction, and good corrosion resistance. Even though dry gasoline does not erode aluminium, pitting of the metal shell has happened in the presence of highly contaminated water on occasion.

Water, iron rust, and other heavy metal corrosion products can be avoided by:

  1. Designing tanks to allow free drainage of water to the sump.
  2. Selecting metals to avoid electrolytic action.
  3. Handling fuel so that it does not pick up water, iron rust, or other heavy metal corrosion products before being introduced into the aircraft fuel tank system.
  4. Applying suitable coatings on the interior of the aircraft.
  5. Using an alclad sheet.

Aircraft Fuel Tank Inerting System

By retaining as non-reactive or inert gas, such as N2, in a confined space, such as an aircraft fuel tank system, an inerting system reduces the likelihood of combustible materials being ignited. An ignition source (heat), fuel, and oxygen are all necessary to start and sustain combustion, hence these 3- ingredients (or individually) can be reduced to prevent combustion. If it is impossible to avoid the presence of an ignition source within a fuel tank, the tank can be rendered non-ignitable by:

  1. Lowering the oxygen concentration in the ullage below the combustion threshold;
  2. Lowering the fuel concentration in the ullage to below the “lower explosive limit” (LEL);
  3. Raising the fuel concentration to above the “upper explosive limit” (UEL).

Flammable vapors in fuel tanks at present are rendered inert by substituting an inert gas such as nitrogen, nitrogen enhanced air, steam, or carbon dioxide to reduce the oxygen concentration in the ullage below the combustion threshold. Alternative strategies include lowering the ullage fuel-air ratio below the LFL or increasing it above the UFL. Due to cost and weight considerations, combat aircraft fuel tank system has long been inerted and self-sealing, which is not the case with military and civilian transport aircraft fuel tank system.

The Handley Page Halifax III and VIII, Short Stirling, and Avro Lincoln B.II were among the first aircraft to use nitrogen inerting systems, which were introduced in 1944. Two more inert fuel tank systems are now in use: a foam suppressant system and an ullage system. The FAA has determined that an ullage system’s additional weight makes it impracticable for use in aircraft.

Sealing Aircraft Fuel Tank | Aircraft Fuel Tank Sealant Application

A self-sealing aircraft fuel tank system is a form of gasoline tank that keeps fuel from leaking and igniting after it has been damaged. It is commonly found in aviation fuel tanks or fuel bladders.

Self sealing tank typically have double layered of rubber and reinforcing fabric, one is vulcanized and other is untreated natural rubber made, these can absorb fuel, swell, and to be expanded when in contact with it. Perforation of aircraft fuel tank system causes seeping of the fuel into the layers, only to have the untreated layer swell and seal the rupture. Self-sealing run-flat tires are also made using a similar concept.

Aircraft Fuel Tank System: Self-sealing; Image Source: High ContrastMe-262, selbstabdichtender KraftstofftankCC BY 3.0 DE

Aircraft Fuel Tank Foam

Several solutions have been introduced to the market to control the oxygen level in an aircraft’s ullage. Aircraft fuel tank system ignites mitigation options generally utilize polyurethane foam to line the cavity of an aircraft fuel tank system’s center, unlike nitrogen inert system that use an ASM with a permeable membrane. This reduces the consequences of fuel vapor ignition and eliminates the risk of an explosion.

The lack of an ASM is one advantage of using an aircraft fuel tank system ignition mitigation solution versus a N2 inerting system. Due to the impact of high amounts of ozone, ASM components require periodic maintenance and, in some cases, complete replacement. Foams have been used to manage pressure after fuel vapor ignition in a variety of military applications, including the United States Air Force and several commercial cargo planes.

Water in Aircraft Fuel Tanks

Water in aircraft fuel tank system continues to be a factor in aviation mishaps and accidents, including fatalities. Fuel must be kept dried or water-free during transportation from the refineries, airport store and refueling equipment’s.

Let’s see how does water enter in the Aircraft Fuel Tank System.

  1. The water will enter into the aircraft fuel tank system by subterranean tank leaks, seals on dome-coverings, floating-roof and hatches by the condensation and precipitation of dissolved water.
  2. During the process of cleaning the aircraft or during rain or snowstorms can lead to water leaking into the aircraft fuel system thru the vents, seals, or poorly fitting fuel caps on filler-port etc.

The best possible ways the above scenarios can be prevented have been summarized below:

  1. Check the drain hoses and man-way gaskets, as well as the water controls on the filter separator, as well as the tank and vessel sumps daily.
  2. After steam cleaning, recirculate the truck and thoroughly inspect the sumps, pressures, and filter quality.
  3. The security, general condition, and sealing of fuel tank filler holes and attachments should all be checked.
  4. This should be ensured that the bladder remains intact to its mountings.
  5. Drain plugs/caps should be provided on gascolaters, strainers, and filters.

Microbiological Contamination In Aircraft Fuel Tanks

Fuel contamination in aircraft fuel tank system by microbes can pose a serious threat to aircraft operations. Corrosion of metallic structure, fuel indicator issues, scavenge system and fuel filter blockage, and sludge development are the most typical issues. These issues impose a significant financial strain on air transportation.

How can bacteria/fungus live in an aircraft fuel tank?

Microbes such as bacteria, fungus, and yeast are the sources of micro biological contamination. These microorganisms dwell in water and can be introduced to fuel through a variety of factors, including changes in relative humanity or failures in fuel handling protocols. Microbes thrive in the water-fuel interface on the tank’s bottom surface, but they can also be found on the tank’s vertical surfaces and in convex structures like pipelines.

Microbiological contamination of aircraft fuel tank system is constantly examined. Aircraft fuel tank system monitoring is recommended by IATA (International Air Transport Association) based on location and experience, but at least once a year. IATA recommends using Easicult TTC and Easicult M as semi-quantitative dipslide tests for contamination monitoring.

EasiTTC identifies bacteria, while EasiM detects yeast and molds. The colony forming unit (CFU) method is one of IATA’s authorized contamination monitoring methods, that dictates these tests. Both tests are dependable, simple to apply, and suited for use in the field by airlines.

Aircraft Fuel Tank Inspection | Aircraft Fuel Tank Maintenance

After a specific amount of flight hours, aircraft maintenance checks are required. The periodical routine check can be conducted overnight or at an airport gate, while others will require the use of a hangar and a long period, keeping the aircraft away from operation. As part of routine maintenance, a technician inspects and adjusts the aircraft fuel tank system and associated equipments. The interior of the tanks must be inspected and modified for a major portion of the work needed in properly checking and altering aviation fuel tanks and their associated systems.

Aircraft Fuel Tank Entry

The aforementioned activities necessitate inspection and maintenance workers physically entering the aircraft fuel tank system, which has numerous environmental dangers. Hazards might include fire and explosion, poisons gas release with a lack of oxygen etc. Operators and responsible maintenance organizations must be equipped with specialized systems to identify, control, or minimizing dangers related with the aircraft fuel tank system entrance. The potential threats that fuel-tank operators may face can be classified into two categories 1) chemical and Physical hazards respectively.

Chemical Hazard

Jet fuel is a flammable liquid that can catch fire under the influence of high-temp and vapor concentration. Fuels are judged too lean to burn if they fall below the Lower Flammability Limit/Lower Explosive Limit.

At excessive concentrations, jet fuel and other hydrocarbons can affect the nervous system may cause health hazards. Chemicals can potentially create long-term health issues like liver and kidney damage and if not properly regulated, cleaning solvent, sealant, and other chemical utilized in fuel-tank operations might irritate the skin.

Physical Hazard

Usually an aircraft fuel tank system has an oblong hole that is less than two feet (0.6 meters) long and one foot (0.3 meters) wide. Even minute portions of any chemical inside such confined areas can prove hazardous by producing a large volume of flammable vapor.

The maintenance person’s head can barely fit in the inboard area of the wing tank. Towards the outboard direction, the tank is only big enough to include the person’s shoulders along with his head. Only a maintenance person’s hands and arms may fit in the most outboard parts of the wing.

Aircraft Fuel Tank Entry Procedures

Before a maintenance person can access an aircraft fuel tank system, he or she must perform several processes. These include electrically grounding and defueling the plane according to industry standards, having enough fire prevention equipment on hand, and deactivating connected aviation systems such as fueling/defueling and fuel transfer systems. To maintain a safe working environment for maintenance staff, three last procedures must be completed:

  1. Make sure you have enough ventilation.
  2. Use the suggested ventilation methods.
  3. Keep an eye on the air in the fuel tanks.

A properly trained personals and entrycrews, consisting of the entry supervisor, standby attendant, and entry men, is the most critical component in preventing harm during fuel-tank operation. The entry supervisor gives his or her approval for the task to be done following protocol. The standby attendant remains outside the gasoline tank and is empowered to command its evacuation if conditions deteriorate and endanger the entry workers.

Personnel who enter the aircraft fuel tank system and conduct the operation are called entry personnel. The members of the ‘aircraft fuel tank system entrance crew’ needed to be trained with the standards for safe working condition, both individually and collectively:

  1. Communication is essential.
  2. Protection for the lungs.
  3. Ventilation and air quality monitoring are also important.
  4. Electrically operated machinery.
  5. Considerations for plane damage.

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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