Sylvie Willis
Fuel transfer from tank to tank in aircraft is a complex process that requires careful consideration of weight, balance, and fuel management. While there is no standard fuel distribution scheme, the location of fuel tanks is generally designed to be near the center of gravity of the aircraft. Fuel calculations and distribution are critical to ensure the aircraft remains within the appropriate weight and balance envelope during flight. Some aircraft, such as the B737, have limited tank-to-tank fuel transfer capabilities and rely on crossfeed options to supply fuel to the engines. In certain cases, fuel transfer can be achieved through the jettison system by manipulating valves, pumps, and crossfeed valves, but this is not a standard practice for all aircraft. It is important to follow specific procedures and guidelines to ensure safe fuel transfer and avoid accidents, such as the accidental transfer of all fuel out of a tank, which has led to crashes in the past.
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What You'll Learn
Single-point fueling systems
The Single-Point Refueling System (SPRS) is a revolutionary approach to refuelling aircraft. This system has been widely adopted in commercial and military aviation due to its efficiency and safety benefits. The SPRS allows for the transfer of fuel to and from an aircraft through a single point of connection, doing away with the traditional method of refuelling through multiple ports and hoses.
The process of refuelling using the SPRS is straightforward. The refuelling hose is connected to the single-point adapter, and the fuel pumps are activated. The fuel is then transferred from the truck to the aircraft's tanks. The fuel level indicators ensure that the tanks are not overfilled. This system is designed to provide an uninterrupted flow of clean fuel from the fuel tanks to the engine.
The SPRS offers several advantages. Firstly, it simplifies the refuelling process by requiring only a single connection point, reducing the time and complexity associated with multiple hoses and connections. Secondly, it provides accurate fuel quantity measurements, vital for flight planning and fuel management. This is achieved through the use of fuel quantity indicating systems (FQIS) that employ sensors to provide real-time fuel level information to pilots. This enables informed decisions regarding fuel consumption and remaining range.
Additionally, the SPRS includes safety features such as automatic shut-off mechanisms and pressure control to prevent over-pressurization of fuel tanks during refuelling. The system also has separate valves for the left and right tanks, ensuring both tanks fill completely. The components are corrosion-protected and powder-coated for added durability.
While the SPRS offers numerous benefits, it is important to note that not all aircraft are compatible with this refuelling method. Some older aircraft models may lack the necessary equipment and infrastructure to support single-point refuelling. However, retrofitting options are available for many modern aircraft designed with SPRS capabilities in mind.
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In-flight fuel transfer
There are two main refuelling systems: the probe-and-drogue method, which is simpler to adapt to existing aircraft; and the flying boom system, which offers faster fuel transfer but requires a dedicated boom operator. The flying boom is a rigid, telescoping tube with movable flight control surfaces that the operator on the tanker aircraft inserts into a receptacle on the receiver aircraft. The boom contains a rigid pipe to transfer fuel, ending in a nozzle with a flexible ball joint that mates with the receptacle.
Some older aircraft, such as the Concorde, required fuel transfer between trim tanks and main transfer tanks to optimise the aircraft's centre of gravity for take-off and flight. This was done manually using pumps in specific tanks, with warnings provided through two separate channels: STANDBY 1 CG pack and STANDBY 2 CG pack.
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Fuel distribution for weight/balance
Fuel distribution for weight and balance is a critical aspect of aircraft operations. The weight of an aircraft includes the fuel, cargo, passengers, and the unloaded weight of the aircraft itself. Balance refers to maintaining these weights within the designed centre of gravity (CG) to ensure safe flight performance.
The location of fuel tanks in an aircraft is generally designed to be near the CG to maintain stability as fuel is burned during flight. Aircraft with multiple fuel tanks aim to load the tanks in a balanced way to stay within CG limits. For example, in a 747 freighter, fuel is placed in the centre tank to act as ballast and bring the CG within the limit. This ballast fuel must be carefully managed to avoid burning it and upsetting the balance.
In some aircraft, such as the A330 and A380, fuel is automatically transferred to trim tanks in the horizontal stabiliser to balance the aircraft and reduce drag. The Concorde aircraft employs a trim transfer system to redistribute fuel in the trim and main transfer tanks, optimising the CG for take-off and flight. This system is automated but can be overridden by pilots in abnormal circumstances requiring rapid fuel transfer.
For smaller, often single-engine aircraft, fuel management is typically left to the pilot. These aircraft may have multiple tanks that must be rotated during flight to avoid running dry. While the quantity of fuel is generally more critical than its location, proper distribution is still important, especially during stalls and spins.
Overall, fuel distribution for weight and balance is a critical aspect of aircraft operations, with specific procedures varying across different aircraft models.
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Crossfeed valves
The fuel system in aircraft typically includes a left and right tank within the corresponding wings, with each tank fuelling the engines on that wing. Crossfeed valves are used to temporarily connect the two sides of the system, allowing fuel to be drawn from one side of the aircraft to the other. This is particularly useful in the case of a fuel imbalance, where one wing tank has more fuel than the other, or if an engine is lost and the remaining engine needs to draw fuel from both wing tanks.
The crossfeed valve is generally kept shut to keep the systems separate, so a leak on one side cannot drain the entire system. However, there are certain scenarios where opening the valve is necessary. For example, if the left fuel pump encounters a problem, the right fuel pump can supply fuel to both engines through the crossfeed that is automatically opened.
The cross-feed system allows the engines to be fed from either the left tank, the right tank, or both tanks (one engine to each tank). The cross-feed pipes feed into the fuel hoppers, not directly into the other tank. While the cross-feed system can be used to fix a fuel imbalance, this is done by using the fuel in the engines, not by actually transferring it from tank to tank.
In-flight fuel transfer is not a normal activity on Boeing aircraft, and is only possible through the jettison system. However, this is not possible on all Boeing models, such as the B757100/200 and B767. On the B747, the only transfer possible in the air is between certain tanks, and the jettison system can be used to transfer fuel down to the standpipe.
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Defueling valves
In the context of the B737, it is noted that there is no capability to transfer fuel from tank to tank during flight. The only option available is to utilise the crossfeed mechanism to supply fuel from a particular tank to the corresponding engine. However, when the aircraft is on the ground, defueling valves come into play. By manually opening the defueling valve, it is possible to transfer fuel from one tank to another. This process requires the defueling valve to be open, and in certain cases, the crossfeed valve must also be open to facilitate the transfer of fuel between tanks.
On the DC10/MD11 aircraft, fuel management is achieved through tank-to-tank transfers, rather than crossfeeding fuel directly to the engine. In this scenario, the defueling valve's role is critical. Even if the defueling valve is "leaking," fuel should not transfer through the refuelling manifold if the fill valves for tanks 1 and 2 are closed. This highlights the importance of properly functioning defueling valves in ensuring safe and controlled fuel transfers.
The GNY Fuel/Defuel Valve is an example of a specialised valve designed for aviation ground refuelling operations. This valve enables a simple and quick transition between fueling and defueling modes by turning the valve handle. The fueling process occurs through the 6" ports, while defueling to the aircraft is facilitated through the 3" ports. The design incorporates Victaulic porting for ease of installation, and the valve is constructed from materials such as aluminium, ductile iron, and stainless steel to ensure durability and reliability.
Additionally, defueling valves are utilised in shipboard aircraft fuelling systems. These valves, such as the Cla-Val Marine Automatic Fueling Valves, control hose-reel stations that deliver fuel to jet planes or helicopters. The same system also facilitates defueling, ensuring a safe and reliable process.
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Frequently asked questions
It is not possible to transfer fuel from tank to tank in a B737. The only option is a crossfeed from tank to engine.
The only transfer possible in the air is: 1R & 4R to 1M and 4M 2R & 3R to 2M and 3M. You can also use the jettison system to transfer 1M to 2M and 4M to 3M down to the standpipe.
On the ground, open the defuelling valve and the concerned tank valves, as well as the #2 fuel SOV (fuel cutoff switch to idle).
The DC10/MD11 accomplishes fuel management by transferring fuel tank to tank rather than crossfeeding tank to engine.
Open all 4 transfer valves and then close the #3 and #4 tank valves while leaving the #1 and #2 valves open.
