Commercial aircraft use an open vent system to connect the ullage, which is the space above the fuel in each tank, to the outside atmosphere. Adequate fuel tank venting across the entire operational flight envelope is essential because it allows the fuel tanks to effectively breathe as the aircraft climbs and descends. Without this venting capability, large pressure differences would develop between the ullage and the surrounding ambient air, creating very high structural loads on the fuel tanks.
Designing the wing structure to withstand such pressure loads would be impractical due to the excessive weight penalties involved. As a result, the vent system plays a critical role in protecting the fuel tank structure from structural failure as the aircraft transitions between ground level and cruise altitude.
During refueling operations, the fuel entering the tanks displaces the air already present inside them. For safety reasons, fuel spillage to the outside must be avoided. To achieve this reliably, a vent box, also known as a surge tank, is incorporated into the vent system. The vent box is designed to capture any fuel that may enter the vent lines during refueling.
On most commercial aircraft, the vent box is located near the wing tip. In typical low wing aircraft designs, which account for the majority of commercial transport aircraft, the wings incorporate significant dihedral. This wing geometry naturally makes the vent box the highest point during flight. Even on aircraft that feature anhedral wings, the vent box is often still positioned outboard. Figure 1 illustrates two different aircraft configurations, one showing a low wing installation with pronounced dihedral typical of commercial transports, and the other showing a high wing configuration with anhedral, which is more commonly associated with military transport aircraft.
As shown in the figure, the vent box location on a high wing aircraft may not represent the highest point during flight. Consequently, these vent systems typically include scavenge pumps to prevent fuel accumulation within the vent box caused by fuel migrating through the tank vent lines. Despite this, the vent box is still positioned outboard so that the air scoop, which connects the vent system to the external airflow, can be located well away from the fuselage. This location helps maximize ram air pressure recovery.
Even with dihedral wings, low wing aircraft must also address the possibility of fuel entering the vent box. During taxi operations, centrifugal forces can push fuel present in the vent lines into the vent box. In addition, wing dihedral is reduced, and in some cases nearly eliminated, while the aircraft is on the ground due to the combined weight of wing fuel and wing mounted engines. This effect is more pronounced on large aircraft with long wingspans. As a result, vent box sizing is often based on accommodating fuel transfer during a predetermined number of taxi turns, assuming the vent lines are completely filled with fuel.
In low wing, dihedral wing designs, scavenge pumps are generally not required. Any fuel that migrates into the vent box during ground operations can be directed back into the fuel tanks after takeoff through the use of check valves.
The positioning of vent openings within each fuel tank must take into account the wide range of aircraft attitudes and acceleration forces experienced during both ground and flight operations. During forward acceleration or pitch up attitudes, fuel is forced toward the rear of the tank, and in swept wing configurations, fuel is also driven outboard. To accommodate these conditions, a vent opening must be located near the wing root and close to the forward boundary of the tank.
Conversely, during forward deceleration or pitch down attitudes, fuel is forced toward the front of the tank, and in swept wings, fuel moves inboard. To address this situation, another vent opening must be positioned aft and outboard within the fuel tank. Figure 2 presents a simplified schematic of a vent system for a three tank aircraft.
In this example, each tank is vented to ensure a continuous breathing path to the external atmosphere for all long term flight attitudes encountered during normal operations. To reduce the likelihood of vent lines becoming filled with fuel, float actuated vent valves are commonly installed. These valves automatically close the vent line whenever fuel is present.
This operating concept is illustrated in Figure 3, which shows a dihedral wing configuration and demonstrates how a float actuated vent valve closes the vent line when fuel enters it. The figure also shows the vent box and its connection to the external airflow, as well as a flame arrestor. The flame arrestor prevents flame propagation into the vent system, which could otherwise occur if a lightning strike ignited fuel vapors exiting the vent inlet.
Flame arrestors must be carefully designed to minimize the risk of icing, since ice accumulation could block the vent system. To ensure continued protection of the fuel tank structure in the event of a blocked vent line, a secondary pressure relief device, such as a burst disk or relief valve, is typically installed. This redundancy ensures that a single failure cannot result in structural damage.
The external air inlet to the vent box typically uses a specially contoured scoop optimized by the National Advisory Committee for Aeronautics (NACA) in 1945. This scoop, commonly referred to as a submerged duct entrance, provides an effective balance between pressure recovery and aerodynamic drag. Efficient dynamic pressure recovery improves boost pump performance margins, particularly during hot and high operating conditions where fuel vapor formation can negatively affect pump performance.
The aircraft forward velocity is recovered in the form of increased pressure and temperature, as described by the equations governing adiabatic, or constant energy, flow of an ideal fluid:
where TT and PT are the total temperature and total pressure, TO and PO are the free stream temperature and pressure, M is the Mach number, and g is the ratio of specific heats, equal to 1.4 for air.
Figure 4 illustrates a NACA scoop along with a graph showing pressure recovery ratios for ideal fluid flow compared with typical NACA scoop performance.
From this data, an aircraft cruising at Mach 0.8 at 35,000 ft, where the static air pressure is approximately 3.46 psia, will experience a vent system ullage pressure between 4.5 and 4.9 psia. This represents a pressure recovery of approximately 1.0 to 1.5 psi.
Figure 5 shows the wing and center tank vent system arrangement of the Airbus A340-600, highlighting the complexity required to provide adequate venting for a modern long range transport aircraft. In this configuration, the wing tank consists of two inboard feed tanks and one outboard tank, each of which must be vented independently. The center tank is equipped with its own dedicated vent line and overpressure protection devices, which in this case are burst disks.
An additional contributor to vent system complexity is the flexibility of the wing structure. When the aircraft is on the ground, the combined weight of fuel and wing mounted engines can shift the highest point of the ullage to a mid span location. The vent system must be capable of accommodating this condition during ground refueling. To address this, bubble studies are routinely performed to determine the ullage location in full or nearly full tanks under various ground and flight conditions.
Scale models of wing tanks are frequently used to qualitatively evaluate vent system behavior for different aircraft attitudes and fuel quantities. These models typically include representative rib structures and may be loaded to simulate operational flight loads.
Figure 6 shows a quarter scale model of the Global Express™ wing and center tank, which was used to assess vent system performance during refueling well before the actual aircraft wing structure was available.
Design requirements typically specify a maximum allowable pressure difference between the ambient external air pressure and the ullage pressure. To demonstrate compliance with these requirements, simulation analyses are performed. The results of these simulations, together with supporting validation data from models and tests, are used to support the aircraft certification process.
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Designing the wing structure to withstand such pressure loads would be impractical due to the excessive weight penalties involved. As a result, the vent system plays a critical role in protecting the fuel tank structure from structural failure as the aircraft transitions between ground level and cruise altitude.
During refueling operations, the fuel entering the tanks displaces the air already present inside them. For safety reasons, fuel spillage to the outside must be avoided. To achieve this reliably, a vent box, also known as a surge tank, is incorporated into the vent system. The vent box is designed to capture any fuel that may enter the vent lines during refueling.
On most commercial aircraft, the vent box is located near the wing tip. In typical low wing aircraft designs, which account for the majority of commercial transport aircraft, the wings incorporate significant dihedral. This wing geometry naturally makes the vent box the highest point during flight. Even on aircraft that feature anhedral wings, the vent box is often still positioned outboard. Figure 1 illustrates two different aircraft configurations, one showing a low wing installation with pronounced dihedral typical of commercial transports, and the other showing a high wing configuration with anhedral, which is more commonly associated with military transport aircraft.
 |
| Figure 1. Vent box locations in typical transport aircraft |
As shown in the figure, the vent box location on a high wing aircraft may not represent the highest point during flight. Consequently, these vent systems typically include scavenge pumps to prevent fuel accumulation within the vent box caused by fuel migrating through the tank vent lines. Despite this, the vent box is still positioned outboard so that the air scoop, which connects the vent system to the external airflow, can be located well away from the fuselage. This location helps maximize ram air pressure recovery.
Even with dihedral wings, low wing aircraft must also address the possibility of fuel entering the vent box. During taxi operations, centrifugal forces can push fuel present in the vent lines into the vent box. In addition, wing dihedral is reduced, and in some cases nearly eliminated, while the aircraft is on the ground due to the combined weight of wing fuel and wing mounted engines. This effect is more pronounced on large aircraft with long wingspans. As a result, vent box sizing is often based on accommodating fuel transfer during a predetermined number of taxi turns, assuming the vent lines are completely filled with fuel.
In low wing, dihedral wing designs, scavenge pumps are generally not required. Any fuel that migrates into the vent box during ground operations can be directed back into the fuel tanks after takeoff through the use of check valves.
The positioning of vent openings within each fuel tank must take into account the wide range of aircraft attitudes and acceleration forces experienced during both ground and flight operations. During forward acceleration or pitch up attitudes, fuel is forced toward the rear of the tank, and in swept wing configurations, fuel is also driven outboard. To accommodate these conditions, a vent opening must be located near the wing root and close to the forward boundary of the tank.
Conversely, during forward deceleration or pitch down attitudes, fuel is forced toward the front of the tank, and in swept wings, fuel moves inboard. To address this situation, another vent opening must be positioned aft and outboard within the fuel tank. Figure 2 presents a simplified schematic of a vent system for a three tank aircraft.
 |
| Figure 2. Simple three tank vent system schematic |
In this example, each tank is vented to ensure a continuous breathing path to the external atmosphere for all long term flight attitudes encountered during normal operations. To reduce the likelihood of vent lines becoming filled with fuel, float actuated vent valves are commonly installed. These valves automatically close the vent line whenever fuel is present.
This operating concept is illustrated in Figure 3, which shows a dihedral wing configuration and demonstrates how a float actuated vent valve closes the vent line when fuel enters it. The figure also shows the vent box and its connection to the external airflow, as well as a flame arrestor. The flame arrestor prevents flame propagation into the vent system, which could otherwise occur if a lightning strike ignited fuel vapors exiting the vent inlet.
 |
| Figure 3. Float actuated vent valve schematic |
Flame arrestors must be carefully designed to minimize the risk of icing, since ice accumulation could block the vent system. To ensure continued protection of the fuel tank structure in the event of a blocked vent line, a secondary pressure relief device, such as a burst disk or relief valve, is typically installed. This redundancy ensures that a single failure cannot result in structural damage.
The external air inlet to the vent box typically uses a specially contoured scoop optimized by the National Advisory Committee for Aeronautics (NACA) in 1945. This scoop, commonly referred to as a submerged duct entrance, provides an effective balance between pressure recovery and aerodynamic drag. Efficient dynamic pressure recovery improves boost pump performance margins, particularly during hot and high operating conditions where fuel vapor formation can negatively affect pump performance.
The aircraft forward velocity is recovered in the form of increased pressure and temperature, as described by the equations governing adiabatic, or constant energy, flow of an ideal fluid:
where TT and PT are the total temperature and total pressure, TO and PO are the free stream temperature and pressure, M is the Mach number, and g is the ratio of specific heats, equal to 1.4 for air.
Figure 4 illustrates a NACA scoop along with a graph showing pressure recovery ratios for ideal fluid flow compared with typical NACA scoop performance.
 |
| Figure 4. NACA scoop recovery pressure |
From this data, an aircraft cruising at Mach 0.8 at 35,000 ft, where the static air pressure is approximately 3.46 psia, will experience a vent system ullage pressure between 4.5 and 4.9 psia. This represents a pressure recovery of approximately 1.0 to 1.5 psi.
Figure 5 shows the wing and center tank vent system arrangement of the Airbus A340-600, highlighting the complexity required to provide adequate venting for a modern long range transport aircraft. In this configuration, the wing tank consists of two inboard feed tanks and one outboard tank, each of which must be vented independently. The center tank is equipped with its own dedicated vent line and overpressure protection devices, which in this case are burst disks.
 |
| Figure 5. A340-600 wing vent arrangement |
An additional contributor to vent system complexity is the flexibility of the wing structure. When the aircraft is on the ground, the combined weight of fuel and wing mounted engines can shift the highest point of the ullage to a mid span location. The vent system must be capable of accommodating this condition during ground refueling. To address this, bubble studies are routinely performed to determine the ullage location in full or nearly full tanks under various ground and flight conditions.
Scale models of wing tanks are frequently used to qualitatively evaluate vent system behavior for different aircraft attitudes and fuel quantities. These models typically include representative rib structures and may be loaded to simulate operational flight loads.
Figure 6 shows a quarter scale model of the Global Express™ wing and center tank, which was used to assess vent system performance during refueling well before the actual aircraft wing structure was available.
 |
| Figure 6. Global Express quarter scale wing tank model |
Vent System Sizing
For commercial aircraft, vent system sizing requirements are generally driven by the emergency descent case. In this scenario, the aircraft must descend rapidly from maximum cruise altitude to below 10,000 ft following a loss of cabin pressurization. The most demanding fuel condition occurs when the tanks are nearly empty, since the ullage volume is at its maximum and vent system airflow is highest.Design requirements typically specify a maximum allowable pressure difference between the ambient external air pressure and the ullage pressure. To demonstrate compliance with these requirements, simulation analyses are performed. The results of these simulations, together with supporting validation data from models and tests, are used to support the aircraft certification process.
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