Fuel systems

Clean fuel to every engine

An aircraft's fuel system must do far more than simply store fuel. It must safely contain thousands of pounds of fuel, accurately measure it, move it between tanks, supply engines under all operating conditions, maintain aircraft balance, and prevent ignition sources from reaching fuel vapors. Because fuel directly affects performance, range and safety, fuel systems are among the most critical systems on a transport aircraft.

The basic objective is simple: deliver clean fuel to every engine, at the correct pressure and flow rate, under all operating conditions.

Fuel storage and tank layout

Most transport aircraft store fuel inside the wing structure itself. These are known as integral tanks because the wing structure forms the walls of the tank. Using the wings as fuel tanks saves weight and places fuel close to the aircraft's center of lift. Typical fuel storage locations include:

• Left wing tank
• Right wing tank
• Center tank
• Horizontal stabilizer (trim tank) on some aircraft

Wing tanks

The wing tanks are the main fuel storage. Fuel carried in the wings counteracts the upward bending caused by lift, so many aircraft keep wing fuel until last to relieve wing-bending loads during flight. Benefits include:

• Efficient weight distribution
• Structural efficiency
• Reduced wing-bending loads
• Natural cooling from airflow

Center tanks

The center tank provides additional fuel capacity for longer flights. Automatic fuel-management systems often handle the sequencing. Center-tank fuel is often used before wing-tank fuel because:

• It keeps wing fuel in place to relieve wing-bending loads.
• It reduces aircraft weight sooner.
• It frees center-tank capacity.
• It optimizes fuel management.

Trim tanks

Some long-range aircraft include a trim tank in the horizontal stabilizer. The trim tank allows fuel to be moved aft during cruise, and fuel may later be transferred forward before landing. Benefits include:

• Reduced trim drag
• Improved fuel efficiency
• Better center-of-gravity management

Fuel feed systems

Fuel must reach the engines under all operating conditions, including climb, cruise, descent, turbulence and negative accelerations within certification limits. Several systems work together to ensure a continuous fuel supply.

Boost pumps

Each tank normally contains electrically powered boost pumps. Positive pressure ensures fuel reaches the engine reliably even at high altitudes. Their functions include:

• Supplying positive fuel pressure
• Preventing vapor formation
• Improving engine-feed reliability
• Supporting engine starting
• Assisting fuel transfer operations

Engine-driven pumps

Each engine contains its own engine-driven fuel pump, which provides the pressure required by the engine fuel-control system. In many aircraft, engine-driven pumps can draw fuel by suction if boost pumps fail. For this reason boost pumps remain the primary fuel-feed method, because suction-feed capability is usually limited by:

• Altitude
• Temperature
• Fuel quantity
• Aircraft certification requirements

Crossfeed systems

A crossfeed system allows fuel from one tank to supply engines on the opposite side of the aircraft. The central component is the crossfeed valve. Many aircraft use automatic fuel-management logic to minimize crew workload. Crossfeed is used to:

• Correct fuel imbalance: aircraft must maintain acceptable lateral balance. If one wing contains significantly more fuel than the other, roll trim changes, control forces increase and structural loads may change. Crossfeed allows fuel consumption to restore balance.
• Handle pump failures: if a boost pump fails, the opposite tank may temporarily supply both engines.
• Accommodate system failures: crossfeed provides flexibility during abnormal operations.

Fuel as a heat sink

Fuel does more than power the engines. It also serves as an important cooling medium. Modern transport aircraft use fuel-to-oil or fuel-to-fluid heat exchangers to remove heat from hydraulic systems, engine oil systems, generator oil systems, integrated drive generators (IDGs) and electronic equipment.

The fuel system therefore contributes significantly to thermal management throughout the aircraft. Fuel absorbs heat efficiently because:

• Large quantities are carried.
• It constantly circulates.
• Fuel temperature remains relatively stable.

Measuring fuel quantity

Knowing how much fuel remains onboard is critical for flight planning, performance calculations, range determination, and weight and balance calculations.

Most transport aircraft use capacitance probes inside each tank. Because fuel and air have different dielectric properties, changes in fuel level alter the probes' electrical capacitance, and the system calculates fuel quantity from those changes.

Fuel mass vs. fuel volume

Aircraft performance depends on fuel mass, not fuel volume. The same volume of fuel can weigh differently depending on fuel temperature, density and type. For this reason, fuel quantity systems use density information to calculate fuel mass rather than simply displaying gallons or liters. Mass determines:

• Range
• Endurance
• Aircraft weight
• Center of gravity

Manual quantity verification

Electronic indications are cross-checked through manual means, which provide an independent confirmation of fuel onboard. Depending on aircraft type, these may include:

• Measuring sticks: calibrated sticks that extend into the tank.
• Magnetic level indicators: external indicators that show fuel level mechanically.
• Drip sticks: common on many airliners for ground verification of fuel quantity.

Water contamination

Clean fuel is essential for safe operation. Water enters fuel systems through condensation, fuel handling operations and environmental exposure. Because water is heavier than fuel, it settles to the lowest points in the tank. Water contamination is one of the most common fuel-system concerns.

Fuel tanks contain low-point sump drains that allow accumulated water to be removed. Regular draining helps prevent:

• Ice formation
• Fuel starvation
• Corrosion
• Microbial growth

Microbial contamination

Microorganisms grow at the fuel-water interface, creating sludge, acid byproducts and debris. Microbial growth is particularly common in warm, humid environments, and regular inspections and treatment programs help control it. It can:

• Block filters
• Corrode components
• Damage coatings
• Restrict fuel flow

Particulate contamination and misfueling

• Particulate contamination: particles can originate from dirt, corrosion, tank deterioration, fueling equipment and component wear. Fuel filters and quality-control procedures prevent contaminants from reaching engines.
• Misfueling: using the wrong fuel grade, such as jet fuel in a gasoline-powered aircraft, an incorrect fuel specification or a contaminated delivery, can cause serious consequences. Fueling procedures include multiple verification steps to prevent these errors.

Fuel tank safety

Modern fuel-tank safety requirements are heavily influenced by accident investigations and decades of operational experience. Fuel vapor is flammable when fuel is present, oxygen is present and an ignition source exists. Safety efforts focus on eliminating ignition sources and reducing flammability.

Ignition source prevention

Fuel tanks contain numerous electrical components, including fuel pumps, quantity probes, wiring and connectors. These components are designed to prevent sparks from reaching fuel vapors, and maintaining these safeguards is critical. Protective measures include:

• Bonding and grounding
• Intrinsically safe designs
• Shielded wiring
• Fault protection systems
• Specialized connectors

Fuel tank inerting

Many modern aircraft use fuel-tank inerting systems. These systems introduce nitrogen-enriched air (NEA) into the tank vapor space, reducing the oxygen concentration below levels that support combustion. Fuel-tank inerting is now a major component of modern transport-aircraft fuel-system design. Benefits include:

• Reduced flammability
• Lower ignition risk
• Improved tank safety

Critical design configuration control limitations (CDCCLs)

Some fuel-tank safety features are so important that their configuration is protected by regulation. These requirements are known as critical design configuration control limitations (CDCCLs). CDCCLs preserve the conditions under which the aircraft was certified. Examples may include:

• Wire separation distances
• Bonding arrangements
• Fuel-pump configurations
• Connector installations
• Protective sleeving

Why CDCCLs matter

A seemingly minor maintenance change could unintentionally increase ignition risk, compromise fault protection or defeat safety features. For that reason, CDCCL-related maintenance instructions are mandatory and must not be altered without approved authorization.

Fuel system maintenance considerations

Fuel-system work demands strict cleanliness standards because even small contaminants can create significant problems later. Technicians routinely inspect:

• Fuel leaks
• Pump operation
• Crossfeed valve function
• Fuel quantity indications
• Tank bonding
• Wiring condition
• Filter contamination
• Water accumulation
• Fuel temperature systems
• Inerting components

Key takeaways

• Fuel is stored primarily in integral wing tanks, with center and trim tanks used on many transport aircraft.
• Boost pumps provide positive fuel pressure. Engine-driven pumps provide the engine's fuel supply pressure.
• Crossfeed systems allow any tank to feed multiple engines and help correct fuel imbalance.
• Fuel serves as both an energy source and an important cooling medium.
• Modern aircraft measure fuel primarily by mass rather than volume.
• Capacitance probes are the most common fuel-quantity sensing devices.
• Water, microbial growth and particulate contamination are major fuel-system concerns.
• Fuel-tank safety focuses on preventing ignition sources and reducing flammability.
• Nitrogen-enriched-air systems reduce oxygen concentration inside fuel tanks.
• CDCCLs protect safety-critical fuel-tank design features and must be followed exactly.

The bottom line

A transport aircraft's fuel system is far more than a collection of tanks and pumps. It is a carefully engineered system that stores fuel safely, supplies engines reliably, maintains aircraft balance, cools other aircraft systems, and protects against contamination and ignition hazards. Modern fuel-system design combines redundancy, precise measurement, contamination control and fuel-tank safety features to ensure that clean fuel reaches the engines while minimizing the risks associated with carrying thousands of gallons of flammable liquid through every phase of flight.